Flutter – Introduction

In general, developing a mobile application is a complex and challenging task. There are many frameworks available to develop a mobile application. Android provides a native framework based on Java language and iOS provides a native framework based on Objective-C / Swift language.

However, to develop an application supporting both the OSs, we need to code in two different languages using two different frameworks. To help overcome this complexity, there exists mobile frameworks supporting both OS. These frameworks range from simple HTML based hybrid mobile application framework (which uses HTML for User Interface and JavaScript for application logic) to complex language specific framework (which do the heavy lifting of converting code to native code). Irrespective of their simplicity or complexity, these frameworks always have many disadvantages, one of the main drawback being their slow performance.

In this scenario, Flutter – a simple and high performance framework based on Dart language, provides high performance by rendering the UI directly in the operating system’s canvas rather than through native framework.

Flutter also offers many ready to use widgets (UI) to create a modern application. These widgets are optimized for mobile environment and designing the application using widgets is as simple as designing HTML.

To be specific, Flutter application is itself a widget. Flutter widgets also supports animations and gestures. The application logic is based on reactive programming. Widget may optionally have a state. By changing the state of the widget, Flutter will automatically (reactive programming) compare the widget’s state (old and new) and render the widget with only the necessary changes instead of re-rendering the whole widget.

We shall discuss the complete architecture in the coming chapters.

Features of Flutter

Flutter framework offers the following features to developers −

• Modern and reactive framework.

• Uses Dart programming language and it is very easy to learn.

• Fast development.

• Beautiful and fluid user interfaces.

• Huge widget catalog.

• Runs same UI for multiple platforms.

• High performance application.

Advantages of Flutter

Flutter comes with beautiful and customizable widgets for high performance and outstanding mobile application. It fulfills all the custom needs and requirements. Besides these, Flutter offers many more advantages as mentioned below −

• Dart has a large repository of software packages which lets you to extend the capabilities of your application.

• Developers need to write just a single code base for both applications (both Android and iOS platforms). Flutter may to be extended to other platform as well in the future.

• Flutter needs lesser testing. Because of its single code base, it is sufficient if we write automated tests once for both the platforms.

• Flutter’s simplicity makes it a good candidate for fast development. Its customization capability and extendibility makes it even more powerful.

• With Flutter, developers has full control over the widgets and its layout.

• Flutter offers great developer tools, with amazing hot reload.

Disadvantages of Flutter

Despite its many advantages, flutter has the following drawbacks in it −

• Since it is coded in Dart language, a developer needs to learn new language (though it is easy to learn).

• Modern framework tries to separate logic and UI as much as possible but, in Flutter, user interface and logic is intermixed. We can overcome this using smart coding and using high level module to separate user interface and logic.

• Flutter is yet another framework to create mobile application. Developers are having a hard time in choosing the right development tools in hugely populated segment.

Flutter – Installation

This chapter will guide you through the installation of Flutter on your local computer in detail.

Installation in Windows

In this section, let us see how to install Flutter SDK and its requirement in a windows system.

Step 1 − Go to URL, https://flutter.dev/docs/get-started/install/windows and download the latest Flutter SDK. As of April 2019, the version is 1.2.1 and the file is flutter_windows_v1.2.1-stable.zip.

Step 2 − Unzip the zip archive in a folder, say C:flutter

Step 3 − Update the system path to include flutter bin directory.

Step 4 − Flutter provides a tool, flutter doctor to check that all the requirement of flutter development is met.

flutter doctor

Step 5 − Running the above command will analyze the system and show its report as shown below −

Doctor summary (to see all details, run flutter doctor -v):
[√] Flutter (Channel stable, v1.2.1, on Microsoft Windows [Version
10.0.17134.706], locale en-US)
[√] Android toolchain - develop for Android devices (Android SDK version
28.0.3)
[√] Android Studio (version 3.2)
[√] VS Code, 64-bit edition (version 1.29.1)
[!] Connected device
! No devices available
! Doctor found issues in 1 category.

The report says that all development tools are available but the device is not connected. We can fix this by connecting an android device through USB or starting an android emulator.

Step 6 − Install the latest Android SDK, if reported by flutter doctor

Step 7 − Install the latest Android Studio, if reported by flutter doctor

Step 8 − Start an android emulator or connect a real android device to the system.

Step 9 − Install Flutter and Dart plugin for Android Studio. It provides startup template to create new Flutter application, an option to run and debug Flutter application in the Android studio itself, etc.,

• Open Android Studio.

• Click File → Settings → Plugins.

• Select the Flutter plugin and click Install.

• Click Yes when prompted to install the Dart plugin.

• Restart Android studio.

Installation in MacOS

To install Flutter on MacOS, you will have to follow the following steps −

Step 1 − Go to URL, https://flutter.dev/docs/get-started/install/macos and download latest Flutter SDK. As of April 2019, the version is 1.2.1 and the file is flutter_macos_v1.2.1- stable.zip.

Step 2 − Unzip the zip archive in a folder, say /path/to/flutter

Step 3 − Update the system path to include flutter bin directory (in ~/.bashrc file).

> export PATH = "$PATH:/path/to/flutter/bin"

Step 4 − Enable the updated path in the current session using below command and then verify it as well.

source ~/.bashrc
source $HOME/.bash_profile
echo $PATH

Flutter provides a tool, flutter doctor to check that all the requirement of flutter development is met. It is similar to the Windows counterpart.

Step 5 − Install latest XCode, if reported by flutter doctor

Step 6 − Install latest Android SDK, if reported by flutter doctor

Step 7 − Install latest Android Studio, if reported by flutter doctor

Step 8 − Start an android emulator or connect a real android device to the system to develop android application.

Step 9 − Open iOS simulator or connect a real iPhone device to the system to develop iOS application.

Step 10 − Install Flutter and Dart plugin for Android Studio. It provides the startup template to create a new Flutter application, option to run and debug Flutter application in the Android studio itself, etc.,

• Open Android Studio

• Click Preferences → Plugins

• Select the Flutter plugin and click Install

• Click Yes when prompted to install the Dart plugin.

• Restart Android studio.

Creating Simple Application in Android Studio

In this chapter, let us create a simple Flutter application to understand the basics of creating a flutter application in the Android Studio.

Step 1 − Open Android Studio

Step 2 − Create Flutter Project. For this, click File → New → New Flutter Project

Step 3 − Select Flutter Application. For this, select Flutter Application and click Next.

Step 4 − Configure the application as below and click Next.

• Project name: hello_app

• Flutter SDK Path: <path_to_flutter_sdk>

• Project Location: <path_to_project_folder>

• Description: Flutter based hello world application

Step 5 − Configure Project.

Set the company domain as flutterapp.tutorialspoint.com and click Finish.

Step 6 − Enter Company domain.

Android Studio creates a fully working flutter application with minimal functionality. Let us check the structure of the application and then, change the code to do our task.

The structure of the application and its purpose is as follows −

Various components of the structure of the application are explained here −

• android − Auto generated source code to create android application

• ios − Auto generated source code to create ios application

• lib − Main folder containing Dart code written using flutter framework

• ib/main.dart − Entry point of the Flutter application

• test − Folder containing Dart code to test the flutter application

• test/widget_test.dart − Sample code

• .gitignore − Git version control file

• .metadata − auto generated by the flutter tools

• .packages − auto generated to track the flutter packages

• .iml − project file used by Android studio

• pubspec.yaml − Used by Pub, Flutter package manager

• pubspec.lock − Auto generated by the Flutter package manager, Pub

• README.md − Project description file written in Markdown format

Step 7 − Replace the dart code in the lib/main.dart file with the below code −

import ''package:flutter/material.dart'';

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
   // This widget is the root of your application.
   @override
   Widget build(BuildContext context) {
      return MaterialApp(
         title: ''Hello World Demo Application'',
         theme: ThemeData(
            primarySwatch: Colors.blue,
         ),
         home: MyHomePage(title: ''Home page''),
      );
   }
}
class MyHomePage extends StatelessWidget {
   MyHomePage({Key key, this.title}) : super(key: key);
   final String title;

   @override
   Widget build(BuildContext context) {
      return Scaffold(
         appBar: AppBar(
            title: Text(this.title),
         ),
         body: Center(
            child:
            Text(
               ''Hello World'',
            )
         ),
      );
   }
}

Let us understand the dart code line by line.

• Line 1 − imports the flutter package, material. The material is a flutter package to create user interface according to the Material design guidelines specified by Android.

• Line 3 − This is the entry point of the Flutter application. Calls runApp function and pass it an object of MyApp class. The purpose of the runApp function is to attach the given widget to the screen.

• Line 5-17 − Widget is used to create UI in flutter framework. StatelessWidget is a widget, which does not maintain any state of the widget. MyApp extends StatelessWidget and overrides its build method. The purpose of the build method is to create a part of the UI of the application. Here, build method uses MaterialApp, a widget to create the root level UI of the application. It has three properties – title, theme and home.

  • title is the title of the application

  • theme is the theme of the widget. Here, we set blue as the overall color of the application using ThemeData class and its property, primarySwatch.

  • home is the inner UI of the application, which we set another widget, MyHomePage

• Line 19 – 38 − MyHomePage is same as MyApp except it returns Scaffold Widget. Scaffold is a top level widget next to MaterialApp widget used to create UI conforming material design. It has two important properties, appBar to show the header of the application and body to show the actual content of the application. AppBar is another widget to render the header of the application and we have used it in appBar property. In body property, we have used Center widget, which centers it child widget. Text is the final and inner most widget to show the text and it is displayed in the center of the screen.

Step 8 − Now, run the application using, Run → Run main.dart

Step 9 − Finally, the output of the application is as follows −

Flutter – Architecture Application

In this chapter, let us discuss the architecture of the Flutter framework.

Widgets

The core concept of the Flutter framework is In Flutter, Everything is a widget. Widgets are basically user interface components used to create the user interface of the application.

In Flutter, the application is itself a widget. The application is the top- level widget and its UI is build using one or more children (widgets), which again build using its children widgets. This composability feature helps us to create a user interface of any complexity.

For example, the widget hierarchy of the hello world application (created in previous chapter) is as specified in the following diagram −

Here the following points are worth notable −

• MyApp is the user created widget and it is build using the Flutter native widget, MaterialApp.

• MaterialApp has a home property to specify the user interface of the home page, which is again a user created widget, MyHomePage.

• MyHomePage is build using another flutter native widget, Scaffold

• Scaffold has two properties – body and appBar

• body is used to specify its main user interface and appBar is used to specify its header user interface

• Header UI is build using flutter native widget, AppBar and Body UI is build using Center widget.

• The Center widget has a property, Child, which refers the actual content and it is build using Text widget

Gestures

Flutter widgets support interaction through a special widget, GestureDetector. GestureDetector is an invisible widget having the ability to capture user interactions such as tapping, dragging, etc., of its child widget. Many native widgets of Flutter support interaction through the use of GestureDetector. We can also incorporate interactive feature into the existing widget by composing it with the GestureDetector widget. We will learn the gestures separately in the upcoming chapters.

Concept of State

Flutter widgets support State maintenance by providing a special widget, StatefulWidget. Widget needs to be derived from StatefulWidget widget to support state maintenance and all other widget should be derived from StatefulWidget. Flutter widgets are reactive in native. This is similar to reactjs and StatefulWidget will be auto re- rendered whenever its internal state is changed. The re-rendering is optimized by finding the difference between old and new widget UI and rendering only the necessary changes

Layers

The most important concept of Flutter framework is that the framework is grouped into multiple category in terms of complexity and clearly arranged in layers of decreasing complexity. A layer is build using its immediate next level layer. The top most layer is widget specific to Android and iOS. The next layer has all flutter native widgets. The next layer is Rendering layer, which is low level renderer component and renders everything in the flutter app. Layers goes down to core platform specific code

The general overview of a layer in Flutter is specified in the below diagram −

The following points summarize the architecture of Flutter −

• In Flutter, everything is a widget and a complex widget is composed of already existing widgets.

• Interactive features can be incorporated whenever necessary using GestureDetector widget.

• The state of a widget can be maintained whenever necessary using StatefulWidget widget.

• Flutter offers layered design so that any layer can be programmed depending on the complexity of the task.

We will discuss all these concepts in detail in the upcoming chapters.

Flutter – Introduction to Dart Programming

Dart is an open-source general-purpose programming language. It is originally developed by Google. Dart is an object-oriented language with C-style syntax. It supports programming concepts like interfaces, classes, unlike other programming languages Dart doesn’t support arrays. Dart collections can be used to replicate data structures such as arrays, generics, and optional typing.

The following code shows a simple Dart program −

void main() {
   print("Dart language is easy to learn");
}

Variables and Data types

Variable is named storage location and Data types simply refers to the type and size of data associated with variables and functions.

Dart uses var keyword to declare the variable. The syntax of var is defined below,

var name = ''Dart'';

The final and const keyword are used to declare constants. They are defined as below −

void main() {
   final a = 12;
   const pi = 3.14;
   print(a);
   print(pi);
}

Dart language supports the following data types −

• Numbers − It is used to represent numeric literals – Integer and Double.

• Strings − It represents a sequence of characters. String values are specified in either single or double quotes.

• Booleans − Dart uses the bool keyword to represent Boolean values – true and false.

• Lists and Maps − It is used to represent a collection of objects. A simple List can be defined as below −.

void main() {
   var list = [1,2,3,4,5];
   print(list);
}

The list shown above produces [1,2,3,4,5] list.

Map can be defined as shown here −

void main() {
   var mapping = {''id'': 1,''name'':''Dart''};
   print(mapping);
}

• Dynamic − If the variable type is not defined, then its default type is dynamic. The following example illustrates the dynamic type variable −

void main() {
   dynamic name = "Dart";
   print(name);
}

Decision Making and Loops

A decision making block evaluates a condition before the instructions are executed. Dart supports If, If..else and switch statements.

Loops are used to repeat a block of code until a specific condition is met. Dart supports for, for..in , while and do..while loops.

Let us understand a simple example about the usage of control statements and loops −

void main() {
   for( var i = 1 ; i <= 10; i++ ) {
      if(i%2==0) {
         print(i);
      }
   }
}

The above code prints the even numbers from 1 to 10.

Functions

A function is a group of statements that together performs a specific task. Let us look into a simple function in Dart as shown here −

void main() {
   add(3,4);
}
void add(int a,int b) {
   int c;
   c = a+b;
   print(c);
}

The above function adds two values and produces 7 as the output.

Object Oriented Programming

Dart is an object-oriented language. It supports object-oriented programming features like classes, interfaces, etc.

A class is a blueprint for creating objects. A class definition includes the following −

• Fields
• Getters and setters
• Constructors
• Functions

Now, let us create a simple class using the above definitions −

class Employee {
   String name;
   
   //getter method
   String get emp_name {
      return name;
   }
   //setter method
   void set emp_name(String name) {
      this.name = name;
   }
   //function definition
   void result() {
      print(name);
   }
}
void main() {
   //object creation
   Employee emp = new Employee();
   emp.name = "employee1";
   emp.result(); //function call
}

Flutter – Introduction to Widgets

As we learned in the earlier chapter, widgets are everything in Flutter framework. We have already learned how to create new widgets in previous chapters.

In this chapter, let us understand the actual concept behind creating the widgets and the different type of widgets available in Flutter framework.

Let us check the Hello World application’s MyHomePage widget. The code for this purpose is as given below −

class MyHomePage extends StatelessWidget { 
   MyHomePage({Key key, this.title}) : super(key: key); 
   
   final String title; 
   @override 
   Widget build(BuildContext context) {
      return Scaffold( 
         appBar: AppBar(title: Text(this.title), ), 
         body: Center(child: Text( ''Hello World'',)),
      );
   }
}

Here, we have created a new widget by extending StatelessWidget.

Note that the StatelessWidget only requires a single method build to be implemented in its derived class. The build method gets the context environment necessary to build the widgets through BuildContext parameter and returns the widget it builds.

In the code, we have used title as one of the constructor argument and also used Key as another argument. The title is used to display the title and Key is used to identify the widget in the build environment.

Here, the build method calls the build method of Scaffold, which in turn calls the build method of AppBar and Center to build its user interface.

Finally, Center build method calls Text build method.

For a better understanding, the visual representation of the same is given below −

Widget Build Visualization

In Flutter, widgets can be grouped into multiple categories based on their features, as listed below −

• Platform specific widgets
• Layout widgets
• State maintenance widgets
• Platform independent / basic widgets

Let us discuss each of them in detail now.

Platform specific widgets

Flutter has widgets specific to a particular platform – Android or iOS.

Android specific widgets are designed in accordance with Material design guideline by Android OS. Android specific widgets are called as Material widgets.

iOS specific widgets are designed in accordance with Human Interface Guidelines by Apple and they are called as Cupertino widgets.

Some of the most used material widgets are as follows −

• Scaffold
• AppBar
• BottomNavigationBar
• TabBar
• TabBarView
• ListTile
• RaisedButton
• FloatingActionButton
• FlatButton
• IconButton
• DropdownButton
• PopupMenuButton
• ButtonBar
• TextField
• Checkbox
• Radio
• Switch
• Slider
• Date & Time Pickers
• SimpleDialog
• AlertDialog

Some of the most used Cupertino widgets are as follows −

• CupertinoButton
• CupertinoPicker
• CupertinoDatePicker
• CupertinoTimerPicker
• CupertinoNavigationBar
• CupertinoTabBar
• CupertinoTabScaffold
• CupertinoTabView
• CupertinoTextField
• CupertinoDialog
• CupertinoDialogAction
• CupertinoFullscreenDialogTransition
• CupertinoPageScaffold
• CupertinoPageTransition
• CupertinoActionSheet
• CupertinoActivityIndicator
• CupertinoAlertDialog
• CupertinoPopupSurface
• CupertinoSlider

Layout widgets

In Flutter, a widget can be created by composing one or more widgets. To compose multiple widgets into a single widget, Flutter provides large number of widgets with layout feature. For example, the child widget can be centered using Center widget.

Some of the popular layout widgets are as follows −

• Container − A rectangular box decorated using BoxDecoration widgets with background, border and shadow.

• Center − Center its child widget.

• Row − Arrange its children in the horizontal direction.

• Column − Arrange its children in the vertical direction.

• Stack − Arrange one above the another.

We will check the layout widgets in detail in the upcoming Introduction to layout widgets chapter.

State maintenance widgets

In Flutter, all widgets are either derived from StatelessWidget or StatefulWidget.

Widget derived from StatelessWidget does not have any state information but it may contain widget derived from StatefulWidget. The dynamic nature of the application is through interactive behavior of the widgets and the state changes during interaction. For example, tapping a counter button will increase / decrease the internal state of the counter by one and reactive nature of the Flutter widget will auto re-render the widget using new state information.

We will learn the concept of StatefulWidget widgets in detail in the upcoming State management chapter.

Platform independent / basic widgets

Flutter provides large number of basic widgets to create simple as well as complex user interface in a platform independent manner. Let us see some of the basic widgets in this chapter.

Text

Text widget is used to display a piece of string. The style of the string can be set by using style property and TextStyle class. The sample code for this purpose is as follows −

Text(''Hello World!'', style: TextStyle(fontWeight: FontWeight.bold))

Text widget has a special constructor, Text.rich, which accepts the child of type TextSpan to specify the string with different style. TextSpan widget is recursive in nature and it accepts TextSpan as its children. The sample code for this purpose is as follows −

Text.rich( 
   TextSpan( 
      children: <TextSpan>[ 
         TextSpan(text: "Hello ", style:  
         TextStyle(fontStyle: FontStyle.italic)),  
         TextSpan(text: "World", style: 
         TextStyle(fontWeight: FontWeight.bold)),  
      ], 
   ), 
)

The most important properties of the Text widget are as follows −

• maxLines, int − Maximum number of lines to show

• overflow, TextOverFlow − Specify how visual overflow is handled using TextOverFlow class

• style, TextStyle − Specify the style of the string using TextStyle class

• textAlign, TextAlign − Alignment of the text like right, left, justify, etc., using TextAlign class

• textDirection, TextDirection − Direction of text to flow, either left-to-right or right-to-left

Image

Image widget is used to display an image in the application. Image widget provides different constructors to load images from multiple sources and they are as follows −

• Image − Generic image loader using ImageProvider

• Image.asset − Load image from flutter project’s assets

• Image.file − Load image from system folder

• Image.memory − Load image from memory

• Image.Network − Load image from network

The easiest option to load and display an image in Flutter is by including the image as assets of the application and load it into the widget on demand.

• Create a folder, assets in the project folder and place the necessary images.

• Specify the assets in the pubspec.yaml as shown below −

flutter: 
   assets: 
      - assets/smiley.png

• Now, load and display the image in the application.

Image.asset(''assets/smiley.png'')

• The complete source code of MyHomePage widget of the hello world application and the result is as shown below −.

class MyHomePage extends StatelessWidget {
   MyHomePage({Key key, this.title}) : super(key: key); 
   final String title; 

   @override 
   Widget build(BuildContext context) {
      return Scaffold( 
         appBar: AppBar( title: Text(this.title), ), 
         body: Center( child: Image.asset("assets/smiley.png")),
      ); 
   }
}

The loaded image is as shown below −

The most important properties of the Image widget are as follows −

• image, ImageProvider − Actual image to load

• width, double − Width of the image

• height, double − Height of the image

• alignment, AlignmentGeometry − How to align the image within its bounds

Icon

Icon widget is used to display a glyph from a font described in IconData class. The code to load a simple email icon is as follows −

Icon(Icons.email)

The complete source code to apply it in hello world application is as follows −

class MyHomePage extends StatelessWidget { 
   MyHomePage({Key key, this.title}) : super(key: key); 
   final String title; 

   @override 
   Widget build(BuildContext context) {
      return Scaffold(
         appBar: AppBar(title: Text(this.title),),
         body: Center( child: Icon(Icons.email)),
      );
   }
}

The loaded icon is as shown below −

Flutter – Introduction to Layouts

Since the core concept of Flutter is Everything is widget, Flutter incorporates a user interface layout functionality into the widgets itself. Flutter provides quite a lot of specially designed widgets like Container, Center, Align, etc., only for the purpose of laying out the user interface. Widgets build by composing other widgets normally use layout widgets. Let use learn the Flutter layout concept in this chapter.

Type of Layout Widgets

Layout widgets can be grouped into two distinct category based on its child −

• Widget supporting a single child
• Widget supporting multiple child

Let us learn both type of widgets and its functionality in the upcoming sections.

Single Child Widgets

In this category, widgets will have only one widget as its child and every widget will have a special layout functionality.

For example, Center widget just centers it child widget with respect to its parent widget and Container widget provides complete flexibility to place it child at any given place inside it using different option like padding, decoration, etc.,

Single child widgets are great options to create high quality widget having single functionality such as button, label, etc.,

The code to create a simple button using Container widget is as follows −

class MyButton extends StatelessWidget {
   MyButton({Key key}) : super(key: key); 

   @override 
   Widget build(BuildContext context) {
      return Container(
         decoration: const BoxDecoration(
            border: Border(
               top: BorderSide(width: 1.0, color: Color(0xFFFFFFFFFF)),
               left: BorderSide(width: 1.0, color: Color(0xFFFFFFFFFF)),
               right: BorderSide(width: 1.0, color: Color(0xFFFF000000)),
               bottom: BorderSide(width: 1.0, color: Color(0xFFFF000000)),
            ),
         ),
         child: Container(
            padding: const
            EdgeInsets.symmetric(horizontal: 20.0, vertical: 2.0),
            decoration: const BoxDecoration(
               border: Border(
                  top: BorderSide(width: 1.0, color: Color(0xFFFFDFDFDF)),
                  left: BorderSide(width: 1.0, color: Color(0xFFFFDFDFDF)),
                  right: BorderSide(width: 1.0, color: Color(0xFFFF7F7F7F)),
                  bottom: BorderSide(width: 1.0, color: Color(0xFFFF7F7F7F)),
               ),
               color: Colors.grey,
            ),
            child: const Text(
               ''OK'',textAlign: TextAlign.center, style: TextStyle(color: Colors.black)
            ), 
         ), 
      ); 
   }
}

Here, we have used two widgets – a Container widget and a Text widget. The result of the widget is as a custom button as shown below −

Let us check some of the most important single child layout widgets provided by Flutter −

• Padding − Used to arrange its child widget by the given padding. Here, padding can be provided by EdgeInsets class.

• Align − Align its child widget within itself using the value of alignment property. The value for alignment property can be provided by FractionalOffset class. The FractionalOffset class specifies the offsets in terms of a distance from the top left.

Some of the possible values of offsets are as follows −

• FractionalOffset(1.0, 0.0) represents the top right.

• FractionalOffset(0.0, 1.0) represents the bottom left.

A sample code about offsets is shown below −

Center(
   child: Container(
      height: 100.0, 
      width: 100.0, 
      color: Colors.yellow, child: Align(
         alignment: FractionalOffset(0.2, 0.6),
         child: Container( height: 40.0, width:
            40.0, color: Colors.red,
         ), 
      ), 
   ), 
)

• FittedBox − It scales the child widget and then positions it according to the specified fit.

• AspectRatio − It attempts to size the child widget to the specified aspect ratio

• ConstrainedBox

• Baseline

• FractinallySizedBox

• IntrinsicHeight

• IntrinsicWidth

• LiimitedBox

• OffStage

• OverflowBox

• SizedBox

• SizedOverflowBox

• Transform

• CustomSingleChildLayout

Our hello world application is using material based layout widgets to design the home page. Let us modify our hello world application to build the home page using basic layout widgets as specified below −

• Container − Generic, single child, box based container widget with alignment, padding, border and margin along with rich styling features.

• Center − Simple, Single child container widget, which centers its child widget.

The modified code of the MyHomePage and MyApp widget is as below −

class MyApp extends StatelessWidget {
   @override
   Widget build(BuildContext context) {
      return MyHomePage(title: "Hello World demo app");
   }
}
class MyHomePage extends StatelessWidget {
   MyHomePage({Key key, this.title}) : super(key: key);
   final String title;
   @override
   Widget build(BuildContext context) {
      return Container(
         decoration: BoxDecoration(color: Colors.white,),
         padding: EdgeInsets.all(25), child: Center(
            child:Text(
               ''Hello World'', style: TextStyle(
                  color: Colors.black, letterSpacing: 0.5, fontSize: 20,
               ),
               textDirection: TextDirection.ltr,
            ),
         )
      );
   }
}

Here,

• Container widget is the top level or root widget. Container is configured using decoration and padding property to layout its content.

• BoxDecoration has many properties like color, border, etc., to decorate the Container widget and here, color is used to set the color of the container.

• padding of the Container widget is set by using dgeInsets class, which provides the option to specify the padding value.

• Center is the child widget of the Container widget. Again, Text is the child of the Center widget. Text is used to show message and Center is used to center the text message with respect to the parent widget, Container.

The final result of the code given above is a layout sample as shown below −

Multiple Child Widgets

In this category, a given widget will have more than one child widgets and the layout of each widget is unique.

For example, Row widget allows the laying out of its children in horizontal direction, whereas Column widget allows laying out of its children in vertical direction. By composing Row and Column, widget with any level of complexity can be built.

Let us learn some of the frequently used widgets in this section.

• Row − Allows to arrange its children in a horizontal manner.

• Column − Allows to arrange its children in a vertical manner.

• ListView − Allows to arrange its children as list.

• GridView − Allows to arrange its children as gallery.

• Expanded − Used to make the children of Row and Column widget to occupy the maximum possible area.

• Table − Table based widget.

• Flow − Flow based widget.

• Stack − Stack based widget.

Advanced Layout Application

In this section, let us learn how to create a complex user interface of product listing with custom design using both single and multiple child layout widgets.

For this purpose, follow the sequence given below −

• Create a new Flutter application in Android studio, product_layout_app.

• Replace the main.dart code with folowing code −

import ''package:flutter/material.dart''; 
void main() => runApp(MyApp()); 

class MyApp extends StatelessWidget {
   // This widget is the root of your application.
   @override 
   Widget build(BuildContext context) {
      return MaterialApp( 
         title: ''Flutter Demo'', theme: ThemeData( 
         primarySwatch: Colors.blue,), 
         home: MyHomePage(title: ''Product layout demo home page''),
      ); 
   } 
} 
class MyHomePage extends StatelessWidget {
   MyHomePage({Key key, this.title}) : super(key: key); 
   final String title; 
      
   @override 
   Widget build(BuildContext context) {
      return Scaffold(
         appBar: AppBar(title: Text(this.title),), 
         body: Center(child: Text( ''Hello World'', )), 
      ); 
   }
}

• Here,

• We have created MyHomePage widget by extending StatelessWidget instead of default StatefulWidget and then removed the relevant code.

• Now, create a new widget, ProductBox according to the specified design as shown below −

• The code for the ProductBox is as follows.

class ProductBox extends StatelessWidget {
   ProductBox({Key key, this.name, this.description, this.price, this.image}) 
      : super(key: key); 
   final String name; 
   final String description; 
   final int price; 
   final String image; 

   Widget build(BuildContext context) {
      return Container(
         padding: EdgeInsets.all(2), height: 120,  child: Card( 
            child: Row(
               mainAxisAlignment: MainAxisAlignment.spaceEvenly, children: <Widget>[
                  Image.asset("assets/appimages/" +image), Expanded(
                     child: Container(
                        padding: EdgeInsets.all(5), child: Column(
                           mainAxisAlignment: MainAxisAlignment.spaceEvenly, 
                              children: <Widget>[ 
                              
                              Text(this.name, style: TextStyle(fontWeight: 
                                 FontWeight.bold)), Text(this.description), 
                              Text("Price: " + this.price.toString()), 
                           ], 
                        )
                     )
                  )
               ]
            )
         )
      );
   }
}

• Please observe the following in the code −

• ProductBox has used four arguments as specified below −

• name – Product name

• description – Product description

• price – Price of the product

• image – Image of the product

• ProductBox uses seven build-in widgets as specified below −

• Container
• Expanded
• Row
• Column
• Card
• Text
• Image

• ProductBox is designed using the above mentioned widget. The arrangement or hierarchy of the widget is specified in the diagram shown below −

• Now, place some dummy image (see below) for product information in the assets folder of the application and configure the assets folder in the pubspec.yaml file as shown below −

assets: 
   - assets/appimages/floppy.png 
   - assets/appimages/iphone.png 
   - assets/appimages/laptop.png 
   - assets/appimages/pendrive.png 
   - assets/appimages/pixel.png 
   - assets/appimages/tablet.png

iPhone.png

Pixel.png

Laptop.png

Tablet.png

Pendrive.png

Floppy.png

Finally, Use the ProductBox widget in the MyHomePage widget as specified below −

class MyHomePage extends StatelessWidget { 
   MyHomePage({Key key, this.title}) : super(key: key); 
   final String title; 

   @override 
   Widget build(BuildContext context) {
      return Scaffold(
         appBar: AppBar(title:Text("Product Listing")), 
         body: ListView(
            shrinkWrap: true, padding: const EdgeInsets.fromLTRB(2.0, 10.0, 2.0, 10.0), 
            children: <Widget> [
               ProductBox(
                  name: "iPhone", 
                  description: "iPhone is the stylist phone ever", 
                  price: 1000, 
                  image: "iphone.png"
               ), 
               ProductBox(
                  name: "Pixel", 
                  description: "Pixel is the most featureful phone ever", 
                  price: 800, 
                  image: "pixel.png"
               ), 
               ProductBox( 
                  name: "Laptop", 
                  description: "Laptop is most productive development tool", 
                  price: 2000, 
                  image: "laptop.png"
               ), 
               ProductBox( 
                  name: "Tablet", 
                  description: "Tablet is the most useful device ever for meeting", 
                  price: 1500, 
                  image: "tablet.png"
               ), 
               ProductBox(
                  name: "Pendrive", 
                  description: "Pendrive is useful storage medium", 
                  price: 100, 
                  image: "pendrive.png"
               ), 
               ProductBox(
                  name: "Floppy Drive", 
                  description: "Floppy drive is useful rescue storage medium", 
                  price: 20, 
                  image: "floppy.png"
               ), 
            ],
         )
      );
   }
}

• Here,we have used ProductBox as children of ListView widget.

• The complete code (main.dart) of the product layout application (product_layout_app) is as follows −

import ''package:flutter/material.dart''; 
void main() => runApp(MyApp()); 

class MyApp extends StatelessWidget { 
   // This widget is the root of your application. 
   @override 
   Widget build(BuildContext context) {
      return MaterialApp(
         title: ''Flutter Demo'', theme: ThemeData(
            primarySwatch: Colors.blue,
         ), 
         home: MyHomePage(title: ''Product layout demo home page''), 
      );
   }
}
class MyHomePage extends StatelessWidget { 
   MyHomePage({Key key, this.title}) : super(key: key); 
   final String title; 
   
   @override 
   Widget build(BuildContext context) { 
      return Scaffold( 
         appBar: AppBar(title: Text("Product Listing")), 
         body: ListView(
            shrinkWrap: true, 
            padding: const EdgeInsets.fromLTRB(2.0, 10.0, 2.0, 10.0), 
            children: <Widget>[ 
               ProductBox(
                  name: "iPhone", 
                  description: "iPhone is the stylist phone ever", 
                  price: 1000, 
                  image: "iphone.png"
               ), 
               ProductBox( 
                  name: "Pixel",    
                  description: "Pixel is the most featureful phone ever", 
                  price: 800, 
                  image: "pixel.png"
               ), 
               ProductBox( 
                  name: "Laptop", 
                  description: "Laptop is most productive development tool", 
                  price: 2000, 
                  image: "laptop.png"
               ), 
               ProductBox( 
                  name: "Tablet", 
                  description: "Tablet is the most useful device ever for meeting", 
                  price: 1500, 
                  image: "tablet.png"
               ), 
               ProductBox( 
                  name: "Pendrive", 
                  description: "Pendrive is useful storage medium", 
                  price: 100, 
                  image: "pendrive.png"
               ), 
               ProductBox(
                  name: "Floppy Drive", 
                  description: "Floppy drive is useful rescue storage medium", 
                  price: 20, 
                  image: "floppy.png"
               ), 
            ],
         )
      );
   }
}
class ProductBox extends StatelessWidget {
   ProductBox({Key key, this.name, this.description, this.price, this.image}) :
      super(key: key); 
   final String name; 
   final String description; 
   final int price; 
   final String image; 
   
   Widget build(BuildContext context) {
      return Container(
         padding: EdgeInsets.all(2), 
         height: 120, 
         child: Card(
            child: Row(
               mainAxisAlignment: MainAxisAlignment.spaceEvenly, 
               children: <Widget>[ 
                  Image.asset("assets/appimages/" + image), 
                  Expanded( 
                     child: Container( 
                        padding: EdgeInsets.all(5), 
                        child: Column(    
                           mainAxisAlignment: MainAxisAlignment.spaceEvenly, 
                           children: <Widget>[ 
                              Text(
                                 this.name, style: TextStyle(
                                    fontWeight: FontWeight.bold
                                 )
                              ),
                              Text(this.description), Text(
                                 "Price: " + this.price.toString()
                              ), 
                           ], 
                        )
                     )
                  )
               ]
            )
         )
      );
   }
}

The final output of the application is as follows −

Flutter – Introduction to Gestures

Gestures are primarily a way for a user to interact with a mobile (or any touch based device) application. Gestures are generally defined as any physical action / movement of a user in the intention of activating a specific control of the mobile device. Gestures are as simple as tapping the screen of the mobile device to more complex actions used in gaming applications.

Some of the widely used gestures are mentioned here −

• Tap − Touching the surface of the device with fingertip for a short period and then releasing the fingertip.

• Double Tap − Tapping twice in a short time.

• Drag − Touching the surface of the device with fingertip and then moving the fingertip in a steady manner and then finally releasing the fingertip.

• Flick − Similar to dragging, but doing it in a speeder way.

• Pinch − Pinching the surface of the device using two fingers.

• Spread/Zoom − Opposite of pinching.

• Panning − Touching the surface of the device with fingertip and moving it in any direction without releasing the fingertip.

Flutter provides an excellent support for all type of gestures through its exclusive widget, GestureDetector. GestureDetector is a non-visual widget primarily used for detecting the user’s gesture. To identify a gesture targeted on a widget, the widget can be placed inside GestureDetector widget. GestureDetector will capture the gesture and dispatch multiple events based on the gesture.

Some of the gestures and the corresponding events are given below −

• Tap

• onTapDown
• onTapUp
• onTap
• onTapCancel

• Double tap

• onDoubleTap

• Long press

• onLongPress

• Vertical drag

• onVerticalDragStart
• onVerticalDragUpdate
• onVerticalDragEnd

• Horizontal drag

• onHorizontalDragStart
• onHorizontalDragUpdate
• onHorizontalDragEnd

• Pan

• onPanStart
• onPanUpdate
• onPanEnd

Now, let us modify the hello world application to include gesture detection feature and try to understand the concept.

• Change the body content of the MyHomePage widget as shown below −

body: Center( 
   child: GestureDetector( 
      onTap: () { 
         _showDialog(context); 
      }, 
      child: Text( ''Hello World'', ) 
   ) 
),

• Observe that here we have placed the GestureDetector widget above the Text widget in the widget hierarchy, captured the onTap event and then finally shown a dialog window.

• Implement the *_showDialog* function to present a dialog when user tabs the hello world message. It uses the generic showDialog and AlertDialog widget to create a new dialog widget. The code is shown below −

// user defined function void _showDialog(BuildContext context) { 
   // flutter defined function 
   showDialog( 
      context: context, builder: (BuildContext context) { 
         // return object of type Dialog
         return AlertDialog( 
            title: new Text("Message"), 
            content: new Text("Hello World"),   
            actions: <Widget>[ 
               new FlatButton( 
                  child: new Text("Close"),  
                  onPressed: () {   
                     Navigator.of(context).pop();  
                  }, 
               ), 
            ], 
         ); 
      }, 
   ); 
}

• The application will reload in the device using Hot Reload feature. Now, simply click the message, Hello World and it will show the dialog as below −

• Now, close the dialog by clicking the close option in the dialog.

• The complete code (main.dart) is as follows −

import ''package:flutter/material.dart''; 
void main() => runApp(MyApp()); 

class MyApp extends StatelessWidget { 
   // This widget is the root of your application.    
   @override 
   Widget build(BuildContext context) {
      return MaterialApp(
         title: ''Hello World Demo Application'', 
         theme: ThemeData( primarySwatch: Colors.blue,), 
         home: MyHomePage(title: ''Home page''), 
      ); 
   }
}
class MyHomePage extends StatelessWidget {
   MyHomePage({Key key, this.title}) : super(key: key); 
   final String title; 
   
   // user defined function 
   void _showDialog(BuildContext context) { 
      // flutter defined function showDialog( 
         context: context, builder: (BuildContext context) { 
            // return object of type Dialog return AlertDialog(
               title: new Text("Message"), 
               content: new Text("Hello World"),   
               actions: <Widget>[
                  new FlatButton(
                     child: new Text("Close"), 
                     onPressed: () {   
                        Navigator.of(context).pop();  
                     }, 
                  ), 
               ],
            );
         },
      );
   }
   @override 
   Widget build(BuildContext context) {
      return Scaffold(
         appBar: AppBar(title: Text(this.title),),
         body: Center(
            child: GestureDetector( 
               onTap: () {
                  _showDialog(context);
               },
            child: Text( ''Hello World'', )
            )
         ),
      );
   }
}

Finally, Flutter also provides a low-level gesture detection mechanism through Listener widget. It will detect all user interactions and then dispatches the following events −

• PointerDownEvent
• PointerMoveEvent
• PointerUpEvent
• PointerCancelEvent

Flutter also provides a small set of widgets to do specific as well as advanced gestures. The widgets are listed below −

• Dismissible − Supports flick gesture to dismiss the widget.

• Draggable − Supports drag gesture to move the widget.

• LongPressDraggable − Supports drag gesture to move a widget, when its parent widget is also draggable.

• DragTarget − Accepts any Draggable widget

• IgnorePointer − Hides the widget and its children from the gesture detection process.

• AbsorbPointer − Stops the gesture detection process itself and so any overlapping widget also can not able to participate in the gesture detection process and hence, no event is raised.

• Scrollable − Support scrolling of the content available inside the widget.

Flutter – State Management

Managing state in an application is one of the most important and necessary process in the life cycle of an application.

Let us consider a simple shopping cart application.

• User will login using their credentials into the application.

• Once user is logged in, the application should persist the logged in user detail in all the screen.

• Again, when the user selects a product and saved into a cart, the cart information should persist between the pages until the user checked out the cart.

• User and their cart information at any instance is called the state of the application at that instance.

A state management can be divided into two categories based on the duration the particular state lasts in an application.

• Ephemeral − Last for a few seconds like the current state of an animation or a single page like current rating of a product. Flutter supports its through StatefulWidget.

• app state − Last for entire application like logged in user details, cart information, etc., Flutter supports its through scoped_model.

Navigation and Routing

In any application, navigating from one page / screen to another defines the work flow of the application. The way that the navigation of an application is handled is called Routing. Flutter provides a basic routing class – MaterialPageRoute and two methods – Navigator.push and Navigator.pop, to define the work flow of an application.

MaterialPageRoute

MaterialPageRoute is a widget used to render its UI by replacing the entire screen with a platform specific animation.

MaterialPageRoute(builder: (context) => Widget())

Here, builder will accepts a function to build its content by suppling the current context of the application.

Navigation.push

Navigation.push is used to navigate to new screen using MaterialPageRoute widget.

Navigator.push( context, MaterialPageRoute(builder: (context) => Widget()), );

Navigation.pop

Navigation.pop is used to navigate to previous screen.

Navigator.pop(context);

Let us create a new application to better understand the navigation concept.

Create a new Flutter application in Android studio, product_nav_app

• Copy the assets folder from product_nav_app to product_state_app and add assets inside the pubspec.yaml file.

flutter:
   assets: 
   - assets/appimages/floppy.png 
   - assets/appimages/iphone.png 
   - assets/appimages/laptop.png 
   - assets/appimages/pendrive.png 
   - assets/appimages/pixel.png 
   - assets/appimages/tablet.png

• Replace the default startup code (main.dart) with our startup code.

import ''package:flutter/material.dart''; 
void main() => runApp(MyApp()); 

class MyApp extends StatelessWidget { 
   // This widget is the root of your application. 
   @override 
   Widget build(BuildContext context) { 
      return MaterialApp( 
         title: ''Flutter Demo'', 
         theme: ThemeData( 
            primarySwatch: Colors.blue, 
         ), 
         home: MyHomePage(
            title: ''Product state demo home page''
         ),
      );
   }
}
class MyHomePage extends StatelessWidget {
   MyHomePage({Key key, this.title}) : super(key: key);
   final String title;
   @override 
   Widget build(BuildContext context) {
      return Scaffold(
         appBar: AppBar(
            title: Text(this.title), 
         ), 
         body: Center(
            child: Text(''Hello World'',)
         ), 
      ); 
   } 
}

• Let us create a Product class to organize the product information.

class Product { 
   final String name; 
   final String description; 
   final int price; 
   final String image; 
   Product(this.name, this.description, this.price, this.image); 
}

• Let us write a method getProducts in the Product class to generate our dummy product records.

static List<Product> getProducts() {
   List<Product> items = <Product>[]; 
   
   items.add(
      Product( 
         "Pixel", 
         "Pixel is the most feature-full phone ever", 800, 
         "pixel.png"
      )
   ); 
   items.add(
      Product(
         "Laptop", 
         "Laptop is most productive development tool", 
         2000, "
         laptop.png"
      )
   ); 
   items.add(
      Product( 
         "Tablet", 
         "Tablet is the most useful device ever for meeting", 
         1500, 
         "tablet.png"
      )
   ); 
   items.add(
      Product( 
         "Pendrive", 
         "Pendrive is useful storage medium",
         100, 
         "pendrive.png"
      )
   ); 
   items.add(
      Product( 
         "Floppy Drive", 
         "Floppy drive is useful rescue storage medium", 
         20, 
         "floppy.png"
      )
   ); 
   return items; 
}
import product.dart in main.dart
import ''Product.dart'';

• Let us include our new widget, RatingBox.

class RatingBox extends StatefulWidget {
   @override 
   _RatingBoxState createState() =>_RatingBoxState(); 
} 
class _RatingBoxState extends State<RatingBox> {
   int _rating = 0; 
   void _setRatingAsOne() {
      setState(() {
         _rating = 1; 
      }); 
   } 
   void _setRatingAsTwo() {
      setState(() {
         _rating = 2; 
      }); 
   }
   void _setRatingAsThree() {
      setState(() {
         _rating = 3;
      });
   }
   Widget build(BuildContext context) {
      double _size = 20; 
      print(_rating); 
      return Row(
         mainAxisAlignment: MainAxisAlignment.end, 
         crossAxisAlignment: CrossAxisAlignment.end, 
         mainAxisSize: MainAxisSize.max, 
         children: <Widget>[
            Container(
               padding: EdgeInsets.all(0), 
               child: IconButton(
                  icon: (
                     _rating >= 1? 
                     Icon( 
                        Icons.star, 
                        size: _size, 
                     ) 
                     : Icon(
                        Icons.star_border, 
                        size: _size, 
                     )
                  ), 
                  color: Colors.red[500], 
                  onPressed: _setRatingAsOne, 
                  iconSize: _size, 
               ), 
            ), 
            Container(
               padding: EdgeInsets.all(0), 
               child: IconButton(
                  icon: (
                     _rating >= 2? 
                     Icon(
                        Icons.star, 
                        size: _size, 
                     ) 
                     : Icon(
                        Icons.star_border, 
                        size: _size, 
                     )
                  ), 
                  color: Colors.red[500], 
                  onPressed: _setRatingAsTwo, 
                  iconSize: _size, 
               ), 
            ), 
            Container(
               padding: EdgeInsets.all(0), 
               child: IconButton(
                  icon: (
                     _rating >= 3 ? 
                     Icon(
                        Icons.star, 
                        size: _size, 
                     ) 
                     : Icon( 
                        Icons.star_border, 
                        size: _size, 
                     )
                  ), 
                  color: Colors.red[500], 
                  onPressed: _setRatingAsThree, 
                  iconSize: _size, 
               ), 
            ), 
         ], 
      ); 
   }
}

• Let us modify our ProductBox widget to work with our new Product class.

class ProductBox extends StatelessWidget {    
   ProductBox({Key key, this.item}) : super(key: key); 
   final Product item; 
   
   Widget build(BuildContext context) {
      return Container(
         padding: EdgeInsets.all(2), 
         height: 140, 
         child: Card( 
            child: Row(
               mainAxisAlignment: MainAxisAlignment.spaceEvenly, 
               children: <Widget>[ 
                  Image.asset("assets/appimages/" + this.item.image), 
                  Expanded(
                     child: Container(
                        padding: EdgeInsets.all(5), 
                        child: Column(
                           mainAxisAlignment: MainAxisAlignment.spaceEvenly, 
                           children: <Widget>[
                              Text(this.item.name, 
                              style: TextStyle(fontWeight: FontWeight.bold)), 
                              Text(this.item.description), 
                              Text("Price: " + this.item.price.toString()), 
                              RatingBox(), 
                           ], 
                        )
                     )
                  )
               ]
            ), 
         )
      ); 
   }
}

Let us rewrite our MyHomePage widget to work with Product model and to list all products using ListView.

class MyHomePage extends StatelessWidget { 
   MyHomePage({Key key, this.title}) : super(key: key); 
   final String title; 
   final items = Product.getProducts(); 
   
   @override 
   Widget build(BuildContext context) { 
      return Scaffold( appBar: AppBar(title: Text("Product Navigation")), 
      body: ListView.builder( 
         itemCount: items.length, 
         itemBuilder: (context, index) {
            return GestureDetector( 
               child: ProductBox(item: items[index]), 
               onTap: () { 
                  Navigator.push( 
                     context, MaterialPageRoute( 
                        builder: (context) => ProductPage(item: items[index]), 
                     ), 
                  ); 
               }, 
            ); 
         }, 
      )); 
   } 
}

Here, we have used MaterialPageRoute to navigate to product details page.

• Now, let us add ProductPage to show the product details.

class ProductPage extends StatelessWidget { 
   ProductPage({Key key, this.item}) : super(key: key); 
   final Product item; 
   
   @override 
   Widget build(BuildContext context) {
      return Scaffold(
         appBar: AppBar( 
            title: Text(this.item.name), 
         ), 
         body: Center(
            child: Container(
               padding: EdgeInsets.all(0), 
               child: Column(
                  mainAxisAlignment: MainAxisAlignment.start, 
                  crossAxisAlignment: CrossAxisAlignment.start, 
                  children: <Widget>[
                     Image.asset("assets/appimages/" + this.item.image), 
                     Expanded(
                        child: Container(
                           padding: EdgeInsets.all(5), 
                           child: Column(
                              mainAxisAlignment: MainAxisAlignment.spaceEvenly, 
                              children: <Widget>[
                                 Text(
                                    this.item.name, style: TextStyle(
                                       fontWeight: FontWeight.bold
                                    )
                                 ), 
                                 Text(this.item.description), 
                                 Text("Price: " + this.item.price.toString()), 
                                 RatingBox(),
                              ], 
                           )
                        )
                     )
                  ]
               ), 
            ), 
         ), 
      ); 
   } 
}

The complete code of the application is as follows −

import ''package:flutter/material.dart''; 
void main() => runApp(MyApp()); 

class Product {
   final String name; 
   final String description; 
   final int price; 
   final String image; 
   Product(this.name, this.description, this.price, this.image); 
   
   static List<Product> getProducts() {
      List<Product> items = <Product>[]; 
      items.add(
         Product(
            "Pixel", 
            "Pixel is the most featureful phone ever", 
            800, 
            "pixel.png"
         )
      );
      items.add(
         Product(
            "Laptop", 
            "Laptop is most productive development tool", 
            2000, 
            "laptop.png"
         )
      ); 
      items.add(
         Product(
            "Tablet", 
            "Tablet is the most useful device ever for meeting", 
            1500, 
            "tablet.png"
         )
      ); 
      items.add(
         Product( 
            "Pendrive", 
            "iPhone is the stylist phone ever", 
            100, 
            "pendrive.png"
         )
      ); 
      items.add(
         Product(
            "Floppy Drive", 
            "iPhone is the stylist phone ever", 
            20, 
            "floppy.png"
         )
      ); 
      items.add(
         Product(
            "iPhone", 
            "iPhone is the stylist phone ever", 
            1000, 
            "iphone.png"
         )
      ); 
      return items; 
   }
}
class MyApp extends StatelessWidget {
   // This widget is the root of your application. 
   @override 
   Widget build(BuildContext context) {
      return MaterialApp(
         title: ''Flutter Demo'', 
         theme: ThemeData( 
            primarySwatch: Colors.blue, 
         ), 
         home: MyHomePage(title: ''Product Navigation demo home page''), 
      ); 
   }
}
class MyHomePage extends StatelessWidget {
   MyHomePage({Key key, this.title}) : super(key: key); 
   final String title; 
   final items = Product.getProducts(); 
   
   @override 
   Widget build(BuildContext context) {
      return Scaffold(
         appBar: AppBar(title: Text("Product Navigation")), 
         body: ListView.builder( 
            itemCount: items.length, 
            itemBuilder: (context, index) { 
               return GestureDetector( 
                  child: ProductBox(item: items[index]), 
                  onTap: () { 
                     Navigator.push( 
                        context, 
                        MaterialPageRoute( 
                           builder: (context) => ProductPage(item: items[index]), 
                        ), 
                     ); 
                  }, 
               ); 
            }, 
         )
      ); 
   }
} 
class ProductPage extends StatelessWidget {
   ProductPage({Key key, this.item}) : super(key: key); 
   final Product item; 
   
   @override 
   Widget build(BuildContext context) {
      return Scaffold(
         appBar: AppBar(
            title: Text(this.item.name), 
         ), 
         body: Center(
            child: Container( 
               padding: EdgeInsets.all(0), 
               child: Column( 
                  mainAxisAlignment: MainAxisAlignment.start, 
                  crossAxisAlignment: CrossAxisAlignment.start, 
                  children: <Widget>[ 
                     Image.asset("assets/appimages/" + this.item.image), 
                     Expanded( 
                        child: Container( 
                           padding: EdgeInsets.all(5), 
                           child: Column( 
                              mainAxisAlignment: MainAxisAlignment.spaceEvenly, 
                              children: <Widget>[ 
                                 Text(this.item.name, style: TextStyle(fontWeight: FontWeight.bold)), 
                                 Text(this.item.description), 
                                 Text("Price: " + this.item.price.toString()), 
                                 RatingBox(), 
                              ], 
                           )
                        )
                     ) 
                  ]
               ), 
            ), 
         ), 
      ); 
   } 
}
class RatingBox extends StatefulWidget { 
   @override 
   _RatingBoxState createState() => _RatingBoxState(); 
} 
class _RatingBoxState extends State<RatingBox> { 
   int _rating = 0;
   void _setRatingAsOne() {
      setState(() {
         _rating = 1; 
      }); 
   }
   void _setRatingAsTwo() {
      setState(() {
         _rating = 2; 
      }); 
   } 
   void _setRatingAsThree() { 
      setState(() {
         _rating = 3; 
      }); 
   }
   Widget build(BuildContext context) {
      double _size = 20; 
      print(_rating); 
      return Row(
         mainAxisAlignment: MainAxisAlignment.end, 
         crossAxisAlignment: CrossAxisAlignment.end, 
         mainAxisSize: MainAxisSize.max, 
         children: <Widget>[
            Container(
               padding: EdgeInsets.all(0), 
               child: IconButton(
                  icon: (
                     _rating >= 1 ? Icon( 
                        Icons.star, 
                        size: _size, 
                     ) 
                     : Icon( 
                        Icons.star_border, 
                        size: _size, 
                     )
                  ), 
                  color: Colors.red[500], 
                  onPressed: _setRatingAsOne, 
                  iconSize: _size, 
               ), 
            ), 
            Container(
               padding: EdgeInsets.all(0), 
               child: IconButton( 
                  icon: (
                     _rating >= 2 ? 
                     Icon( 
                        Icons.star, 
                        size: _size, 
                     ) 
                     : Icon( 
                        Icons.star_border, 
                        size: _size, 
                     )
                  ), 
                  color: Colors.red[500], 
                  onPressed: _setRatingAsTwo, 
                  iconSize: _size, 
               ), 
            ), 
            Container(
               padding: EdgeInsets.all(0), 
               child: IconButton(
                  icon: (
                     _rating >= 3 ? 
                     Icon( 
                        Icons.star, 
                        size: _size, 
                     )
                     : Icon( 
                        Icons.star_border, 
                        size: _size, 
                     )
                  ), 
                  color: Colors.red[500], 
                  onPressed: _setRatingAsThree, 
                  iconSize: _size, 
               ), 
            ), 
         ], 
      ); 
   } 
} 
class ProductBox extends StatelessWidget {
   ProductBox({Key key, this.item}) : super(key: key); 
   final Product item; 
   
   Widget build(BuildContext context) {
      return Container(
         padding: EdgeInsets.all(2), 
         height: 140, 
         child: Card(
            child: Row(
               mainAxisAlignment: MainAxisAlignment.spaceEvenly, 
               children: <Widget>[ 
                  Image.asset("assets/appimages/" + this.item.image), 
                  Expanded( 
                     child: Container( 
                        padding: EdgeInsets.all(5), 
                        child: Column( 
                           mainAxisAlignment: MainAxisAlignment.spaceEvenly, 
                           children: <Widget>[ 
                              Text(this.item.name, style: TextStyle(fontWeight: FontWeight.bold)), Text(this.item.description), 
                              Text("Price: " + this.item.price.toString()), 
                              RatingBox(), 
                           ], 
                        )
                     )
                  ) 
               ]
            ), 
         )
      ); 
   } 
}

Run the application and click any one of the product item. It will show the relevant details page. We can move to home page by clicking back button. The product list page and product details page of the application are shown as follows −

Flutter – Animation

Animation is a complex procedure in any mobile application. In spite of its complexity, Animation enhances the user experience to a new level and provides a rich user interaction. Due to its richness, animation becomes an integral part of modern mobile application. Flutter framework recognizes the importance of Animation and provides a simple and intuitive framework to develop all types of animations.

Introduction

Animation is a process of showing a series of images / picture in a particular order within a specific duration to give an illusion of movement. The most important aspects of the animation are as follows −

• Animation have two distinct values: Start value and End value. The animation starts from Start value and goes through a series of intermediate values and finally ends at End values. For example, to animate a widget to fade away, the initial value will be the full opacity and the final value will be the zero opacity.

• The intermediate values may be linear or non-linear (curve) in nature and it can be configured. Understand that the animation works as it is configured. Each configuration provides a different feel to the animation. For example, fading a widget will be linear in nature whereas bouncing of a ball will be non-linear in nature.

• The duration of the animation process affects the speed (slowness or fastness) of the animation.

• The ability to control the animation process like starting the animation, stopping the animation, repeating the animation to set number of times, reversing the process of animation, etc.,

• In Flutter, animation system does not do any real animation. Instead, it provides only the values required at every frame to render the images.

Animation Based Classes

Flutter animation system is based on Animation objects. The core animation classes and its usage are as follows −

Animation

Generates interpolated values between two numbers over a certain duration. The most common Animation classes are −

• Animation<double> − interpolate values between two decimal number

• Animation<Color> − interpolate colors between two color

• Animation<Size> − interpolate sizes between two size

• AnimationController − Special Animation object to control the animation itself. It generates new values whenever the application is ready for a new frame. It supports linear based animation and the value starts from 0.0 to 1.0

controller = AnimationController(duration: const Duration(seconds: 2), vsync: this);

Here, controller controls the animation and duration option controls the duration of the animation process. vsync is a special option used to optimize the resource used in the animation.

CurvedAnimation

Similar to AnimationController but supports non-linear animation. CurvedAnimation can be used along with Animation object as below −

controller = AnimationController(duration: const Duration(seconds: 2), vsync: this); 
animation = CurvedAnimation(parent: controller, curve: Curves.easeIn)

Tween<T>

Derived from Animatable<T> and used to generate numbers between any two numbers other than 0 and 1. It can be used along with Animation object by using animate method and passing actual Animation object.

AnimationController controller = AnimationController( 
   duration: const Duration(milliseconds: 1000), 
vsync: this); Animation<int> customTween = IntTween(
   begin: 0, end: 255).animate(controller);

• Tween can also used along with CurvedAnimation as below −

AnimationController controller = AnimationController(
   duration: const Duration(milliseconds: 500), vsync: this); 
final Animation curve = CurvedAnimation(parent: controller, curve: Curves.easeOut); 
Animation<int> customTween = IntTween(begin: 0, end: 255).animate(curve);

Here, controller is the actual animation controller. curve provides the type of non-linearity and the customTween provides custom range from 0 to 255.

Work flow of the Flutter Animation

The work flow of the animation is as follows −

• Define and start the animation controller in the initState of the StatefulWidget.

AnimationController(duration: const Duration(seconds: 2), vsync: this); 
animation = Tween<double>(begin: 0, end: 300).animate(controller); 
controller.forward();

• Add animation based listener, addListener to change the state of the widget.

animation = Tween<double>(begin: 0, end: 300).animate(controller) ..addListener(() {
   setState(() { 
      // The state that has changed here is the animation object’s value. 
   }); 
});

• Build-in widgets, AnimatedWidget and AnimatedBuilder can be used to skip this process. Both widget accepts Animation object and get current values required for the animation.

• Get the animation values during the build process of the widget and then apply it for width, height or any relevant property instead of the original value.

child: Container( 
   height: animation.value, 
   width: animation.value, 
   child: <Widget>, 
)

Working Application

Let us write a simple animation based application to understand the concept of animation in Flutter framework.

• Create a new Flutter application in Android studio, product_animation_app.

• Copy the assets folder from product_nav_app to product_animation_app and add assets inside the pubspec.yaml file.

flutter: 
   assets: 
   - assets/appimages/floppy.png 
   - assets/appimages/iphone.png 
   - assets/appimages/laptop.png 
   - assets/appimages/pendrive.png 
   - assets/appimages/pixel.png 
   - assets/appimages/tablet.png

• Remove the default startup code (main.dart).

• Add import and basic main function.

import ''package:flutter/material.dart''; 
void main() => runApp(MyApp());

• Create the MyApp widget derived from StatefulWidgtet.

class MyApp extends StatefulWidget { 
   _MyAppState createState() => _MyAppState(); 
}

• Create _MyAppState widget and implement initState and dispose in addition to default build method.

class _MyAppState extends State<MyApp> with SingleTickerProviderStateMixin { 
   Animation<double> animation; 
   AnimationController controller; 
   @override void initState() {
      super.initState(); 
      controller = AnimationController(
         duration: const Duration(seconds: 10), vsync: this
      ); 
      animation = Tween<double>(begin: 0.0, end: 1.0).animate(controller); 
      controller.forward(); 
   } 
   // This widget is the root of your application. 
   @override 
   Widget build(BuildContext context) {
      controller.forward(); 
      return MaterialApp(
         title: ''Flutter Demo'',
         theme: ThemeData(primarySwatch: Colors.blue,), 
         home: MyHomePage(title: ''Product layout demo home page'', animation: animation,)
      ); 
   } 
   @override 
   void dispose() {
      controller.dispose();
      super.dispose();
   }
}

Here,

• In initState method, we have created an animation controller object (controller), an animation object (animation) and started the animation using controller.forward.

• In dispose method, we have disposed the animation controller object (controller).

• In build method, send animation to MyHomePage widget through constructor. Now, MyHomePage widget can use the animation object to animate its content.

• Now, add ProductBox widget

class ProductBox extends StatelessWidget {
   ProductBox({Key key, this.name, this.description, this.price, this.image})
      : super(key: key);
   final String name; 
   final String description; 
   final int price; 
   final String image; 
   
   Widget build(BuildContext context) {
      return Container(
         padding: EdgeInsets.all(2), 
         height: 140, 
         child: Card( 
            child: Row( 
               mainAxisAlignment: MainAxisAlignment.spaceEvenly, 
               children: <Widget>[ 
                  Image.asset("assets/appimages/" + image), 
                  Expanded( 
                     child: Container( 
                        padding: EdgeInsets.all(5), 
                        child: Column( 
                           mainAxisAlignment: MainAxisAlignment.spaceEvenly, 
                           children: <Widget>[ 
                              Text(this.name, style: 
                                 TextStyle(fontWeight: FontWeight.bold)), 
                              Text(this.description), 
                                 Text("Price: " + this.price.toString()), 
                           ], 
                        )
                     )
                  )
               ]
            )
         )
      ); 
   }
}

• Create a new widget, MyAnimatedWidget to do simple fade animation using opacity.

class MyAnimatedWidget extends StatelessWidget { 
   MyAnimatedWidget({this.child, this.animation}); 
      
   final Widget child; 
   final Animation<double> animation; 
   
   Widget build(BuildContext context) => Center( 
   child: AnimatedBuilder(
      animation: animation, 
      builder: (context, child) => Container( 
         child: Opacity(opacity: animation.value, child: child), 
      ), 
      child: child), 
   ); 
}

• Here, we have used AniatedBuilder to do our animation. AnimatedBuilder is a widget which build its content while doing the animation at the same time. It accepts a animation object to get current animation value. We have used animation value, animation.value to set the opacity of the child widget. In effect, the widget will animate the child widget using opacity concept.

• Finally, create the MyHomePage widget and use the animation object to animate any of its content.

class MyHomePage extends StatelessWidget {
   MyHomePage({Key key, this.title, this.animation}) : super(key: key); 
   
   final String title; 
   final Animation<double> 
   animation; 
   
   @override 
   Widget build(BuildContext context) {
      return Scaffold(
         appBar: AppBar(title: Text("Product Listing")),body: ListView(
            shrinkWrap: true,
            padding: const EdgeInsets.fromLTRB(2.0, 10.0, 2.0, 10.0), 
            children: <Widget>[
               FadeTransition(
                  child: ProductBox(
                     name: "iPhone", 
                     description: "iPhone is the stylist phone ever", 
                     price: 1000, 
                     image: "iphone.png"
                  ), opacity: animation
               ), 
               MyAnimatedWidget(child: ProductBox(
                  name: "Pixel", 
                  description: "Pixel is the most featureful phone ever", 
                  price: 800, 
                  image: "pixel.png"
               ), animation: animation), 
               ProductBox(
                  name: "Laptop", 
                  description: "Laptop is most productive development tool", 
                  price: 2000, 
                  image: "laptop.png"
               ), 
               ProductBox(
                  name: "Tablet", 
                  description: "Tablet is the most useful device ever for meeting", 
                  price: 1500, 
                  image: "tablet.png"
               ), 
               ProductBox(
                  name: "Pendrive", 
                  description: "Pendrive is useful storage medium", 
                  price: 100, 
                  image: "pendrive.png"
               ),
               ProductBox(
                  name: "Floppy Drive", 
                  description: "Floppy drive is useful rescue storage medium", 
                  price: 20, 
                  image: "floppy.png"
               ),
            ],
         )
      );
   }
}

Here, we have used FadeAnimation and MyAnimationWidget to animate the first two items in the list. FadeAnimation is a build-in animation class, which we used to animate its child using opacity concept.

• The complete code is as follows −

import ''package:flutter/material.dart''; 
void main() => runApp(MyApp()); 

class MyApp extends StatefulWidget { 
   _MyAppState createState() => _MyAppState(); 
} 
class _MyAppState extends State<MyApp> with SingleTickerProviderStateMixin {
   Animation<double> animation; 
   AnimationController controller; 
   
   @override 
   void initState() {
      super.initState(); 
      controller = AnimationController(
         duration: const Duration(seconds: 10), vsync: this); 
      animation = Tween<double>(begin: 0.0, end: 1.0).animate(controller); 
      controller.forward(); 
   } 
   // This widget is the root of your application. 
   @override 
   Widget build(BuildContext context) {
      controller.forward(); 
      return MaterialApp( 
         title: ''Flutter Demo'', theme: ThemeData(primarySwatch: Colors.blue,), 
         home: MyHomePage(title: ''Product layout demo home page'', animation: animation,) 
      ); 
   } 
   @override 
   void dispose() {
      controller.dispose();
      super.dispose(); 
   } 
}
class MyHomePage extends StatelessWidget { 
   MyHomePage({Key key, this.title, this.animation}): super(key: key);
   final String title; 
   final Animation<double> animation; 
   
   @override 
   Widget build(BuildContext context) {
      return Scaffold(
         appBar: AppBar(title: Text("Product Listing")), 
         body: ListView(
            shrinkWrap: true, 
            padding: const EdgeInsets.fromLTRB(2.0, 10.0, 2.0, 10.0), 
            children: <Widget>[
               FadeTransition(
                  child: ProductBox(
                     name: "iPhone", 
                     description: "iPhone is the stylist phone ever", 
                     price: 1000, 
                     image: "iphone.png"
                  ), 
                  opacity: animation
               ), 
               MyAnimatedWidget(
                  child: ProductBox( 
                     name: "Pixel", 
                     description: "Pixel is the most featureful phone ever", 
                     price: 800, 
                     image: "pixel.png"
                  ), 
                  animation: animation
               ), 
               ProductBox( 
                  name: "Laptop", 
                  description: "Laptop is most productive development tool", 
                  price: 2000, 
                  image: "laptop.png"
               ), 
               ProductBox(
                  name: "Tablet",
                  description: "Tablet is the most useful device ever for meeting",
                  price: 1500, 
                  image: "tablet.png"
               ), 
               ProductBox(
                  name: "Pendrive", 
                  description: "Pendrive is useful storage medium", 
                  price: 100, 
                  image: "pendrive.png"
               ), 
               ProductBox(
                  name: "Floppy Drive", 
                  description: "Floppy drive is useful rescue storage medium", 
                  price: 20, 
                  image: "floppy.png"
               ), 
            ], 
         )
      ); 
   } 
} 
class ProductBox extends StatelessWidget { 
   ProductBox({Key key, this.name, this.description, this.price, this.image}) :
      super(key: key);
   final String name; 
   final String description; 
   final int price; 
   final String image; 
   Widget build(BuildContext context) {
      return Container(
         padding: EdgeInsets.all(2), 
         height: 140, 
         child: Card(
            child: Row(
               mainAxisAlignment: MainAxisAlignment.spaceEvenly, 
               children: <Widget>[ 
                  Image.asset("assets/appimages/" + image), 
                  Expanded(
                     child: Container( 
                        padding: EdgeInsets.all(5), 
                        child: Column( 
                           mainAxisAlignment: MainAxisAlignment.spaceEvenly, 
                           children: <Widget>[ 
                              Text(
                                 this.name, style: TextStyle(
                                    fontWeight: FontWeight.bold
                                 )
                              ), 
                              Text(this.description), Text(
                                 "Price: " + this.price.toString()
                              ), 
                           ], 
                        )
                     )
                  ) 
               ]
            )
         )
      ); 
   } 
}
class MyAnimatedWidget extends StatelessWidget { 
   MyAnimatedWidget({this.child, this.animation}); 
   final Widget child; 
   final Animation<double> animation; 
 
   Widget build(BuildContext context) => Center( 
      child: AnimatedBuilder(
         animation: animation, 
         builder: (context, child) => Container( 
            child: Opacity(opacity: animation.value, child: child), 
         ), 
         child: child
      ), 
   ); 
}

• Compile and run the application to see the results. The initial and final version of the application is as follows −

Flutter – Writing Android Specific Code

Flutter provides a general framework to access platform specific feature. This enables the developer to extend the functionality of the Flutter framework using platform specific code. Platform specific functionality like camera, battery level, browser, etc., can be accessed easily through the framework.

The general idea of accessing the platform specific code is through simple messaging protocol. Flutter code, Client and the platform code and Host binds to a common Message Channel. Client sends message to the Host through the Message Channel. Host listens on the Message Channel, receives the message and does the necessary functionality and finally, returns the result to the Client through Message Channel.

The platform specific code architecture is shown in the block diagram given below −

The messaging protocol uses a standard message codec (StandardMessageCodec class) that supports binary serialization of JSON-like values such as numbers, strings, boolean, etc., The serialization and de-serialization works transparently between the client and the host.

Let us write a simple application to open a browser using Android SDK and understand how

• Create a new Flutter application in Android studio, flutter_browser_app

• Replace main.dart code with below code −

import ''package:flutter/material.dart''; 
void main() => runApp(MyApp()); 
class MyApp extends StatelessWidget { 
   @override 
   Widget build(BuildContext context) {
      return MaterialApp(
         title: ''Flutter Demo'', 
         theme: ThemeData( 
            primarySwatch: Colors.blue, 
         ), 
         home: MyHomePage(title: ''Flutter Demo Home Page''),
      );
   }
}
class MyHomePage extends StatelessWidget { 
   MyHomePage({Key key, this.title}) : super(key: key); 
   final String title; 
   
   @override 
   Widget build(BuildContext context) {
      return Scaffold(
         appBar: AppBar(
            title: Text(this.title), 
         ), 
         body: Center(
            child: RaisedButton( 
               child: Text(''Open Browser''), 
               onPressed: null, 
            ), 
         ), 
      ); 
   }
}

• Here, we have created a new button to open the browser and set its onPressed method as null.

• Now, import the following packages −

import ''dart:async''; 
import ''package:flutter/services.dart'';

• Here, services.dart include the functionality to invoke platform specific code.

• Create a new message channel in the MyHomePage widget.

static const platform = const 
MethodChannel(''flutterapp.tutorialspoint.com/browser'');

• Write a method, _openBrowser to invoke platform specific method, openBrowser method through message channel.

Future<void> _openBrowser() async { 
   try {
      final int result = await platform.invokeMethod(
         ''openBrowser'', <String, String>{ 
            ''url'': "https://flutter.dev" 
         }
      ); 
   } 
   on PlatformException catch (e) { 
      // Unable to open the browser 
      print(e); 
   }
}

Here, we have used platform.invokeMethod to invoke openBrowser (explained in coming steps). openBrowser has an argument, url to open a specific url.

• Change the value of onPressed property of the RaisedButton from null to _openBrowser.

onPressed: _openBrowser,

• Open MainActivity.java (inside the android folder) and import the required library −

import android.app.Activity; 
import android.content.Intent; 
import android.net.Uri; 
import android.os.Bundle; 

import io.flutter.app.FlutterActivity; 
import io.flutter.plugin.common.MethodCall; 
import io.flutter.plugin.common.MethodChannel; 
import io.flutter.plugin.common.MethodChannel.MethodCallHandler; 
import io.flutter.plugin.common.MethodChannel.Result; 
import io.flutter.plugins.GeneratedPluginRegistrant;

• Write a method, openBrowser to open a browser

private void openBrowser(MethodCall call, Result result, String url) { 
   Activity activity = this; 
   if (activity == null) { 
      result.error("ACTIVITY_NOT_AVAILABLE", 
      "Browser cannot be opened without foreground 
      activity", null); 
      return; 
   } 
   Intent intent = new Intent(Intent.ACTION_VIEW); 
   intent.setData(Uri.parse(url)); 
   
   activity.startActivity(intent); 
   result.success((Object) true); 
}

• Now, set channel name in the MainActivity class −

private static final String CHANNEL = "flutterapp.tutorialspoint.com/browser";

• Write android specific code to set message handling in the onCreate method −

new MethodChannel(getFlutterView(), CHANNEL).setMethodCallHandler( 
   new MethodCallHandler() { 
   @Override 
   public void onMethodCall(MethodCall call, Result result) { 
      String url = call.argument("url"); 
      if (call.method.equals("openBrowser")) {
         openBrowser(call, result, url); 
      } else { 
         result.notImplemented(); 
      } 
   } 
});

Here, we have created a message channel using MethodChannel class and used MethodCallHandler class to handle the message. onMethodCall is the actual method responsible for calling the correct platform specific code by the checking the message. onMethodCall method extracts the url from message and then invokes the openBrowser only when the method call is openBrowser. Otherwise, it returns notImplemented method.

The complete source code of the application is as follows −

main.dart

MainActivity.java

package com.tutorialspoint.flutterapp.flutter_browser_app; 

import android.app.Activity; 
import android.content.Intent; 
import android.net.Uri; 
import android.os.Bundle; 
import io.flutter.app.FlutterActivity; 
import io.flutter.plugin.common.MethodCall; 
import io.flutter.plugin.common.MethodChannel.Result; 
import io.flutter.plugins.GeneratedPluginRegistrant; 

public class MainActivity extends FlutterActivity { 
   private static final String CHANNEL = "flutterapp.tutorialspoint.com/browser"; 
   @Override 
   protected void onCreate(Bundle savedInstanceState) { 
      super.onCreate(savedInstanceState); 
      GeneratedPluginRegistrant.registerWith(this); 
      new MethodChannel(getFlutterView(), CHANNEL).setMethodCallHandler(
         new MethodCallHandler() {
            @Override 
            public void onMethodCall(MethodCall call, Result result) {
               String url = call.argument("url"); 
               if (call.method.equals("openBrowser")) { 
                  openBrowser(call, result, url); 
               } else { 
                  result.notImplemented(); 
               }
            }
         }
      ); 
   }
   private void openBrowser(MethodCall call, Result result, String url) {
      Activity activity = this; if (activity == null) {
         result.error(
            "ACTIVITY_NOT_AVAILABLE", "Browser cannot be opened without foreground activity", null
         ); 
         return; 
      } 
      Intent intent = new Intent(Intent.ACTION_VIEW); 
      intent.setData(Uri.parse(url)); 
      activity.startActivity(intent); 
      result.success((Object) true); 
   }
}

main.dart

import ''package:flutter/material.dart''; 
import ''dart:async''; 
import ''package:flutter/services.dart''; 

void main() => runApp(MyApp()); 
class MyApp extends StatelessWidget {
   @override 
   Widget build(BuildContext context) {
      return MaterialApp(
         title: ''Flutter Demo'', 
         theme: ThemeData( 
            primarySwatch: Colors.blue, 
         ), 
         home: MyHomePage(
            title: ''Flutter Demo Home Page''
         ), 
      ); 
   }
}
class MyHomePage extends StatelessWidget {
   MyHomePage({Key key, this.title}) : super(key: key); 
   final String title; 
   static const platform = const MethodChannel(''flutterapp.tutorialspoint.com/browser''); 
   Future<void> _openBrowser() async {
      try {
         final int result = await platform.invokeMethod(''openBrowser'', <String, String>{ 
            ''url'': "https://flutter.dev" 
         });
      }
      on PlatformException catch (e) { 
         // Unable to open the browser print(e); 
      } 
   }
   @override 
   Widget build(BuildContext context) {
      return Scaffold( 
         appBar: AppBar( 
            title: Text(this.title), 
         ), 
         body: Center(
            child: RaisedButton( 
               child: Text(''Open Browser''), 
               onPressed: _openBrowser, 
            ), 
         ),
      );
   }
}

Run the application and click the Open Browser button and you can see that the browser is launched. The Browser app – Home page is as shown in the screenshot here −

Flutter – Writing IOS Specific Code

Accessing iOS specific code is similar to that on Android platform except that it uses iOS specific languages – Objective-C or Swift and iOS SDK. Otherwise, the concept is same as that of the Android platform.

Let us write the same application as in the previous chapter for iOS platform as well.

• Let us create a new application in Android Studio (macOS), flutter_browser_ios_app

• Follow steps 2 – 6 as in previous chapter.

• Start XCode and click File → Open

• Choose the xcode project under ios directory of our flutter project.

• Open AppDelegate.m under Runner → Runner path. It contains the following code −

#include "AppDelegate.h" 
#include "GeneratedPluginRegistrant.h" 
@implementation AppDelegate 

- (BOOL)application:(UIApplication *)application
   didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {
      // [GeneratedPluginRegistrant registerWithRegistry:self];
      // Override point for customization after application launch.
      return [super application:application didFinishLaunchingWithOptions:launchOptions];
   } 
@end

• We have added a method, openBrowser to open browser with specified url. It accepts single argument, url.

- (void)openBrowser:(NSString *)urlString { 
   NSURL *url = [NSURL URLWithString:urlString]; 
   UIApplication *application = [UIApplication sharedApplication]; 
   [application openURL:url]; 
}

• In didFinishLaunchingWithOptions method, find the controller and set it in controller variable.

FlutterViewController* controller = (FlutterViewController*)self.window.rootViewController;

• In didFinishLaunchingWithOptions method, set the browser channel as flutterapp.tutorialspoint.com/browse −

FlutterMethodChannel* browserChannel = [
   FlutterMethodChannel methodChannelWithName:
   @"flutterapp.tutorialspoint.com/browser" binaryMessenger:controller];

• Create a variable, weakSelf and set current class −

__weak typeof(self) weakSelf = self;

• Now, implement setMethodCallHandler. Call openBrowser by matching call.method. Get url by invoking call.arguments and pass it while calling openBrowser.

[browserChannel setMethodCallHandler:^(FlutterMethodCall* call, FlutterResult result) {
   if ([@"openBrowser" isEqualToString:call.method]) { 
      NSString *url = call.arguments[@"url"];   
      [weakSelf openBrowser:url]; 
   } else { result(FlutterMethodNotImplemented); } 
}];

• The complete code is as follows −

#include "AppDelegate.h" 
#include "GeneratedPluginRegistrant.h" 
@implementation AppDelegate 

- (BOOL)application:(UIApplication *)application 
   didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {
   
   // custom code starts 
   FlutterViewController* controller = (FlutterViewController*)self.window.rootViewController; 
   FlutterMethodChannel* browserChannel = [
      FlutterMethodChannel methodChannelWithName:
      @"flutterapp.tutorialspoint.com /browser" binaryMessenger:controller]; 
   
   __weak typeof(self) weakSelf = self; 
   [browserChannel setMethodCallHandler:^(
      FlutterMethodCall* call, FlutterResult result) { 
      
      if ([@"openBrowser" isEqualToString:call.method]) { 
         NSString *url = call.arguments[@"url"];
         [weakSelf openBrowser:url]; 
      } else { result(FlutterMethodNotImplemented); } 
   }]; 
   // custom code ends 
   [GeneratedPluginRegistrant registerWithRegistry:self]; 
   
   // Override point for customization after application launch. 
   return [super application:application didFinishLaunchingWithOptions:launchOptions]; 
}
- (void)openBrowser:(NSString *)urlString { 
   NSURL *url = [NSURL URLWithString:urlString]; 
   UIApplication *application = [UIApplication sharedApplication]; 
   [application openURL:url]; 
} 
@end

• Open project setting.

• Go to Capabilities and enable Background Modes.

• Add *Background fetch and Remote Notification**.

• Now, run the application. It works similar to Android version but the Safari browser will be opened instead of chrome.

Flutter – Introduction to Package

Dart’s way of organizing and sharing a set of functionality is through Package. Dart Package is simply sharable libraries or modules. In general, the Dart Package is same as that of Dart Application except Dart Package does not have application entry point, main.

The general structure of Package (consider a demo package, my_demo_package) is as below −

• lib/src/* − Private Dart code files.

• lib/my_demo_package.dart − Main Dart code file. It can be imported into an application as −

import ''package:my_demo_package/my_demo_package.dart''

• Other private code file may be exported into the main code file (my_demo_package.dart), if necessary as shown below −

export src/my_private_code.dart

• lib/* − Any number of Dart code files arranged in any custom folder structure. The code can be accessed as,

import ''package:my_demo_package/custom_folder/custom_file.dart''

• pubspec.yaml − Project specification, same as that of application,

All Dart code files in the Package are simply Dart classes and it does not have any special requirement for a Dart code to include it in a Package.

Types of Packages

Since Dart Packages are basically a small collection of similar functionality, it can be categorized based on its functionality.

Dart Package

Generic Dart code, which can be used in both web and mobile environment. For example, english_words is one such package which contains around 5000 words and has basic utility functions like nouns (list nouns in the English), syllables (specify number of syllables in a word.

Flutter Package

Generic Dart code, which depends on Flutter framework and can be used only in mobile environment. For example, fluro is a custom router for flutter. It depends on the Flutter framework.

Flutter Plugin

Generic Dart code, which depends on Flutter framework as well as the underlying platform code (Android SDK or iOS SDK). For example, camera is a plugin to interact with device camera. It depends on the Flutter framework as well as the underlying framework to get access to camera.

Using a Dart Package

Dart Packages are hosted and published into the live server, https://pub.dartlang.org. Also, Flutter provides simple tool, pub to manage Dart Packages in the application. The steps needed to use as Package is as follows −

• Include the package name and the version needed into the pubspec.yaml as shown below −

dependencies: english_words: ^3.1.5

• The latest version number can be found by checking the online server.

• Install the package into the application by using the following command −

flutter packages get

• While developing in the Android studio, Android Studio detects any change in the pubspec.yaml and displays an Android studio package alert to the developer as shown below −

• Dart Packages can be installed or updated in Android Studio using the menu options.

• Import the necessary file using the command shown below and start working −

import ''package:english_words/english_words.dart'';

• Use any method available in the package,

nouns.take(50).forEach(print);

• Here, we have used nouns function to get and print the top 50 words.

Develop a Flutter Plugin Package

Developing a Flutter Plugin is similar to developing a Dart application or Dart Package. The only exception is that the plugin is going to use System API (Android or iOS) to get the required platform specific functionality.

As we have already learned how to access platform code in the previous chapters, let us develop a simple plugin, my_browser to understand the plugin development process. The functionality of the my_browser plugin is to allow the application to open the given website in the platform specific browser.

• Start Android Studio.

• Click File → New Flutter Project and select Flutter Plugin option.

• You can see a Flutter plugin selection window as shown here −

• Enter my_browser as project name and click Next.

• Enter the plugin name and other details in the window as shown here −

• Enter company domain, flutterplugins.tutorialspoint.com in the window shown below and then click on Finish. It will generate a startup code to develop our new plugin.

• Open my_browser.dart file and write a method, openBrowser to invoke platform specific openBrowser method.

Future<void> openBrowser(String urlString) async { 
   try {
      final int result = await _channel.invokeMethod(
         ''openBrowser'', <String, String>{ ''url'': urlString }
      );
   }
   on PlatformException catch (e) { 
      // Unable to open the browser print(e); 
   } 
}

• Open MyBrowserPlugin.java file and import the following classes −

import android.app.Activity; 
import android.content.Intent; 
import android.net.Uri; 
import android.os.Bundle;

• Here, we have to import library required for opening a browser from Android.

• Add new private variable mRegistrar of type Registrar in MyBrowserPlugin class.

private final Registrar mRegistrar;

• Here, Registrar is used to get context information of the invoking code.

• Add a constructor to set Registrar in MyBrowserPlugin class.

private MyBrowserPlugin(Registrar registrar) { 
   this.mRegistrar = registrar; 
}

• Change registerWith to include our new constructor in MyBrowserPlugin class.

public static void registerWith(Registrar registrar) { 
   final MethodChannel channel = new MethodChannel(registrar.messenger(), "my_browser"); 
   MyBrowserPlugin instance = new MyBrowserPlugin(registrar); 
   channel.setMethodCallHandler(instance); 
}

• Change the onMethodCall to include openBrowser method in MyBrowserPlugin class.

@Override 
public void onMethodCall(MethodCall call, Result result) { 
   String url = call.argument("url");
   if (call.method.equals("getPlatformVersion")) { 
      result.success("Android " + android.os.Build.VERSION.RELEASE); 
   } 
   else if (call.method.equals("openBrowser")) { 
      openBrowser(call, result, url); 
   } else { 
      result.notImplemented(); 
   } 
}

• Write the platform specific openBrowser method to access browser in MyBrowserPlugin class.

private void openBrowser(MethodCall call, Result result, String url) { 
   Activity activity = mRegistrar.activity(); 
   if (activity == null) {
      result.error("ACTIVITY_NOT_AVAILABLE", 
      "Browser cannot be opened without foreground activity", null); 
      return; 
   } 
   Intent intent = new Intent(Intent.ACTION_VIEW); 
   intent.setData(Uri.parse(url)); 
   activity.startActivity(intent); 
   result.success((Object) true); 
}

• The complete source code of the my_browser plugin is as follows −

my_browser.dart

import ''dart:async''; 
import ''package:flutter/services.dart''; 

class MyBrowser {
   static const MethodChannel _channel = const MethodChannel(''my_browser''); 
   static Future<String> get platformVersion async { 
      final String version = await _channel.invokeMethod(''getPlatformVersion''); return version; 
   } 
   Future<void> openBrowser(String urlString) async { 
      try {
         final int result = await _channel.invokeMethod(
            ''openBrowser'', <String, String>{''url'': urlString}); 
      } 
      on PlatformException catch (e) { 
         // Unable to open the browser print(e); 
      }
   }
}

MyBrowserPlugin.java

package com.tutorialspoint.flutterplugins.my_browser; 

import io.flutter.plugin.common.MethodCall; 
import io.flutter.plugin.common.MethodChannel; 
import io.flutter.plugin.common.MethodChannel.MethodCallHandler; 
import io.flutter.plugin.common.MethodChannel.Result; 
import io.flutter.plugin.common.PluginRegistry.Registrar; 
import android.app.Activity; 
import android.content.Intent; 
import android.net.Uri; 
import android.os.Bundle; 

/** MyBrowserPlugin */ 
public class MyBrowserPlugin implements MethodCallHandler {
   private final Registrar mRegistrar; 
   private MyBrowserPlugin(Registrar registrar) { 
      this.mRegistrar = registrar; 
   } 
   /** Plugin registration. */
   public static void registerWith(Registrar registrar) {
      final MethodChannel channel = new MethodChannel(
         registrar.messenger(), "my_browser"); 
      MyBrowserPlugin instance = new MyBrowserPlugin(registrar); 
      channel.setMethodCallHandler(instance); 
   } 
   @Override 
   public void onMethodCall(MethodCall call, Result result) { 
      String url = call.argument("url"); 
      if (call.method.equals("getPlatformVersion")) { 
         result.success("Android " + android.os.Build.VERSION.RELEASE); 
      } 
      else if (call.method.equals("openBrowser")) { 
         openBrowser(call, result, url); 
      } else { 
         result.notImplemented(); 
      } 
   } 
   private void openBrowser(MethodCall call, Result result, String url) { 
      Activity activity = mRegistrar.activity(); 
      if (activity == null) {
         result.error("ACTIVITY_NOT_AVAILABLE",
            "Browser cannot be opened without foreground activity", null); 
         return; 
      }
      Intent intent = new Intent(Intent.ACTION_VIEW); 
      intent.setData(Uri.parse(url)); 
      activity.startActivity(intent); 
      result.success((Object) true); 
   } 
}

• Create a new project, my_browser_plugin_test to test our newly created plugin.

• Open pubspec.yaml and set my_browser as a plugin dependency.

dependencies: 
   flutter: 
      sdk: flutter 
   my_browser: 
      path: ../my_browser

• Android studio will alert that the pubspec.yaml is updated as shown in the Android studio package alert given below −

• Click Get dependencies option. Android studio will get the package from Internet and properly configure it for the application.

• Open main.dart and include my_browser plugin as below −

import ''package:my_browser/my_browser.dart'';

• Call the openBrowser function from my_browser plugin as shown below −

onPressed: () => MyBrowser().openBrowser("https://flutter.dev"),

• The complete code of the main.dart is as follows −

import ''package:flutter/material.dart''; 
import ''package:my_browser/my_browser.dart''; 

void main() => runApp(MyApp()); 

class MyApp extends StatelessWidget { 
   @override 
   Widget build(BuildContext context) {
      return MaterialApp( 
         title: ''Flutter Demo'', 
         theme: ThemeData( 
            primarySwatch: Colors.blue, 
         ), 
         home: MyHomePage(
            title: ''Flutter Demo Home Page''
         ), 
      );,
   }
} 
class MyHomePage extends StatelessWidget { 
   MyHomePage({Key key, this.title}) : super(key: key); 
   final String title; 
   @override 
   Widget build(BuildContext context) {
      return Scaffold(
         appBar: AppBar( 
            title: Text(this.title), 
         ), 
         body: Center(
            child: RaisedButton(
               child: Text(''Open Browser''), 
               onPressed: () => MyBrowser().openBrowser("https://flutter.dev"), 
            ),
         ), 
      ); 
   }
}

• Run the application and click the Open Browser button and see that the browser is launched. You can see a Browser app – Home page as shown in the screenshot shown below −

You can see a Browser app – Browser screen as shown in the screenshot shown below −

print(await http.read(''https://flutter.dev/''));

var client = new http.Client(); 
try { 
   print(await client.get(''https://flutter.dev/'')); 
} 
finally { 
   client.close(); 
}

flutter: 
   assets: 
      - assets/appimages/floppy.png 
      - assets/appimages/iphone.png 
      - assets/appimages/laptop.png 
      - assets/appimages/pendrive.png 
      - assets/appimages/pixel.png 
      - assets/appimages/tablet.png

dependencies: 
   http: ^0.12.0+2

import ''dart:async''; 
import ''dart:convert''; 
import ''package:http/http.dart'' as http;

[ 
   { 
      "name": "iPhone", 
      "description": "iPhone is the stylist phone ever", 
      "price": 1000, 
      "image": "iphone.png" 
   }, 
   { 
      "name": "Pixel", 
      "description": "Pixel is the most feature phone ever", 
      "price": 800, 
      "image": "pixel.png"
   }, 
   { 
      "name": "Laptop", 
      "description": "Laptop is most productive development tool", 
      "price": 2000, 
      "image": "laptop.png" 
   }, 
   { 
      "name": "Tablet", 
      "description": "Tablet is the most useful device ever for meeting", 
      "price": 1500, 
      "image": "tablet.png" 
   }, 
   { 
      "name": "Pendrive", 
      "description": "Pendrive is useful storage medium", 
      "price": 100, 
      "image": "pendrive.png" 
   }, 
   { 
      "name": "Floppy Drive", 
      "description": "Floppy drive is useful rescue storage medium", 
      "price": 20, 
      "image": "floppy.png" 
   } 
]

cd /path/to/JSONWebServer

npm install -g http-server

http-server . -p 8000 

Starting up http-server, serving . 
Available on: 
   http://192.168.99.1:8000
   http://127.0.0.1:8000 
   Hit CTRL-C to stop the server

factory Product.fromJson(Map<String, dynamic> data) {
   return Product(
      data[''name''],
      data[''description''], 
      data[''price''],
      data[''image''],
   );
}

class Product {
   final String name; 
   final String description;
   final int price;
   final String image; 
   
   Product(this.name, this.description, this.price, this.image); 
   factory Product.fromMap(Map<String, dynamic> json) { 
      return Product( 
         json[''name''], 
         json[''description''], 
         json[''price''], 
         json[''image''], 
      );
   }
}

List<Product> parseProducts(String responseBody) { 
   final parsed = json.decode(responseBody).cast<Map<String, dynamic>>(); 
   return parsed.map<Product>((json) =>Product.fromJson(json)).toList(); 
} 
Future<List<Product>> fetchProducts() async { 
   final response = await http.get(''http://192.168.1.2:8000/products.json''); 
   if (response.statusCode == 200) { 
      return parseProducts(response.body); 
   } else { 
      throw Exception(''Unable to fetch products from the REST API'');
   } 
}

class MyApp extends StatelessWidget { 
   final Future<List<Product>> products; 
   MyApp({Key key, this.products}) : super(key: key); 
   ...

class MyHomePage extends StatelessWidget { 
   final String title; 
   final Future<ListList<Product>> products; 
   MyHomePage({Key key, this.title, this.products}) : super(key: key); 
   ...

home: MyHomePage(title: ''Product Navigation demo home page'', products: products),

void main() => runApp(MyApp(fetchProduct()));

class ProductBoxList extends StatelessWidget { 
   final List<Product> items;
   ProductBoxList({Key key, this.items}); 
   
   @override 
   Widget build(BuildContext context) {
      return ListView.builder(
         itemCount: items.length,
         itemBuilder: (context, index) {
            return GestureDetector(
               child: ProductBox(item: items[index]), 
               onTap: () {
                  Navigator.push(
                     context, MaterialPageRoute(
                        builder: (context) =gt; ProductPage(item: items[index]), 
                     ), 
                  ); 
               }, 
            ); 
         }, 
      ); 
   } 
}

Widget build(BuildContext context) { 
   return Scaffold(
      appBar: AppBar(title: Text("Product Navigation")),
      body: Center(
         child: FutureBuilder<List<Product>>(
            future: products, builder: (context, snapshot) {
               if (snapshot.hasError) print(snapshot.error); 
               return snapshot.hasData ? ProductBoxList(items: snapshot.data)
               
               // return the ListView widget : 
               Center(child: CircularProgressIndicator()); 
            }, 
         ), 
      )
   ); 
}

import ''package:flutter/material.dart''; 
import ''dart:async''; 
import ''dart:convert''; 
import ''package:http/http.dart'' as http; 
import ''Product.dart''; 

void main() => runApp(MyApp(products: fetchProducts())); 

List<Product> parseProducts(String responseBody) { 
   final parsed = json.decode(responseBody).cast<Map<String, dynamic>>(); 
   return parsed.map<Product>((json) => Product.fromMap(json)).toList(); 
} 
Future<List<Product>> fetchProducts() async { 
   final response = await http.get(''http://192.168.1.2:8000/products.json''); 
   if (response.statusCode == 200) { 
      return parseProducts(response.body); 
   } else { 
      throw Exception(''Unable to fetch products from the REST API''); 
   } 
}
class MyApp extends StatelessWidget {
   final Future<List<Product>> products; 
   MyApp({Key key, this.products}) : super(key: key); 
   
   // This widget is the root of your application. 
   @override 
   Widget build(BuildContext context) {
      return MaterialApp(
         title: ''Flutter Demo'', 
         theme: ThemeData( 
            primarySwatch: Colors.blue, 
         ), 
         home: MyHomePage(title: ''Product Navigation demo home page'', products: products), 
      ); 
   }
}
class MyHomePage extends StatelessWidget { 
   final String title; 
   final Future<List<Product>> products; 
   MyHomePage({Key key, this.title, this.products}) : super(key: key); 
   
   // final items = Product.getProducts();
   @override 
   Widget build(BuildContext context) { 
      return Scaffold(
         appBar: AppBar(title: Text("Product Navigation")), 
         body: Center(
            child: FutureBuilder<List<Product>>(
               future: products, builder: (context, snapshot) {
                  if (snapshot.hasError) print(snapshot.error); 
                  return snapshot.hasData ? ProductBoxList(items: snapshot.data) 
                  
                  // return the ListView widget : 
                  Center(child: CircularProgressIndicator()); 
               },
            ),
         )
      );
   }
}
class ProductBoxList extends StatelessWidget {
   final List<Product> items; 
   ProductBoxList({Key key, this.items}); 
   
   @override 
   Widget build(BuildContext context) {
      return ListView.builder(
         itemCount: items.length, 
         itemBuilder: (context, index) { 
            return GestureDetector( 
               child: ProductBox(item: items[index]), 
               onTap: () { 
                  Navigator.push(
                     context, MaterialPageRoute( 
                        builder: (context) => ProductPage(item: items[index]), 
                     ), 
                  ); 
               }, 
            ); 
         }, 
      ); 
   } 
} 
class ProductPage extends StatelessWidget { 
   ProductPage({Key key, this.item}) : super(key: key); 
   final Product item; 
   @override 
   Widget build(BuildContext context) {
      return Scaffold(
         appBar: AppBar(title: Text(this.item.name),), 
         body: Center( 
            child: Container(
               padding: EdgeInsets.all(0), 
               child: Column( 
                  mainAxisAlignment: MainAxisAlignment.start, 
                  crossAxisAlignment: CrossAxisAlignment.start, 
                  children: <Widget>[
                     Image.asset("assets/appimages/" + this.item.image), 
                     Expanded( 
                        child: Container( 
                           padding: EdgeInsets.all(5), 
                           child: Column( 
                              mainAxisAlignment: MainAxisAlignment.spaceEvenly, 
                              children: <Widget>[ 
                                 Text(this.item.name, style: 
                                    TextStyle(fontWeight: FontWeight.bold)), 
                                 Text(this.item.description), 
                                 Text("Price: " + this.item.price.toString()), 
                                 RatingBox(), 
                              ], 
                           )
                        )
                     ) 
                  ]
               ), 
            ), 
         ), 
      ); 
   } 
}
class RatingBox extends StatefulWidget { 
   @override 
   _RatingBoxState createState() =>_RatingBoxState(); 
} 
class _RatingBoxState extends State<RatingBox> { 
   int _rating = 0; 
   void _setRatingAsOne() {
      setState(() { 
         _rating = 1; 
      }); 
   }
   void _setRatingAsTwo() {
      setState(() {
         _rating = 2; 
      }); 
   }
   void _setRatingAsThree() { 
      setState(() {
         _rating = 3; 
      }); 
   }
   Widget build(BuildContext context) {
      double _size = 20; 
      print(_rating); 
      return Row(
         mainAxisAlignment: MainAxisAlignment.end, 
         crossAxisAlignment: CrossAxisAlignment.end, 
         mainAxisSize: MainAxisSize.max, 
         
         children: <Widget>[
            Container(
               padding: EdgeInsets.all(0), 
               child: IconButton( 
                  icon: (
                     _rating >= 1 
                     ? Icon(Icons.star, ize: _size,) 
                     : Icon(Icons.star_border, size: _size,)
                  ), 
                  color: Colors.red[500], onPressed: _setRatingAsOne, iconSize: _size, 
               ), 
            ), 
            Container(
               padding: EdgeInsets.all(0), 
               child: IconButton(
                  icon: (
                     _rating >= 2 
                     ? Icon(Icons.star, size: _size,) 
                     : Icon(Icons.star_border, size: _size, )
                  ), 
                  color: Colors.red[500], 
                  onPressed: _setRatingAsTwo, 
                  iconSize: _size, 
               ), 
            ), 
            Container(
               padding: EdgeInsets.all(0), 
               child: IconButton(
                  icon: (
                     _rating >= 3 ? 
                     Icon(Icons.star, size: _size,)
                     : Icon(Icons.star_border, size: _size,)
                  ), 
                  color: Colors.red[500], 
                  onPressed: _setRatingAsThree, 
                  iconSize: _size, 
               ), 
            ), 
         ], 
      ); 
   } 
}
class ProductBox extends StatelessWidget {
   ProductBox({Key key, this.item}) : super(key: key); 
   final Product item; 
   
   Widget build(BuildContext context) {
      return Container(
         padding: EdgeInsets.all(2), height: 140, 
         child: Card(
            child: Row( 
               mainAxisAlignment: MainAxisAlignment.spaceEvenly, 
               children: <Widget>[
                  Image.asset("assets/appimages/" + this.item.image), 
                  Expanded( 
                     child: Container( 
                        padding: EdgeInsets.all(5), 
                        child: Column( 
                           mainAxisAlignment: MainAxisAlignment.spaceEvenly, 
                           children: <Widget>[ 
                              Text(this.item.name, style:TextStyle(fontWeight: FontWeight.bold)), 
                              Text(this.item.description), 
                              Text("Price: " + this.item.price.toString()), 
                              RatingBox(), 
                           ], 
                        )
                     )
                  )
               ]
            ), 
         )
      ); 
   } 
}

flutter: 
   assets: 
      - assets/appimages/floppy.png 
      - assets/appimages/iphone.png 
      - assets/appimages/laptop.png 
      - assets/appimages/pendrive.png 
      - assets/appimages/pixel.png 
      - assets/appimages/tablet.png

dependencies: sqflite: any

dependencies: path_provider: any

class Product { 
   final int id; 
   final String name; 
   final String description; 
   final int price; 
   final String image; 
   static final columns = ["id", "name", "description", "price", "image"]; 
   Product(this.id, this.name, this.description, this.price, this.image); 
   factory Product.fromMap(Map<String, dynamic> data) {
      return Product( 
         data[''id''], 
         data[''name''], 
         data[''description''], 
         data[''price''], 
         data[''image''], 
      ); 
   } 
   Map<String, dynamic> toMap() => {
      "id": id, 
      "name": name, 
      "description": description, 
      "price": price, 
      "image": image 
   }; 
}

import ''dart:async''; 
import ''dart:io''; 
import ''package:path/path.dart''; 
import ''package:path_provider/path_provider.dart''; 
import ''package:sqflite/sqflite.dart''; 
import ''Product.dart'';

class SQLiteDbProvider { 
   SQLiteDbProvider._(); 
   static final SQLiteDbProvider db = SQLiteDbProvider._(); 
   static Database _database; 
}

SQLiteDBProvoider.db.<emthod>

Future<Database> get database async { 
   if (_database != null) 
   return _database; 
   _database = await initDB(); 
   return _database; 
}
initDB() async { 
   Directory documentsDirectory = await getApplicationDocumentsDirectory(); 
   String path = join(documentsDirectory.path, "ProductDB.db"); 
   return await openDatabase(
      path, 
      version: 1,
      onOpen: (db) {}, 
      onCreate: (Database db, int version) async {
         await db.execute(
            "CREATE TABLE Product ("
            "id INTEGER PRIMARY KEY,"
            "name TEXT,"
            "description TEXT,"
            "price INTEGER," 
            "image TEXT" ")"
         ); 
         await db.execute(
            "INSERT INTO Product (''id'', ''name'', ''description'', ''price'', ''image'') 
            values (?, ?, ?, ?, ?)", 
            [1, "iPhone", "iPhone is the stylist phone ever", 1000, "iphone.png"]
         ); 
         await db.execute(
            "INSERT INTO Product (''id'', ''name'', ''description'', ''price'', ''image'') 
            values (?, ?, ?, ?, ?)", 
            [2, "Pixel", "Pixel is the most feature phone ever", 800, "pixel.png"]
         ); 
         await db.execute(
            "INSERT INTO Product (''id'', ''name'', ''description'', ''price'', ''image'') 
            values (?, ?, ?, ?, ?)", 
            [3, "Laptop", "Laptop is most productive development tool", 2000, "laptop.png"]
         ); 
         await db.execute( 
            "INSERT INTO Product (''id'', ''name'', ''description'', ''price'', ''image'') 
            values (?, ?, ?, ?, ?)", 
            [4, "Tablet", "Laptop is most productive development tool", 1500, "tablet.png"]
         );
         await db.execute( 
            "INSERT INTO Product 
            (''id'', ''name'', ''description'', ''price'', ''image'') 
            values (?, ?, ?, ?, ?)", 
            [5, "Pendrive", "Pendrive is useful storage medium", 100, "pendrive.png"]
         );
         await db.execute( 
            "INSERT INTO Product 
            (''id'', ''name'', ''description'', ''price'', ''image'') 
            values (?, ?, ?, ?, ?)", 
            [6, "Floppy Drive", "Floppy drive is useful rescue storage medium", 20, "floppy.png"]
         ); 
      }
   ); 
}

Future<List<Product>> getAllProducts() async { 
   final db = await database; 
   List<Map> 
   results = await db.query("Product", columns: Product.columns, orderBy: "id ASC"); 
   
   List<Product> products = new List(); 
   results.forEach((result) { 
      Product product = Product.fromMap(result); 
      products.add(product); 
   }); 
   return products; 
}

Future<Product> getProductById(int id) async {
   final db = await database; 
   var result = await db.query("Product", where: "id = ", whereArgs: [id]); 
   return result.isNotEmpty ? Product.fromMap(result.first) : Null; 
}

insert(Product product) async { 
   final db = await database; 
   var maxIdResult = await db.rawQuery(
      "SELECT MAX(id)+1 as last_inserted_id FROM Product");

   var id = maxIdResult.first["last_inserted_id"]; 
   var result = await db.rawInsert(
      "INSERT Into Product (id, name, description, price, image)" 
      " VALUES (?, ?, ?, ?, ?)", 
      [id, product.name, product.description, product.price, product.image] 
   ); 
   return result; 
}
update(Product product) async { 
   final db = await database; 
   var result = await db.update("Product", product.toMap(), 
   where: "id = ?", whereArgs: [product.id]); return result; 
} 
delete(int id) async { 
   final db = await database; 
   db.delete("Product", where: "id = ?", whereArgs: [id]); 
}

import ''dart:async''; 
import ''dart:io''; 
import ''package:path/path.dart''; 
import ''package:path_provider/path_provider.dart''; 
import ''package:sqflite/sqflite.dart''; 
import ''Product.dart''; 

class SQLiteDbProvider {
   SQLiteDbProvider._(); 
   static final SQLiteDbProvider db = SQLiteDbProvider._(); 
   static Database _database; 
   
   Future<Database> get database async {
      if (_database != null) 
      return _database; 
      _database = await initDB(); 
      return _database; 
   } 
   initDB() async {
      Directory documentsDirectory = await 
      getApplicationDocumentsDirectory(); 
      String path = join(documentsDirectory.path, "ProductDB.db"); 
      return await openDatabase(
         path, version: 1, 
         onOpen: (db) {}, 
         onCreate: (Database db, int version) async {
            await db.execute(
               "CREATE TABLE Product (" 
               "id INTEGER PRIMARY KEY," 
               "name TEXT," 
               "description TEXT," 
               "price INTEGER," 
               "image TEXT"")"
            ); 
            await db.execute(
               "INSERT INTO Product (''id'', ''name'', ''description'', ''price'', ''image'') 
               values (?, ?, ?, ?, ?)", 
               [1, "iPhone", "iPhone is the stylist phone ever", 1000, "iphone.png"]
            ); 
            await db.execute( 
               "INSERT INTO Product (''id'', ''name'', ''description'', ''price'', ''image'') 
               values (?, ?, ?, ?, ?)", 
               [2, "Pixel", "Pixel is the most feature phone ever", 800, "pixel.png"]
            );
            await db.execute(
               "INSERT INTO Product (''id'', ''name'', ''description'', ''price'', ''image'') 
               values (?, ?, ?, ?, ?)", 
               [3, "Laptop", "Laptop is most productive development tool", 2000, "laptop.png"]
            ); 
            await db.execute( 
               "INSERT INTO Product (''id'', ''name'', ''description'', ''price'', ''image'') 
               values (?, ?, ?, ?, ?)", 
               [4, "Tablet", "Laptop is most productive development tool", 1500, "tablet.png"]
            ); 
            await db.execute( 
               "INSERT INTO Product (''id'', ''name'', ''description'', ''price'', ''image'') 
               values (?, ?, ?, ?, ?)", 
               [5, "Pendrive", "Pendrive is useful storage medium", 100, "pendrive.png"]
            );
            await db.execute( 
               "INSERT INTO Product (''id'', ''name'', ''description'', ''price'', ''image'') 
               values (?, ?, ?, ?, ?)", 
               [6, "Floppy Drive", "Floppy drive is useful rescue storage medium", 20, "floppy.png"]
            ); 
         }
      ); 
   }
   Future<List<Product>> getAllProducts() async {
      final db = await database; 
      List<Map> results = await db.query(
         "Product", columns: Product.columns, orderBy: "id ASC"
      ); 
      List<Product> products = new List();   
      results.forEach((result) {
         Product product = Product.fromMap(result); 
         products.add(product); 
      }); 
      return products; 
   } 
   Future<Product> getProductById(int id) async {
      final db = await database; 
      var result = await db.query("Product", where: "id = ", whereArgs: [id]); 
      return result.isNotEmpty ? Product.fromMap(result.first) : Null; 
   } 
   insert(Product product) async { 
      final db = await database; 
      var maxIdResult = await db.rawQuery("SELECT MAX(id)+1 as last_inserted_id FROM Product"); 
      var id = maxIdResult.first["last_inserted_id"]; 
      var result = await db.rawInsert(
         "INSERT Into Product (id, name, description, price, image)" 
         " VALUES (?, ?, ?, ?, ?)", 
         [id, product.name, product.description, product.price, product.image] 
      ); 
      return result; 
   } 
   update(Product product) async { 
      final db = await database; 
      var result = await db.update(
         "Product", product.toMap(), where: "id = ?", whereArgs: [product.id]
      ); 
      return result; 
   } 
   delete(int id) async { 
      final db = await database; 
      db.delete("Product", where: "id = ?", whereArgs: [id]);
   } 
}

void main() {
   runApp(MyApp(products: SQLiteDbProvider.db.getAllProducts())); 
}

class Product { 
   final String name; 
   final String description; 
   final int price; 
   final String image; 
   
   Product(this.name, this.description, this.price, this.image); 
   factory Product.fromMap(Map<String, dynamic> json) {
      return Product( 
         json[''name''], 
         json[''description''], 
         json[''price''], 
         json[''image''], 
      ); 
   }
}

flutter:
   assets: 
   - assets/appimages/floppy.png 
   - assets/appimages/iphone.png 
   - assets/appimages/laptop.png 
   - assets/appimages/pendrive.png 
   - assets/appimages/pixel.png 
   - assets/appimages/tablet.png

dependencies: cloud_firestore: ^0.9.13+1

apply plugin: ''com.google.gms.google-services''

buildscript {
   repositories { 
      // ... 
   } 
   dependencies { 
      // ... 
      classpath ''com.google.gms:google-services:3.2.1'' // new 
   } 
}

android {
   defaultConfig { 
      ... 
      multiDexEnabled true 
   } 
   ...
}
dependencies {
   ... 
   compile ''com.android.support: multidex:1.0.3'' 
}

import ''package:cloud_firestore/cloud_firestore.dart'';

Stream<QuerySnapshot> fetchProducts() { 
   return Firestore.instance.collection(''product'').snapshots(); }

@override 
Widget build(BuildContext context) {
   return Scaffold(
      appBar: AppBar(title: Text("Product Navigation")), 
      body: Center(
         child: StreamBuilder<QuerySnapshot>(
            stream: products, builder: (context, snapshot) {
               if (snapshot.hasError) print(snapshot.error); 
               if(snapshot.hasData) {
                  List<DocumentSnapshot> 
                  documents = snapshot.data.documents; 
                  
                  List<Product> 
                  items = List<Product>(); 
                  
                  for(var i = 0; i < documents.length; i++) { 
                     DocumentSnapshot document = documents[i]; 
                     items.add(Product.fromMap(document.data)); 
                  } 
                  return ProductBoxList(items: items);
               } else { 
                  return Center(child: CircularProgressIndicator()); 
               }
            }, 
         ), 
      )
   ); 
}

Locale en_locale = Locale(''en'', ''US'')

Locale es_locale = Locale(''es'', ''AR'')

class CustomLocalizations { 
   CustomLocalizations(this.locale); 
   final Locale locale; 
   static CustomLocalizations of(BuildContext context) { 
      return Localizations.of<CustomLocalizations>(context, CustomLocalizations); 
   } 
   static Map<String, Map<String, String>> _resources = {
      ''en'': {
         ''title'': ''Demo'', 
         ''message'': ''Hello World'' 
      }, 
      ''es'': {
         ''title'': ''Manifestación'', 
         ''message'': ''Hola Mundo'', 
      }, 
   }; 
   String get title { 
      return _resources[locale.languageCode][''title'']; 
   }
   String get message { 
      return _resources[locale.languageCode][''message'']; 
   } 
}

@override 
bool isSupported(Locale locale) => [''en'', ''es''].contains(locale.languageCode);

@override 
Future<CustomLocalizations> load(Locale locale) { 
   return SynchronousFuture<CustomLocalizations>(CustomLocalizations(locale)); 
}

@override 
bool shouldReload(CustomLocalizationsDelegate old) => false;

class CustomLocalizationsDelegate extends 
LocalizationsDelegate<CustomLocalizations> { 
   const CustomLocalizationsDelegate(); 
   @override 
   bool isSupported(Locale locale) => [''en'', ''es''].contains(locale.languageCode);
   @override 
   Future<CustomLocalizations> load(Locale locale) { 
      return SynchronousFuture<CustomLocalizations>(CustomLocalizations(locale));
   } 
   @override bool shouldReload(CustomLocalizationsDelegate old) => false; 
}

dependencies: 
   flutter: 
      sdk: flutter 
   flutter_localizations:
      sdk: flutter

import ''package:flutter_localizations/flutter_localizations.dart''; 
import ''package:flutter/foundation.dart'' show SynchronousFuture;

class CustomLocalizations { 
   CustomLocalizations(this.locale); 
   final Locale locale; 
   static CustomLocalizations of(BuildContext context) {
      return Localizations.of<CustomLocalizations>(context, CustomLocalizations); 
   }
   static Map<String, Map<String, String>> _resources = {
      ''en'': {
         ''title'': ''Demo'', 
         ''message'': ''Hello World'' 
      }, 
      ''es'': { 
         ''title'': ''Manifestación'', 
         ''message'': ''Hola Mundo'', 
      }, 
   }; 
   String get title { 
      return _resources[locale.languageCode][''title'']; 
   } 
   String get message { 
      return _resources[locale.languageCode][''message'']; 
   } 
}
class CustomLocalizationsDelegate extends
LocalizationsDelegate<CustomLocalizations> {
   const CustomLocalizationsDelegate();
   
   @override 
   bool isSupported(Locale locale) => [''en'', ''es''].contains(locale.languageCode); 
   
   @override 
   Future<CustomLocalizations> load(Locale locale) { 
      return SynchronousFuture<CustomLocalizations>(CustomLocalizations(locale)); 
   } 
   @override bool shouldReload(CustomLocalizationsDelegate old) => false; 
}

localizationsDelegates: [
   const CustomLocalizationsDelegate(),   
   GlobalMaterialLocalizations.delegate, 
   GlobalWidgetsLocalizations.delegate, 
], 
supportedLocales: [
   const Locale(''en'', ''''),
   const Locale(''es'', ''''), 
],

class MyHomePage extends StatelessWidget {
   MyHomePage({Key key, this.title}) : super(key: key); 
   final String title; 
   @override 
   Widget build(BuildContext context) {
      return Scaffold(
         appBar: AppBar(title: Text(CustomLocalizations .of(context) .title), ), 
         body: Center(
            child: Column(
               mainAxisAlignment: MainAxisAlignment.center, 
               children: <Widget>[ 
                  Text( CustomLocalizations .of(context) .message, ), 
               ], 
            ), 
         ),
      );
   }
}

dependencies: 
   flutter: 
      sdk: flutter 
   flutter_localizations: 
      sdk: flutter 
   intl: ^0.15.7 
   intl_translation: ^0.17.3

import ''package:intl/intl.dart'';

class CustomLocalizations { 
   static Future<CustomLocalizations> load(Locale locale) {
      final String name = locale.countryCode.isEmpty ? locale.languageCode : locale.toString(); 
      final String localeName = Intl.canonicalizedLocale(name); 
      
      return initializeMessages(localeName).then((_) {
         Intl.defaultLocale = localeName; 
         return CustomLocalizations(); 
      }); 
   } 
   static CustomLocalizations of(BuildContext context) { 
      return Localizations.of<CustomLocalizations>(context, CustomLocalizations); 
   } 
   String get title {
      return Intl.message( 
         ''Demo'', 
         name: ''title'', 
         desc: ''Title for the Demo application'', 
      ); 
   }
   String get message{
      return Intl.message(
         ''Hello World'', 
         name: ''message'', 
         desc: ''Message for the Demo application'', 
      ); 
   }
}
class CustomLocalizationsDelegate extends 
LocalizationsDelegate<CustomLocalizations> {
   const CustomLocalizationsDelegate();
   
   @override
   bool isSupported(Locale locale) => [''en'', ''es''].contains(locale.languageCode); 
   @override 
   Future<CustomLocalizations> load(Locale locale) { 
      return CustomLocalizations.load(locale); 
   } 
   @override 
   bool shouldReload(CustomLocalizationsDelegate old) => false; 
}

import ''l10n/messages_all.dart'';

flutter packages pub run intl_translation:extract_to_arb --output-
   dir=lib/l10n lib/main.dart

{
   "@@last_modified": "2019-04-19T02:04:09.627551", 
   "title": "Demo", 
   "@title": {
      "description": "Title for the Demo application", 
      "type": "text", 
      "placeholders": {} 
   }, 
   "message": "Hello World", 
   "@message": {
      "description": "Message for the Demo 
      application", 
      "type": "text", 
      "placeholders": {} 
   }
}

{
   "@@last_modified": "2019-04-19T02:04:09.627551",  
   "title": "Manifestación", 
   "@title": {
      "description": "Title for the Demo application", 
      "type": "text", 
      "placeholders": {} 
   },
   "message": "Hola Mundo",
   "@message": {
      "description": "Message for the Demo application", 
      "type": "text", 
      "placeholders": {} 
   } 
}

flutter packages pub run intl_translation:generate_from_arb 
--output-dir=libl10n --no-use-deferred-loading 
libmain.dart libl10nintl_en.arb libl10nintl_es.arb

testWidgets(''test description: find a widget'', ''<test code>'');

testWidgets(''finds a specific instance'', (WidgetTester tester) async { 
   await tester.pumpWidget(MaterialApp( 
      home: Scaffold( 
         body: Text(''Hello''), 
      ), 
   )); 
});

find.text(''Hello'')

find.byKey(''home'')

find.byWidget(homeWidget)

expect(find.text(''Hello''), findsOneWidget);

expect(find.text(''Hello World''), findsNothing);

expect(find.text(''Save''), findsWidgets);

expect(find.text(''Save''), findsNWidgets(2));

testWidgets(''finds hello widget'', (WidgetTester tester) async { 
   await tester.pumpWidget(MaterialApp( 
      home: Scaffold( 
         body: Text(''Hello''), 
      ), 
   )); 
   expect(find.text(''Hello''), findsOneWidget); 
});

testWidgets(''Counter increments smoke test'', (WidgetTester tester) async {
   // Build our app and trigger a frame. 
   await tester.pumpWidget(MyApp()); 
   
   // Verify that our counter starts at 0. 
   expect(find.text(''0''), findsOneWidget); 
   expect(find.text(''1''), findsNothing); 
   
   // Tap the ''+'' icon and trigger a frame. 
   await tester.tap(find.byIcon(Icons.add)); 
   await tester.pump(); 
   
   // Verify that our counter has incremented. 
   expect(find.text(''0''), findsNothing); 
   expect(find.text(''1''), findsOneWidget); 
});

await tester.tap(find.byIcon(Icons.add)); 
await tester.pump(); 

expect(find.text(''2''), findsOneWidget);

cd /path/to/my/application 
flutter build apk

Initializing gradle...                                            8.6s 
Resolving dependencies...                                        19.9s 
Calling mockable JAR artifact transform to create file: 
/Users/.gradle/caches/transforms-1/files-1.1/android.jar/ 
c30932f130afbf3fd90c131ef9069a0b/android.jar with input 
/Users/Library/Android/sdk/platforms/android-28/android.jar 
Running Gradle task ''assembleRelease''... 
Running Gradle task ''assembleRelease''... 
Done                                                             85.7s 
Built build/app/outputs/apk/release/app-release.apk (4.8MB).

flutter install

flutter build appbundle

flutter build ios

Building com.example.MyApp for device (ios-release)... 
Automatically signing iOS for device deployment 
using specified development team in Xcode project: 
Running Xcode build...                                   23.5s 
......................

flutter packages pub global activate devtools

Resolving dependencies... 
+ args 1.5.1 
+ async 2.2.0
+ charcode 1.1.2 
+ codemirror 0.5.3+5.44.0 
+ collection 1.14.11 
+ convert 2.1.1 
+ devtools 0.0.16 
+ devtools_server 0.0.2 
+ http 0.12.0+2 
+ http_parser 3.1.3 
+ intl 0.15.8 
+ js 0.6.1+1 
+ meta 1.1.7 
+ mime 0.9.6+2 
.................. 
.................. 
Installed executable devtools. 
Activated devtools 0.0.16.

flutter packages pub global run devtools

Serving DevTools at http://127.0.0.1:9100

flutter run --observatory-port=9200

http://localhost:9100/?port=9200

flutter upgrade

flutter packages upgrade

Running "flutter packages upgrade" in my_app... 7.4s

flutter run --track-widget-creation

Launching lib/main.dart on iPhone X in debug mode... 
─Assembling Flutter resources...                       3.6s 
Compiling, linking and signing...                      6.8s 
Xcode build done.                                     14.2s 
2,904ms (!)
To hot reload changes while running, press "r". To hot restart (and rebuild state), press "R". 
An Observatory debugger and profiler on iPhone X is available at: http://127.0.0.1:50399/ 
For a more detailed help message, press "h". To detach, press "d"; to quit, press "q".

dependencies: 
   flutter: 
      sdk: flutter 
   sqflite: ^1.1.0 
   path_provider: ^0.5.0+1 
   scoped_model: ^1.0.1 
   intl: any

factory Expense.fromMap(Map<String, dynamic> data) { 
   return Expense( 
      data[''id''], 
      data[''amount''], 
      DateTime.parse(data[''date'']),    
      data[''category''] 
   ); 
}

Map<String, dynamic> toMap() => { 
   "id" : id, 
   "amount" : amount, 
   "date" : date.toString(), 
   "category" : category, 
};

import ''package:intl/intl.dart''; class Expense {
   final int id; 
   final double amount; 
   final DateTime date; 
   final String category; 
   String get formattedDate { 
      var formatter = new DateFormat(''yyyy-MM-dd''); 
      return formatter.format(this.date); 
   } 
   static final columns = [''id'', ''amount'', ''date'', ''category''];
   Expense(this.id, this.amount, this.date, this.category); 
   factory Expense.fromMap(Map<String, dynamic> data) { 
      return Expense( 
         data[''id''], 
         data[''amount''], 
         DateTime.parse(data[''date'']), data[''category''] 
      ); 
   }
   Map<String, dynamic> toMap() => {
      "id" : id, 
      "amount" : amount, 
      "date" : date.toString(), 
      "category" : category, 
   }; 
}

Future<List<Expense>> getAllExpenses() async { 
   final db = await database; 
   
   List<Map> results = await db.query(
      "Expense", columns: Expense.columns, orderBy: "date DESC"
   );
   List<Expense> expenses = new List(); 
   results.forEach((result) {
      Expense expense = Expense.fromMap(result); 
      expenses.add(expense); 
   }); 
   return expenses; 
}

Future<Expense> getExpenseById(int id) async {
   final db = await database;
   var result = await db.query("Expense", where: "id = ", whereArgs: [id]);
   
   return result.isNotEmpty ? 
   Expense.fromMap(result.first) : Null; 
}

Future<double> getTotalExpense() async {
   final db = await database; 
   List<Map> list = await db.rawQuery(
      "Select SUM(amount) as amount from expense"
   );
   return list.isNotEmpty ? list[0]["amount"] : Null; 
}

Future<Expense> insert(Expense expense) async { 
   final db = await database; 
   var maxIdResult = await db.rawQuery(
      "SELECT MAX(id)+1 as last_inserted_id FROM Expense"
   );
   var id = maxIdResult.first["last_inserted_id"]; 
   var result = await db.rawInsert(
      "INSERT Into Expense (id, amount, date, category)" 
      " VALUES (?, ?, ?, ?)", [
         id, expense.amount, expense.date.toString(), expense.category
      ]
   ); 
   return Expense(id, expense.amount, expense.date, expense.category); 
}

update(Expense product) async {
   final db = await database; 
   
   var result = await db.update("Expense", product.toMap(), 
   where: "id = ?", whereArgs: [product.id]); 
   return result; 
}

delete(int id) async {
   final db = await database;
   db.delete("Expense", where: "id = ?", whereArgs: [id]); 
}

import ''dart:async''; 
import ''dart:io''; 
import ''package:path/path.dart''; 
import ''package:path_provider/path_provider.dart''; 
import ''package:sqflite/sqflite.dart''; 
import ''Expense.dart''; 

class SQLiteDbProvider {
   SQLiteDbProvider._(); 
   static final SQLiteDbProvider db = SQLiteDbProvider._(); 
   
   static Database _database; Future<Database> get database async { 
      if (_database != null) 
         return _database; 
      _database = await initDB(); 
      return _database; 
   } 
   initDB() async {
      Directory documentsDirectory = await getApplicationDocumentsDirectory(); 
      String path = join(documentsDirectory.path, "ExpenseDB2.db"); 
      return await openDatabase(
         path, version: 1, onOpen:(db){}, onCreate: (Database db, int version) async {
            await db.execute(
               "CREATE TABLE Expense (
                  ""id INTEGER PRIMARY KEY," "amount REAL," "date TEXT," "category TEXT""
               )
            "); 
            await db.execute(
               "INSERT INTO Expense (''id'', ''amount'', ''date'', ''category'') 
               values (?, ?, ?, ?)",[1, 1000, ''2019-04-01 10:00:00'', "Food"]
            );
            /*await db.execute(
               "INSERT INTO Product (''id'', ''name'', ''description'', ''price'', ''image'') 
               values (?, ?, ?, ?, ?)", [
                  2, "Pixel", "Pixel is the most feature phone ever", 800, "pixel.png"
               ]
            ); 
            await db.execute(
               "INSERT INTO Product (''id'', ''name'', ''description'', ''price'', ''image'') 
               values (?, ?, ?, ?, ?)", [
                  3, "Laptop", "Laptop is most productive development tool", 2000, "laptop.png"
               ]
            );
            await db.execute(
               "INSERT INTO Product (''id'', ''name'', ''description'', ''price'', ''image'') 
               values (?, ?, ?, ?, ?)", [
                  4, "Tablet", "Laptop is most productive development tool", 1500, "tablet.png"
               ]
            );
            await db.execute(
               "INSERT INTO Product (''id'', ''name'', ''description'', ''price'', ''image'') 
               values (?, ?, ?, ?, ?)", [
                  5, "Pendrive", "iPhone is the stylist phone ever", 100, "pendrive.png"
               ]
            ); 
            await db.execute(
               "INSERT INTO Product (''id'', ''name'', ''description'', ''price'', ''image'') 
               values (?, ?, ?, ?, ?)", [
                  6, "Floppy Drive", "iPhone is the stylist phone ever", 20, "floppy.png"
               ]
            ); */ 
         }
      );
   }
   Future<List<Expense>> getAllExpenses() async {
      final db = await database; 
      List<Map> 
      results = await db.query(
         "Expense", columns: Expense.columns, orderBy: "date DESC"
      );
      List<Expense> expenses = new List(); 
      results.forEach((result) {
         Expense expense = Expense.fromMap(result);
         expenses.add(expense);
      }); 
      return expenses; 
   } 
   Future<Expense> getExpenseById(int id) async {
      final db = await database;
      var result = await db.query("Expense", where: "id = ", whereArgs: [id]); 
      return result.isNotEmpty ? Expense.fromMap(result.first) : Null; 
   }
   Future<double> getTotalExpense() async {
      final db = await database;
      List<Map> list = await db.rawQuery(
         "Select SUM(amount) as amount from expense"
      );
      return list.isNotEmpty ? list[0]["amount"] : Null; 
   }
   Future<Expense> insert(Expense expense) async {
      final db = await database; 
      var maxIdResult = await db.rawQuery(
         "SELECT MAX(id)+1 as last_inserted_id FROM Expense"
      );
      var id = maxIdResult.first["last_inserted_id"]; 
      var result = await db.rawInsert(
         "INSERT Into Expense (id, amount, date, category)" 
         " VALUES (?, ?, ?, ?)", [
            id, expense.amount, expense.date.toString(), expense.category
         ]
      );
      return Expense(id, expense.amount, expense.date, expense.category); 
   }
   update(Expense product) async {
      final db = await database; 
      var result = await db.update(
         "Expense", product.toMap(), where: "id = ?", whereArgs: [product.id]
      ); 
      return result; 
   }
   delete(int id) async {
      final db = await database;
      db.delete("Expense", where: "id = ?", whereArgs: [id]);
   }
}

double get totalExpense {
   double amount = 0.0; 
   for(var i = 0; i < _items.length; i++) { 
      amount = amount + _items[i].amount; 
   } 
   return amount; 
}

void load() {
   Future<List<Expense>> 
   list = SQLiteDbProvider.db.getAllExpenses(); 
   list.then( (dbItems) {
      for(var i = 0; i < dbItems.length; i++) { 
         _items.add(dbItems[i]); 
      } notifyListeners(); 
   });
}

Expense byId(int id) { 
   for(var i = 0; i < _items.length; i++) { 
      if(_items[i].id == id) { 
         return _items[i]; 
      } 
   }
   return null; 
}

void add(Expense item) {
   SQLiteDbProvider.db.insert(item).then((val) { 
      _items.add(val); notifyListeners(); 
   }); 
}

void update(Expense item) {
   bool found = false;
   for(var i = 0; i < _items.length; i++) {
      if(_items[i].id == item.id) {
         _items[i] = item; 
         found = true; 
         SQLiteDbProvider.db.update(item); break; 
      } 
   }
   if(found) notifyListeners(); 
}

void delete(Expense item) { 
   bool found = false; 
   for(var i = 0; i < _items.length; i++) {
      if(_items[i].id == item.id) {
         found = true; 
         SQLiteDbProvider.db.delete(item.id); 
         _items.removeAt(i); break; 
      }
   }
   if(found) notifyListeners(); 
}

import ''dart:collection''; 
import ''package:scoped_model/scoped_model.dart''; 
import ''Expense.dart''; 
import ''Database.dart''; 

class ExpenseListModel extends Model { 
   ExpenseListModel() { 
      this.load(); 
   } 
   final List<Expense> _items = []; 
   UnmodifiableListView<Expense> get items => 
   UnmodifiableListView(_items); 
   
   /*Future<double> get totalExpense { 
      return SQLiteDbProvider.db.getTotalExpense(); 
   }*/ 
   
   double get totalExpense {
      double amount = 0.0;
      for(var i = 0; i < _items.length; i++) { 
         amount = amount + _items[i].amount; 
      } 
      return amount; 
   }
   void load() {
      Future<List<Expense>> list = SQLiteDbProvider.db.getAllExpenses(); 
      list.then( (dbItems) {
         for(var i = 0; i < dbItems.length; i++) {
            _items.add(dbItems[i]); 
         } 
         notifyListeners(); 
      }); 
   }
   Expense byId(int id) {
      for(var i = 0; i < _items.length; i++) { 
         if(_items[i].id == id) { 
            return _items[i]; 
         } 
      }
      return null; 
   }
   void add(Expense item) {
      SQLiteDbProvider.db.insert(item).then((val) {
         _items.add(val);
         notifyListeners();
      }); 
   }
   void update(Expense item) {
      bool found = false; 
      for(var i = 0; i < _items.length; i++) {
         if(_items[i].id == item.id) {
            _items[i] = item; 
            found = true; 
            SQLiteDbProvider.db.update(item); 
            break; 
         }
      }
      if(found) notifyListeners(); 
   }
   void delete(Expense item) {
      bool found = false; 
      for(var i = 0; i < _items.length; i++) {
         if(_items[i].id == item.id) {
            found = true; 
            SQLiteDbProvider.db.delete(item.id); 
            _items.removeAt(i); break; 
         }
      }
      if(found) notifyListeners(); 
   }
}

import ''package:flutter/material.dart''; 
import ''package:scoped_model/scoped_model.dart''; 
import ''ExpenseListModel.dart''; 
import ''Expense.dart'';

void main() { 
   final expenses = ExpenseListModel(); 
   runApp(
      ScopedModel<ExpenseListModel>(model: expenses, child: MyApp(),)
   );
}

class MyApp extends StatelessWidget {
   // This widget is the root of your application. 
   @override 
   Widget build(BuildContext context) {
      return MaterialApp(
         title: ''Expense'',
         theme: ThemeData(
            primarySwatch: Colors.blue, 
         ), 
         home: MyHomePage(title: ''Expense calculator''), 
      );
   }
}

class MyHomePage extends StatelessWidget { 
   MyHomePage({Key key, this.title}) : super(key: key); 
   final String title; 
   @override 
   Widget build(BuildContext context) {
      return Scaffold(
         appBar: AppBar( 
            title: Text(this.title), 
         ), 
         body: ScopedModelDescendant<ExpenseListModel>(
            builder: (context, child, expenses) {
               return ListView.separated(
                  itemCount: expenses.items == null ? 1 
                  : expenses.items.length + 1, 
                  itemBuilder: (context, index) { 
                     if (index == 0) { 
                        return ListTile(
                           title: Text("Total expenses: " 
                           + expenses.totalExpense.toString(), 
                           style: TextStyle(fontSize: 24,
                           fontWeight: FontWeight.bold),) 
                        );
                     } else {
                        index = index - 1; 
                        return Dismissible( 
                           key: Key(expenses.items[index].id.toString()), 
                              onDismissed: (direction) { 
                              expenses.delete(expenses.items[index]); 
                              Scaffold.of(context).showSnackBar(
                                 SnackBar(
                                    content: Text(
                                       "Item with id, " 
                                       + expenses.items[index].id.toString() + 
                                       " is dismissed"
                                    )
                                 )
                              ); 
                           },
                           child: ListTile( onTap: () { 
                              Navigator.push(
                                 context, MaterialPageRoute(
                                    builder: (context) => FormPage(
                                       id: expenses.items[index].id,
                                       expenses: expenses, 
                                    )
                                 )
                              );
                           }, 
                           leading: Icon(Icons.monetization_on), 
                           trailing: Icon(Icons.keyboard_arrow_right), 
                           title: Text(expenses.items[index].category + ": " + 
                           expenses.items[index].amount.toString() + 
                           " nspent on " + expenses.items[index].formattedDate, 
                           style: TextStyle(fontSize: 18, fontStyle: FontStyle.italic),))
                        ); 
                     }
                  },
                  separatorBuilder: (context, index) { 
                     return Divider(); 
                  }, 
               );
            },
         ),
         floatingActionButton: ScopedModelDescendant<ExpenseListModel>(
            builder: (context, child, expenses) {
               return FloatingActionButton( onPressed: () {
                  Navigator.push( 
                     context, MaterialPageRoute(
                        builder: (context) => ScopedModelDescendant<ExpenseListModel>(
                           builder: (context, child, expenses) { 
                              return FormPage( id: 0, expenses: expenses, ); 
                           }
                        )
                     )
                  ); 
                  // expenses.add(new Expense( 
                     // 2, 1000, DateTime.parse(''2019-04-01 11:00:00''), ''Food'')
                  ); 
                  // print(expenses.items.length); 
               },
               tooltip: ''Increment'', child: Icon(Icons.add), ); 
            }
         )
      );
   }
}

class FormPage extends StatefulWidget { 
   FormPage({Key key, this.id, this.expenses}) : super(key: key); 
   final int id; 
   final ExpenseListModel expenses; 
   
   @override _FormPageState createState() => _FormPageState(id: id, expenses: expenses); 
}
class _FormPageState extends State<FormPage> {
   _FormPageState({Key key, this.id, this.expenses}); 
   
   final int id; 
   final ExpenseListModel expenses; 
   final scaffoldKey = GlobalKey<ScaffoldState>(); 
   final formKey = GlobalKey<FormState>(); 
   
   double _amount; 
   DateTime _date; 
   String _category; 
   
   void _submit() {
      final form = formKey.currentState; 
      if (form.validate()) {
         form.save(); 
         if (this.id == 0) expenses.add(Expense(0, _amount, _date, _category)); 
            else expenses.update(Expense(this.id, _amount, _date, _category)); 
         Navigator.pop(context); 
      }
   }
   @override 
   Widget build(BuildContext context) {
      return Scaffold(
         key: scaffoldKey, appBar: AppBar(
            title: Text(''Enter expense details''),
         ), 
         body: Padding(
            padding: const EdgeInsets.all(16.0), 
            child: Form(
               key: formKey, child: Column(
                  children: [
                     TextFormField( 
                        style: TextStyle(fontSize: 22), 
                        decoration: const InputDecoration( 
                           icon: const Icon(Icons.monetization_on), 
                           labelText: ''Amount'', 
                           labelStyle: TextStyle(fontSize: 18)
                        ), 
                        validator: (val) {
                           Pattern pattern = r''^[1-9]d*(.d+)?$''; 
                           RegExp regex = new RegExp(pattern); 
                           if (!regex.hasMatch(val)) 
                           return ''Enter a valid number''; else return null; 
                        }, 
                        initialValue: id == 0 
                        ? '''' : expenses.byId(id).amount.toString(), 
                        onSaved: (val) => _amount = double.parse(val), 
                     ), 
                     TextFormField( 
                        style: TextStyle(fontSize: 22), 
                        decoration: const InputDecoration( 
                           icon: const Icon(Icons.calendar_today),
                           hintText: ''Enter date'', 
                           labelText: ''Date'', 
                           labelStyle: TextStyle(fontSize: 18), 
                        ), 
                        validator: (val) {
                           Pattern pattern = r''^((?:19|20)dd)[- /.]
                              (0[1-9]|1[012])[- /.](0[1-9]|[12][0-9]|3[01])$''; 
                           RegExp regex = new RegExp(pattern); 
                           if (!regex.hasMatch(val)) 
                              return ''Enter a valid date''; 
                           else return null; 
                        },
                        onSaved: (val) => _date = DateTime.parse(val), 
                        initialValue: id == 0 
                        ? '''' : expenses.byId(id).formattedDate, 
                        keyboardType: TextInputType.datetime, 
                     ),
                     TextFormField(
                        style: TextStyle(fontSize: 22), 
                        decoration: const InputDecoration(
                           icon: const Icon(Icons.category),
                           labelText: ''Category'', 
                           labelStyle: TextStyle(fontSize: 18)
                        ),
                        onSaved: (val) => _category = val, 
                        initialValue: id == 0 ? '''' 
                        : expenses.byId(id).category.toString(),
                     ), 
                     RaisedButton( 
                        onPressed: _submit, 
                        child: new Text(''Submit''), 
                     ), 
                  ],
               ),
            ),
         ),
      );
   }
}

import ''package:flutter/material.dart''; 
import ''package:scoped_model/scoped_model.dart''; 
import ''ExpenseListModel.dart''; 
import ''Expense.dart''; 

void main() { 
   final expenses = ExpenseListModel(); 
   runApp(
      ScopedModel<ExpenseListModel>(
         model: expenses, child: MyApp(), 
      )
   ); 
}
class MyApp extends StatelessWidget {
   // This widget is the root of your application. 
   @override
   Widget build(BuildContext context) {
      return MaterialApp(
         title: ''Expense'',
         theme: ThemeData(
            primarySwatch: Colors.blue, 
         ), 
         home: MyHomePage(title: ''Expense calculator''), 
      );
   }
}
class MyHomePage extends StatelessWidget {
   MyHomePage({Key key, this.title}) : super(key: key);
   final String title;

   @override 
   Widget build(BuildContext context) {
      return Scaffold(
         appBar: AppBar(
            title: Text(this.title),
         ),
         body: ScopedModelDescendant<ExpenseListModel>(
            builder: (context, child, expenses) { 
               return ListView.separated(
                  itemCount: expenses.items == null ? 1 
                  : expenses.items.length + 1, itemBuilder: (context, index) { 
                     if (index == 0) { 
                        return ListTile( title: Text("Total expenses: " 
                        + expenses.totalExpense.toString(), 
                        style: TextStyle(fontSize: 24,fontWeight: 
                        FontWeight.bold),) ); 
                     } else {
                        index = index - 1; return Dismissible(
                           key: Key(expenses.items[index].id.toString()), 
                           onDismissed: (direction) {
                              expenses.delete(expenses.items[index]); 
                              Scaffold.of(context).showSnackBar(
                                 SnackBar(
                                    content: Text(
                                       "Item with id, " + 
                                       expenses.items[index].id.toString() 
                                       + " is dismissed"
                                    )
                                 )
                              );
                           }, 
                           child: ListTile( onTap: () {
                              Navigator.push( context, MaterialPageRoute(
                                 builder: (context) => FormPage(
                                    id: expenses.items[index].id, expenses: expenses, 
                                 )
                              ));
                           }, 
                           leading: Icon(Icons.monetization_on), 
                           trailing: Icon(Icons.keyboard_arrow_right), 
                           title: Text(expenses.items[index].category + ": " + 
                           expenses.items[index].amount.toString() + " nspent on " + 
                           expenses.items[index].formattedDate, 
                           style: TextStyle(fontSize: 18, fontStyle: FontStyle.italic),))
                        );
                     }
                  }, 
                  separatorBuilder: (context, index) {
                     return Divider(); 
                  },
               ); 
            },
         ),
         floatingActionButton: ScopedModelDescendant<ExpenseListModel>(
            builder: (context, child, expenses) {
               return FloatingActionButton(
                  onPressed: () {
                     Navigator.push(
                        context, MaterialPageRoute(
                           builder: (context)
                           => ScopedModelDescendant<ExpenseListModel>(
                              builder: (context, child, expenses) { 
                                 return FormPage( id: 0, expenses: expenses, ); 
                              }
                           )
                        )
                     );
                     // expenses.add(
                        new Expense(
                           // 2, 1000, DateTime.parse(''2019-04-01 11:00:00''), ''Food''
                        )
                     );
                     // print(expenses.items.length); 
                  },
                  tooltip: ''Increment'', child: Icon(Icons.add), 
               );
            }
         )
      );
   } 
}
class FormPage extends StatefulWidget {
   FormPage({Key key, this.id, this.expenses}) : super(key: key); 
   final int id; 
   final ExpenseListModel expenses; 
   
   @override 
   _FormPageState createState() => _FormPageState(id: id, expenses: expenses); 
}
class _FormPageState extends State<FormPage> {
   _FormPageState({Key key, this.id, this.expenses}); 
   final int id; 
   final ExpenseListModel expenses; 
   final scaffoldKey = GlobalKey<ScaffoldState>(); 
   final formKey = GlobalKey<FormState>(); 
   double _amount; DateTime _date; 
   String _category;
   void _submit() {
      final form = formKey.currentState; 
      if (form.validate()) {
         form.save(); 
         if (this.id == 0) expenses.add(Expense(0, _amount, _date, _category)); 
         else expenses.update(Expense(this.id, _amount, _date, _category)); 
         Navigator.pop(context); 
      } 
   } 
   @override 
   Widget build(BuildContext context) {
      return Scaffold(
         key: scaffoldKey, appBar: AppBar( 
            title: Text(''Enter expense details''), 
         ), 
         body: Padding(
            padding: const EdgeInsets.all(16.0),
            child: Form(
               key: formKey, child: Column(
                  children: [
                     TextFormField(
                        style: TextStyle(fontSize: 22), 
                        decoration: const InputDecoration( 
                           icon: const Icon(Icons.monetization_on), 
                           labelText: ''Amount'', 
                           labelStyle: TextStyle(fontSize: 18)
                        ), 
                        validator: (val) {
                           Pattern pattern = r''^[1-9]d*(.d+)?$''; 
                           RegExp regex = new RegExp(pattern); 
                           if (!regex.hasMatch(val)) return ''Enter a valid number''; 
                           else return null; 
                        },
                        initialValue: id == 0 ? '''' 
                        : expenses.byId(id).amount.toString(), 
                        onSaved: (val) => _amount = double.parse(val), 
                     ),
                     TextFormField(
                        style: TextStyle(fontSize: 22), 
                        decoration: const InputDecoration(
                           icon: const Icon(Icons.calendar_today), 
                           hintText: ''Enter date'', 
                           labelText: ''Date'', 
                           labelStyle: TextStyle(fontSize: 18), 
                        ),
                        validator: (val) {
                           Pattern pattern = r''^((?:19|20)dd)[- /.]
                           (0[1-9]|1[012])[- /.](0[1-9]|[12][0-9]|3[01])$''; 
                           RegExp regex = new RegExp(pattern); 
                           if (!regex.hasMatch(val)) return ''Enter a valid date''; 
                           else return null; 
                        },
                        onSaved: (val) => _date = DateTime.parse(val), 
                        initialValue: id == 0 ? '''' : expenses.byId(id).formattedDate, 
                        keyboardType: TextInputType.datetime, 
                     ),
                     TextFormField(
                        style: TextStyle(fontSize: 22), 
                        decoration: const InputDecoration(
                           icon: const Icon(Icons.category), 
                           labelText: ''Category'', 
                           labelStyle: TextStyle(fontSize: 18)
                        ), 
                        onSaved: (val) => _category = val, 
                        initialValue: id == 0 ? '''' : expenses.byId(id).category.toString(), 
                     ),
                     RaisedButton(
                        onPressed: _submit, 
                        child: new Text(''Submit''), 
                     ),
                  ],
               ),
            ),
         ),
      );
   }
}