Freeing memory

Delete all references to objects to make sure that garbage collection is triggered.

You cannot launch the garbage collector directly in the release version of Flash Player. To make sure that an object is garbage collected, delete all references to the object. Keep in mind that the old delete operator used in ActionScript 1.0 and 2.0 behaves differently in ActionScript 3.0. It can only be used to delete dynamic properties on a dynamic object.

For example, the following code sets a Sprite reference to null :

var mySprite:Sprite = new Sprite(); 
 
// Set the reference to null, so that the garbage collector removes  
// it from memory 
mySprite = null;

Remember that when an object is set to null , it is not necessarily removed from memory. Sometimes the garbage collector does not run, if available memory is not considered low enough. Garbage collection is not predictable. Memory allocation, rather than object deletion, triggers garbage collection. When the garbage collector runs, it finds graphs of objects that haven't been collected yet. It detects inactive objects in the graphs by finding objects that reference each other, but that the application no longer uses. Inactive objects detected this way are deleted.

In large applications, this process can be CPU-intensive and can affect performance and generate a noticeable slowdown in the application. Try to limit garbage collection passes by reusing objects as much as possible. Also, set references to null, when possible, so that the garbage collector spends less processing time finding the objects. Think of garbage collection as insurance, and always manage object lifetimes explicitly, when possible.

The garbage collector can be launched using the System.gc() method, available in Adobe AIR and in the debug version of Flash Player. The profiler bundled with Adobe® Flash® Builder™ allows you to start the garbage collector manually. Running the garbage collector allows you to see how your application responds and whether objects are correctly deleted from memory.

Fortunately, the amount of memory used by bitmaps can be instantly reduced. For example, the BitmapData class includes a dispose() method. The next example creates a BitmapData instance of 1.8 MB. The current memory in use grows to 1.8 MB, and the System.totalMemory property returns a smaller value:

trace(System.totalMemory / 1024); 
// output: 43100 
  
// Create a BitmapData instance 
var image:BitmapData = new BitmapData(800, 600); 
 
trace(System.totalMemory / 1024); 
// output: 44964 

Next, the BitmapData is manually removed (disposed) from memory and the memory use is once again checked:

trace(System.totalMemory / 1024); 
// output: 43100 
  
// Create a BitmapData instance 
var image:BitmapData = new BitmapData(800, 600); 
 
trace(System.totalMemory / 1024); 
// output: 44964 
 
image.dispose(); 
image = null; 
 
trace(System.totalMemory / 1024); 
// output: 43084 

Although the dispose() method removes the pixels from memory, the reference must still be set to null to release it completely. Always call the dispose() method and set the reference to null when you no longer need a BitmapData object, so that memory is freed immediately.

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