Kivy is a Python library that helps you to build cross-platform GUI applications. Any GUI application is event driven, wherein the flow of the program is not sequential (top to bottom), but is decided by the user interactions with the widgets on the interface. User actions such as clicking a button, “selecting an item from a list” or “choosing an option from available radio buttons”, etc. are called events.

A GUI-based program anticipates all possible events that can occur in and around its environment and dispatch them to appropriate handler functions as and when (and if) a certain event occurs.

When the run() method of Kivy”s App object is called, the application starts an “event listening” loop and triggers appropriate callback functions, one each for a type of event.

Global Event Dispatcher

The Kivy framework includes the EventDispatcher class. It dispatches the event object to the widget tree and the event is propagated through the widget hierarchy. When a widget that is in a position to process the event, its associated callback handler is triggered. The Widget, Animation and Clock classes are examples of event dispatchers.

Clock is a global event dispatcher that allows you to schedule and trigger events at specific intervals. It defines methods such as ”schedule_once()” and ”schedule_interval()” to register functions or methods to be called after a certain delay or at regular intervals. This mechanism is useful for handling timed events, animation updates, and other recurring tasks in your app.

Example

In the following example, we have put a label and two buttons in the widget tree. The button captioned “Start” schedules an event to occur periodically after every one second. The schedule_interval() function is written with the following syntax −

Clock.schedule_interval(callback, timeout)

On the other hand, the second button captioned as “Stop” calls the unscheduled() method which removes the scheduled event.

from kivy.app import App
from kivy.uix.label import Label
from kivy.uix.button import Button
from kivy.uix.boxlayout import BoxLayout
from kivy.clock import Clock
from kivy.config import Config

# Configuration
Config.set(''graphics'', ''width'', ''720'')
Config.set(''graphics'', ''height'', ''400'')
Config.set(''graphics'', ''resizable'', ''1'')

class DemoApp(App):
   def build(self):
      lo = BoxLayout(orientation=''vertical'')
      self.l1 = Label(text="0", font_size=100)
      self.b1 = Button(text="start", font_size=100, color=(0, 0, 1, 1))
      
      self.b1.bind(on_press=self.onstart)
      self.b2 = Button(text="Stop", font_size=100, color=(1, 0, 0, 1))
      self.b2.bind(on_press=self.onstop)
      lo.add_widget(self.l1)
      lo.add_widget(self.b1)
      lo.add_widget(self.b2)
      return lo

   def onstart(self, event):
      print("started")
      Clock.schedule_interval(self.update_label, 1)

   def onstop(self, event):
      Clock.unschedule(self.update_label)

   def update_label(self, event):
      self.l1.text = str(int(self.l1.text) + 1)

if __name__ == ''__main__'':
   DemoApp().run()

Output

Run the above program. The label initially shows “0”. Click the Start button. The label starts incrementing the number on it with each second. Press the “Stop” button to unschedule the event.

Widget Events

Most of the widgets Kivy have built-in event handling capabilities. In fact, each widget is designed to handle specific type of events. For example, the Button widget handles the event caused by clicking it. You can register event handlers for specific events that occur on a widget, such as button clicks, touch events, or keyboard events.

Usually, the event handlers are defined as methods in your widget”s class or App class. They generally are prefixed with ”on_” followed by the name of the event. For example, ”on_press” for a button press event.

When an event occurs, Kivy automatically calls the corresponding event handler method, passing relevant information about the event as arguments. Inside the event handler, you can define the desired behavior or actions to perform.

Events Associated with Widgets

Given below is a list of the events associated with some of the most commonly used widgets −

Button

• on_press − Triggered when the button is pressed.

• on_release − Triggered when the button is released.

• on_touch_down − Triggered when a touch event starts on the button.

• on_touch_up − Triggered when a touch event ends on the button.

TextInput

• on_text_validate − Triggered when the user finishes editing the text input (by pressing Enter or Return).

• on_focus − Triggered when the text input receives or loses focus.

• on_text − Triggered whenever the text in the input field changes.

CheckBox

• on_active − Triggered when the checkbox is checked or unchecked.

Slider

• on_value − Triggered when the slider”s value changes.

• on_touch_down − Triggered when a touch event starts on the slider.

• on_touch_up − Triggered when a touch event ends on the slider.

ToggleButton

• on_state − Triggered when the toggle button”s state changes (toggled on or off).

• on_press − Triggered when the toggle button is pressed.

ListView

• on_select − Triggered when an item in the list view is selected.

FileChooser

• on_selection − Triggered when a file or directory is selected in the file chooser.

Switch

• on_active − Triggered when the switch is toggled on or off.

Video

• on_load − Triggered when the video finishes loading.

• on_play − Triggered when the video starts playing.

• on_pause − Triggered when the video is paused.

• on_stop − Triggered when the video stops playing.

Spinner

• on_text − Triggered when an item is selected from the spinner.

ActionButton

• on_press − Triggered when the action button is pressed.

Examples − Event Handling in Kivy

These events will be discussed when we explain each of the widgets in Kivy framework. However, for this chapter, two examples of event handling are given here.

Example 1

The first example shows the “on_press” event of a Button widget. In the code below, we have a Label and a Button widget, arranged in a BoxLayout. To process the on_press event, we bind the button with onstart() method defined in the DemoApp class

from kivy.app import App
from kivy.uix.label import Label
from kivy.uix.button import Button
from kivy.uix.boxlayout import BoxLayout
from kivy.config import Config

# Configuration
Config.set(''graphics'', ''width'', ''720'')
Config.set(''graphics'', ''height'', ''400'')
Config.set(''graphics'', ''resizable'', ''1'')

class DemoApp(App):
   def build(self):
      lo = BoxLayout(orientation=''vertical'')
      self.l1 = Label(text="0", font_size=100)
      self.b1 = Button(text = "start", font_size = 100, color=(1,0,0,1))
      self.b1.bind(on_press=self.onstart)
      lo.add_widget(self.l1)
      lo.add_widget(self.b1)
      return lo

   def onstart(self, event):
      print ("started")
      self.l1.text = str(int(self.l1.text)+1)

if __name__ == ''__main__'':
   DemoApp().run()

Output

Save the above code as “demo.py” and run it from command line or a Python IDE. The program starts with the label showing 0 as its caption. Every time the button is clicked, the caption increments by 1.

Example 2

In this example, we have a TextInput widget and a Label. The TextInput is bound to a onkey() method. Each keystroke input by the user is reflected on the Label.

from kivy.app import App
from kivy.uix.label import Label
from kivy.uix.textinput import TextInput
from kivy.uix.boxlayout import BoxLayout

from kivy.clock import Clock
from kivy.core.window import Window

Window.size = (720,400)

class DemoApp(App):
   def build(self):
      lo = BoxLayout(orientation=''vertical'')
      self.t1 = TextInput(multiline=False)
      self.l1 = Label(font_size=50)
      self.t1.bind(text=self.onkey)

      lo.add_widget(self.t1)
      lo.add_widget(self.l1)
      return lo
      
   def onkey(self, event, value):
      print ("press")
      self.l1.text = value

if __name__ == ''__main__'':
   DemoApp().run()

Output

Run the program and enter some text in the text box. As you type, the Label caption is updated.

It is also possible to define custom events and event handlers to handle application-specific events. To do so, you need to subclass the ”EventDispatcher” class and define the desired properties and methods.

Custom events can be dispatched and handled using the ”trigger()” and ”on_event_name” pattern similar to widget events.