Flash CS3 - how to clean memory

Question

I have a question how can I clear/free memory in flash? For example I am finishing game and I want to start from beginning and if I will just jump to the first frame all the objects there are still in this memory, is any possibility to force cleaning memory?

Can I free memory for an object? for example I removeChild(something) - and I want to free memory for an object as I will reuse it?

Can anybody explain me how the engine works?

Thanks

Answer 1

I would encourage you to read Chapter 14, Garbage Collection in the "Moock Book" (Essential ActionScript 3.0 by O'Reilly Publishing).

The short answer to your question is that you're not in control of de-allocation, the garbage collector is. In a garbage-collected language like AS3 or Java, you don't have manual control over allocation and de-allocation of memory like you do in lower level languages; there are no AS3 equivalents to things like delete in C++ or free in C. Your goal should not be controlling when you destroy things, but rather not forgetting to remove references to things you no longer need around and making sure you disable things that you intend for garbage collection.

Memory leaks in AS3 typically come from a mixture of newbie misunderstanding (like thinking removeChild or setting a reference to null destroys objects), and from not keeping good track of references to objects--especially where strong listeners are involved.

A previous respondent posted this:

myObject = null;

What this does is remove a reference to the object that the variable myObject was holding. Nothing more. You need to know a lot more about the situation in order to be able to say whether this assignment even makes the object in question eligible for garbage collection, especially how many other variable are holding references to the object. And the object might already be eligible for garbage collection even if you didn't set the reference to null (i.e. if myObject has no connection to a GC root).

Suffice to say, the entire GC mechanism is more complex than can be satisfactorily explained in a StackOverflow post. That's why it has a whole chapter in the Moock Book, and even that book does not go into implemenation detail or great detail about when exactly the Flash Player does its ref counting deletions or mark and sweep passes.

The most important things to remember are, IMHO:

1. When you intend to "kill" an object, give it a cleanUp() or destroy() function where you do things like stop all its timers, sounds, remove listeners, etc. An object will continue to exist and execute code until it gets GC'd. And the Flash Player defers GC as long as it can--it's usually triggered when the Player needs to allocate more RAM from your system, because allocating memory is about the only thing that's more time consuming than doing the mark and sweep GC pass.

2. Read about weak vs strong listeners. Basically, when you have a weak listener, the listener reference is one that is ignored by mark-and-sweep GC, so it alone will not prevent an object from getting collected. But don't listen to anyone who tells you "always use weak" or "always use strong listeners and manually remove them" because there are times where each is appropriate, and that's why the choice is yours.

Answer 2

Clearing memory is a tricky thing with Flash, the proper way ow implementing it setting up objects properly in the first play for easy clearing, rather than forcing deletion. When you want to remove an object from memory, you do it by removing any reference to it, and then flash marks it for garbage collection. Then Flash at a later point removes the object from memory.

In order for the object to be ready for data collection it cannot have any connection to another object.

so if you have an object that has a single connection to a MovieClip and the movie clip has no other relation, then if you set it to null, you will remove it.

if you, however, have two objects that point to it, if you remove one link by setting it to null, the MovieClip will not be removed.

furhtermore, if you have a 2 or more movie clips that have a network of connections, removing those objects requires these connections be broken as-well. For example if you have a level with many characters and listeners set up, removing the lavel movieClip will not clear it.

one way of breaking these connections is adding onRemovedFromStage Events, that remove further children, listeners and objects. I've started using the casaLib extension of movieclip - CasaMovieClip, that has a function called removeChildrenAndDestroy. this makes it a bit easier, but would take a while to implement on an older project.

Anyhow, you'll find there are many sites discussing this, a good place to start is grant skinner's blog

Answer 3

removeChild() will remove object from stage, but will still keep it in memory. You will have to null the object like this myObject = null if you wish to get rid of it entirely. You might not need to do that thought. Just removing it from stage and removing all associated events will be sufficient in most cases.