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Symbols in Dart are opaque, dynamic string name used in reflecting out metadata from a library. Simply put, symbols are a way to store the relationship between a human readable string and a string that is optimized to be used by computers.
Reflection is a mechanism to get metadata of a type at runtime like the number of methods in a class, the number of constructors it has or the number of parameters in a function. You can even invoke a method of the type which is loaded at runtime.
In Dart reflection specific classes are available in the dart:mirrors package. This library works in both web applications and command line applications.
Syntax
Symbol obj = new Symbol(''name''); // expects a name of class or function or library to reflect
The name must be a valid public Dart member name, public constructor name, or library name.
Example
Consider the following example. The code declares a class Foo in a library foo_lib. The class defines the methods m1, m2, and m3.
Foo.dart
library foo_lib; // libarary name can be a symbol class Foo { // class name can be a symbol m1() { // method name can be a symbol print("Inside m1"); } m2() { print("Inside m2"); } m3() { print("Inside m3"); } }
The following code loads Foo.dart library and searches for Foo class, with help of Symbol type. Since we are reflecting the metadata from the above library the code imports dart:mirrors library.
FooSymbol.dart
import ''dart:core''; import ''dart:mirrors''; import ''Foo.dart''; main() { Symbol lib = new Symbol("foo_lib"); //library name stored as Symbol Symbol clsToSearch = new Symbol("Foo"); // class name stored as Symbol if(checkIf_classAvailableInlibrary(lib, clsToSearch)) // searches Foo class in foo_lib library print("class found.."); } bool checkIf_classAvailableInlibrary(Symbol libraryName, Symbol className) { MirrorSystem mirrorSystem = currentMirrorSystem(); LibraryMirror libMirror = mirrorSystem.findLibrary(libraryName); if (libMirror != null) { print("Found Library"); print("checkng...class details.."); print("No of classes found is : ${libMirror.declarations.length}"); libMirror.declarations.forEach((s, d) => print(s)); if (libMirror.declarations.containsKey(className)) return true; return false; } }
Note that the line libMirror.declarations.forEach((s, d) => print(s)); will iterate across every declaration in the library at runtime and prints the declarations as type of Symbol.
This code should produce the following output −
Found Library checkng...class details.. No of classes found is : 1 Symbol("Foo") // class name displayed as symbol class found.
Example: Display the number of instance methods of a class
Let us now consider displaying the number of instance methods in a class. The predefined class ClassMirror helps us to achieve the same.
import ''dart:core''; import ''dart:mirrors''; import ''Foo.dart''; main() { Symbol lib = new Symbol("foo_lib"); Symbol clsToSearch = new Symbol("Foo"); reflect_InstanceMethods(lib, clsToSearch); } void reflect_InstanceMethods(Symbol libraryName, Symbol className) { MirrorSystem mirrorSystem = currentMirrorSystem(); LibraryMirror libMirror = mirrorSystem.findLibrary(libraryName); if (libMirror != null) { print("Found Library"); print("checkng...class details.."); print("No of classes found is : ${libMirror.declarations.length}"); libMirror.declarations.forEach((s, d) => print(s)); if (libMirror.declarations.containsKey(className)) print("found class"); ClassMirror classMirror = libMirror.declarations[className]; print("No of instance methods found is ${classMirror.instanceMembers.length}"); classMirror.instanceMembers.forEach((s, v) => print(s)); } }
This code should produce the following output −
Found Library checkng...class details.. No of classes found is : 1 Symbol("Foo") found class No of instance methods found is 8 Symbol("==") Symbol("hashCode") Symbol("toString") Symbol("noSuchMethod") Symbol("runtimeType") Symbol("m1") Symbol("m2") Symbol("m3")
Convert Symbol to String
You can convert the name of a type like class or library stored in a symbol back to string using MirrorSystem class. The following code shows how you can convert a symbol to a string.
import ''dart:mirrors''; void main(){ Symbol lib = new Symbol("foo_lib"); String name_of_lib = MirrorSystem.getName(lib); print(lib); print(name_of_lib); }
It should produce the following output −
Symbol("foo_lib") foo_lib
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