Memory Leak in Flex 3.2 Lists & Grids

Ok…so to say that there’s a memory leak in Lists and Grids is a bit redundant as they all inherit from one class, ListBase, which is where the problem is, but a bit of redundancy can be eye-catching.

Anyway…recently I was tracking down an issue filed by one of our testers, and stumbled across a nasty little memory leak in the 3.2 SDK.  Adobe’s already corrected the problem in the 3.3 nightlies, but I thought I’d post my findings here on the chance that anyone else needs the fix in their 3.2 SDK.

The problem is a small class called StageEventProxy.  SystemManager creates an instance of StageEventProxy passing to it your listener whenever one of the following types of events is listened for from SystemManager:

• MouseEvent.MOUSE_MOVE
• MouseEvent.MOUSE_UP
• MouseEvent.MOUSE_DOWN
• Event.ACTIVATE
• Event.DEACTIVATE

The problem is that StageEventProxy keeps a hard reference to your listener function.  This means that when garbage collection runs, that listener and the object its attached to will never get marked for garbage collection.

Now, you’re probably thinking, “I’ve never added any of those listeners to SystemManager so I’m free and clear.  Woot!”  Unfortunately you’re not off the hook just yet.  Adobe adds a MOUSE_UP listener to SystemManager in ListBase’s mouseDownHandler():

systemManager.getSandboxRoot().addEventListener(MouseEvent.MOUSE_UP, mouseUpHandler, true, 0, true);

The consequence of this line is that any object that’s an instance of a sub-class of this class (a quick search through the source code revealed List, DataGrid, AdvancedDataGrid, and TileList) will never be garbage collected once the user has clicked on any of the rows in the list. Additionally, because all UIComponents keep a hard reference to their parent component, the garbage collector will also neglect to clean up any of your list’s ancestors in the document hierarchy.

The implications of that for most people are that lists in pop-ups will cause the entirety of your pop-up to never get garbage collected. Here at Workday its a much larger issue, however. Our pages are dynamically created based on XML page definitions we get from the server. These pages are also dynamically unloaded when a new page is received. As a result of the patch for this fix, we’ve seen dramatically reduced memory usage over time in our Flex applications.

Here’s the culprit so you can see what was going on:

////////////////////////////////////////////////////////////////////////////////
//
//  ADOBE SYSTEMS INCORPORATED
//  Copyright 2003-2006 Adobe Systems Incorporated
//  All Rights Reserved.
//
//  NOTICE: Adobe permits you to use, modify, and distribute this file
//  in accordance with the terms of the license agreement accompanying it.
//
////////////////////////////////////////////////////////////////////////////////

package mx.managers.systemClasses
{

[ExcludeClass]

import flash.display.Stage;
import flash.events.Event;

/**
 * An object that filters stage
 */
public class StageEventProxy
{
	private var listener:Function;

	public function StageEventProxy(listener:Function)
	{
		this.listener = listener;
	}

	public function stageListener(event:Event):void
	{
		if (event.target is Stage)
			listener(event);
	}

}

}

Here’s the updated version Adobe’s including in the 3.3 SDK:

////////////////////////////////////////////////////////////////////////////////
//
//  ADOBE SYSTEMS INCORPORATED
//  Copyright 2003-2006 Adobe Systems Incorporated
//  All Rights Reserved.
//
//  NOTICE: Adobe permits you to use, modify, and distribute this file
//  in accordance with the terms of the license agreement accompanying it.
//
////////////////////////////////////////////////////////////////////////////////

package mx.managers.systemClasses
{

[ExcludeClass]

import flash.display.Stage;
import flash.events.Event;
import flash.utils.Dictionary;

/**
 * An object that filters stage
 */
public class StageEventProxy
{
	private var weakRef: Dictionary = new Dictionary(true);

	public function StageEventProxy(listener:Function)
	{
		this.weakRef[listener] = 1;
	}

	public function stageListener(event:Event):void
	{
		if (event.target is Stage)
		{
			for (var p:* in weakRef)
			{
				var f:Function = p as Function;
				f(event);
			}
		}
	}

}

}

And finally…Here’s a patch file you can drop in your Lib folder that will solve the problem: PATCH FILE