Dart Symbol

In this tutorial you will learn about the Dart Symbol and its application with practical example.

Dart Symbol object used to refer an operator or identifier declared in a Dart program. Dart symbol are commonly used in APIs that refer to identifiers by name, because an identifier name can changes but not identifier symbols.

Table Of Contents−

Dart Symbol
Display Number of Instance Methods In a Class

Dart Convert Symbol to String

Dart symbols are opaque, dynamic string name used in reflecting out metadata from a library. Symbols is basically means to save the relationship between a human readable string and a string that is optimized to be used by computers.

Reflection is a mechanism that used to get metadata of a type at run-time for example – the number of methods in a class, the number of constructors it has or the number of parameters in a function.

In Dart, all of the reflection related classes are available in the dart:mirrors package. This library can be used with web applications as well as command line applications.

Syntax:-

Symbol for an identifier can be created using a hash (#) followed by the identifier name.

Symbol obj = new Symbol("name");

1	Symbol obj = new Symbol("name");

Here, the name must be a valid class, function, public member name, public constructor name, or library name.

Example:-

Lets create a file Foo.dart and put the following code in it –

Foo.dart

library foo_lib;   
// libarary name can be a symbol   

// class name can be a symbol
class Foo {         

// method name can be a symbol   
   m1() {        
      print("Inside m1"); 
   } 
   m2() { 
      print("Inside m2"); 
   } 
   m3() { 
      print("Inside m3"); 
   } 
}

library foo_lib;

// libarary name can be a symbol

// class name can be a symbol

class Foo {

// method name can be a symbol

m1() {

print("Inside m1");

}

m2() {

print("Inside m2");

}

m3() {

print("Inside m3");

}

Here, we have declared a class Foo in a library foo_lib, and defined three methods m1, m2, and m3 in class Foo.

Now, create another dart file FooSymbol.dart and put the following code in it –

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; 
   } 
}

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;

}

In the above code we are loading Foo.dart library along with the dart:mirrors library because we will be reflecting the metadata from the above library. In the above code we are basically searching for the Foo class in foo_lib using Symbol.

The following line of code iterate through every declaration in the library at runtime and prints the declarations as type of Symbol.

libMirror.declarations.forEach((s, d) => print(s));

1	libMirror.declarations.forEach((s, d) => print(s));

When we run the above Dart Program, we will see the following output –

Output:-

Found Library 
checkng...class details.. 
No of classes found is : 1 
Symbol("Foo") // class name displayed as symbol  
class found.

Found Library

checkng...class details..

No of classes found is : 1

Symbol("Foo") // class name displayed as symbol

class found.

Display Number of Instance Methods In a Class

Example:-

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)); 
   } 
}

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));

}

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")

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")

Dart Convert Symbol to String

The MirrorSystem class available in dart:mirrors library allow us to convert a Symbol back to string.

Example:-

import 'dart:mirrors'; 
void main(){ 
   Symbol lib = new Symbol("foo_lib"); 
   String name_of_lib = MirrorSystem.getName(lib); 
   print(lib); 
   print(name_of_lib); 
}

import 'dart:mirrors';

void main(){

Symbol lib = new Symbol("foo_lib");

String name_of_lib = MirrorSystem.getName(lib);

print(lib);

print(name_of_lib);

}

Output:-

Symbol("foo_lib")   
foo_lib

1 2	Symbol("foo_lib") foo_lib

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