AS3 Memory management when instantiating extended classes

I'm developing an AS3 application which has some memory leaks I can't find, so I'd like to ask some n开发者_开发问答ewbie questions about memory management.

Imagine I have a class named BaseClass, and some classes that extend this one, such as ClassA, ClassB, etc.

I declare a variable:

myBaseClass:BaseClass = new ClassA();

After a while, I use it to instantiate a new object:

myBaseClass = new ClassB(); 

some time after

myBaseClass = new ClassC(); 

and the same thing keeps happening every x millis, triggered by a timer.

Is there any memory problem here? Are the unused instances correctly deleted by the garbage collector?

Thanks!

Assuming you have no other references to the instance (or, possibly, its contents), the garbage collector will clean them up. However, the time before cleanup is, as far as I know, indeterminate (there might be some hard timeline in use, but I've never seen it documented). If you're creating a huge number of instances, you might use up a lot of memory before the first ever gets cleaned up.

There is an AS call (the name of which escapes me at the moment) to force a GC run, but it shouldn't normally be necessary. If you find it necessary, you almost certainly need to rethink how your application works.

If myBaseClass is the only stored reference to

new ClassA()

and you assign something else to myBaseClass

new ClassB()

then there will be no reference pointing to ClassA and the garbage collector should find it when it runs.

However, if you give myBaseClass to a library or class that stores its own reference to that object, when you reassign myBaseClass there will still be a reference pointing to the ClassA and the garbage collector will not clean it up, creating a memory leak. Normally a well written library will provide you with a way to remove the reference. e.g.

var child:Sprite = new Sprite()
// one reference to the new Sprite

stage.addChild(child); // assume stage stores reference
// two references to the new Sprite

/**
 * assume the following:
 *
 * child = null; 
 *
 * one reference would still remain in stage
 * garbage collector will not clean up the sprite
 */

stage.removeChild(child); // assume stage clears reference
// one reference left
child = null;
// no reference to the sprite, garbage collector will clean it up

Hope this clears things up a bit. Essentially you want to keep track of how many references there are to an object, if you suspect it to be leaking memory.

I know that FlashDevelop has a profiler that is quite useful for finding these bugs. Also, you need to know that when the garbage collector runs... it will free up memory and give a saw tooth pattern to your memory profiler.

• Good example code for demonstrating profiling in Actionscript 3
• AS3 Memory Leak Example