What are good memory management techniques in Flash/as3

Question

Hello!

So I am pretty familiar with memory management in Java, C and C++; however, in flash what constructs are there for memory management? I assume flash has a sort of virtual machine like java, and I have been assuming that things get garbage collected when they are set to null. I am not sure if this is actually the case though. Also is there a way to force garbage collection in Flash? Any other tips?

Thanks

Answer 1

Flash bytecode is run by the AVM (Actionscript Virtual Machine). In general terms (and without being an expert in Java or the internals of the Flash Player), I think it's safe to say that the AVM model is somewhat analogue to the JVM model (source code is compiled to bytecode, which is run by the VM; in AVM, at least, some of it is interpreted and some is JIT compiled to native code before execution, etc).

AVM is, as you said, garbage collected, so basically memory allocation and deallocation is managed for you by the GC. When an object becomes unreachable, it's eligible for GC (which doesn't mean it is collected right away).

There's a way to force a GC cycle, only available in the debug version of the player, and also a hack, unofficial and undocumented, but you can find some links about it in google (try GC hack flash LocalConnection or something along those lines). Forcing a GC is almost always a bad idea though.

I've recently come across this blog post that explains how the GC works in some deatil, with references to the AVM C++ source code (that part of the player is open source, so you can check it out for more in depth info if you're so inclined). http://jpauclair.net/2009/12/23/tamarin-part-iii-current-garbage-collector-in-flash-10-0/

Answer 2

On a very specific note: memory leaks can become rampant in your code when using EventListeners. The most common example I've seen in AS/Flex tutorials for adding listeners looks like this:

button.addEventListener(MouseEvent.CLICK, doSomething);

This works just fine, but ignores one salient point: the listener is strongly referenced. This means when the component containing this button is GC'd, the listener persists and maintains a reference to the button, meaning it also won't be harvested.

To mitigate this, you can do one of two things:

button.addEventListener(MouseEvent.CLICK, doSomething, false, 0, true);

Here is Adobe's description of the 3 extra fields. Note what they say about strong references:

A strong reference (the default) prevents your listener from being garbage-collected. A weak reference does not.

The other option is to create a destructor in your code so when a component which uses EventListeners removes them before being torn down:

button.removeEventListener(MouseEvent.CLICK, doSomething);

Answer 3

In addition to what has already been answered it is good idea to use a library like Mr Doob's Actionscript Performance Monitor which will activley display the current memory usuage. Useful for detecting and fixing memory leaks.