Ted On Flash: Simple Method Closure in AS3

Simple Method Closure in AS3

DIGG IT! 4 Comments Published Friday, January 15, 2010 at 3:23 PM .

Method closures allow you to bind variables into the scope of an anonymous function. Watch the value of local variable 'i' in the example below in the returned anonymous function. It is a bit twisted but results show the scope of local variable 'i' is bound in the returned function from newCounter.

CODE AT PASTIE


package
{
 import flash.display.Sprite;
 
 public class ClosureAS3 extends Sprite
 {
  public function ClosureAS3()
  {
   init() //giv-em-the-jit :)
  }
  
  public function init():void
  {
   //create a counter
   var counter1:Function = newCounter();
   trace( counter1() ); //1
   trace( counter1() ); //2
   trace( counter1() ); //3
   var counter2:Function = newCounter();
   trace( counter2() ); //1
   trace( counter2() ); //2
   trace( counter1() ); //4 --> scope of i is still with counter1...cool! :)
  }
  
  public function newCounter():Function
  {
   var i:int = 0; //variable i gets bound into returned anonymous function via method Closure
   return function():int
   {
    //i is available to the scope of the anonymous function
    i=i+1;
    return i;
   }
  }  
 }
}

Scary, functional AS3. :)

Cheers,

Ted :)

4 Responses to “Simple Method Closure in AS3”

# johnny January 15, 2010 4:59 PM

Scary good, or scary bad?
# Dirk January 18, 2010 4:13 AM

you're invoking the function at assignment time already which is not necessary (i.e. you do not need to add parenthesis when you assign the function reference)

// just take the reference without invoking it
var counter1:Function = newCounter;
# Marcus January 18, 2010 8:31 AM

How do you remove the closure (and the variable it refers to) from memory?
# Ted Patrick January 18, 2010 11:42 AM

Dirk,

Everytime newCounter is called a new anonymous function is created and returned. As the function is returned the local variable i becomes bound creating the closure. Although i was a local variable it is now scoped within the new function and isn't local or global. If I returned a reference to newCounter directly, the closure will not get created.

I checked with an engineer on player team who works on the VM in regards to closures and its effects on memory and reference. Provided that the object bound into the closure has no references to other objects, the closure and all variables will garbage collect when all references to the function are destroyed.

Ted Patrick - Demos & MAX @ Adobe Systems

Note: This is the personal blog of Ted Patrick. The opinions and statements voiced here are my own.

Simple Method Closure in AS3

4 Responses to “Simple Method Closure in AS3”

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