Dart Programming - Symbol

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