I was working on a project recently and I discovered (or possibly misunderstood – see disclaimer) that Flex does not cache modules correctly, or – at the very least – it does not do it effectively according to my tests.

My test was conducted using Flex SDK 3.4.0.9271, Firefox 3.5.7 and Flex Builder Professional 3.0.214193

According to this page (under the Preloading modules heading), Adobe asserts the following:

When a module is loaded by the Flex application for the first time, the module’s SWF file is transferred across the network and stored in the browser’s cache. If the Flex application unloads that module, but then later reloads it, there should be less wait time because Flash Player loads the module from the cache rather than across the network.

Module SWF files, like all SWF files, reside in the browser’s cache unless and until a user clears them. As a result, modules can be loaded by the main application across several sessions, reducing load time; but this depends on how frequently the browser’s cache is flushed.

I found the above claims to be demonstrably false, or at least just plain inefficient…

Let me qualify this:
According to my tests (conducted with my Firefox browser cache turned both on and off), i found that the application’s memory usage keeps growing exponentially when switching between two loaded modules. There was a variance (as you’d imagine) when the browser cache was left on but nonetheless, the memory usage keeps growing steadily.

In the code sample below, I have extended the functionality of the mx.modules.ModuleLoader class, and added a Dictionary (with a little logic) to manage the (ostensibly) effective caching of previously loaded modules. I have not tested this class extensively, but all the tests that I conducted seemed to produce a significant memory and speed improvement.

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package
{
	import flash.display.DisplayObject;
	import flash.display.DisplayObjectContainer;
	import flash.utils.ByteArray;
	import flash.utils.Dictionary;
 
	import mx.events.FlexEvent;
	import mx.events.ModuleEvent;
	import mx.modules.IModuleInfo;
	import mx.modules.ModuleLoader;
	import mx.modules.ModuleManager;
 
	/**
	 * This class manages the loading, unloading and caching of Flex Modules
	 * This is a modified version of the mx.modules.ModuleLoader class
	 *
	 * @author Danny Kopping - danny@ria-coder.com
	 */
	public class CachedModuleLoader extends ModuleLoader
	{
		private var map:Dictionary = new Dictionary();
 
		private var _url:String = null;
		private var module:IModuleInfo;
		private var loadRequested:Boolean = false;
 
		public function CachedModuleLoader()
		{
			super();
		}
 
		override public function set url(value:String):void
		{
			if (value == _url)
				return;
 
			if (module)
			{
				module.removeEventListener(ModuleEvent.PROGRESS, moduleProgressHandler);
				module.removeEventListener(ModuleEvent.SETUP, moduleSetupHandler);
				module.removeEventListener(ModuleEvent.READY, moduleReadyHandler);
				module.removeEventListener(ModuleEvent.ERROR, moduleErrorHandler);
				module.removeEventListener(ModuleEvent.UNLOAD, moduleUnloadHandler);
 
				//module.release();
				module = null;
 
				if (child)
				{
					removeChild(child);
					//child = null;
				}
			}
 
			_url = value;
 
			dispatchEvent(new FlexEvent(FlexEvent.URL_CHANGED));
			removeAllChildren();
 
			if (_url != null && loadRequested)
			{
				if(!map[_url])
					loadModule();
				else
				{
 
					child = map[_url];
					addChild(child);
				}
			}
		}
 
		override public function get url():String
		{
			return _url;
		}
 
		override public function createComponentsFromDescriptors(recurse:Boolean = true):void
		{
			super.createComponentsFromDescriptors(recurse);
 
			loadRequested = true;
			loadModule();
		}
 
		override public function loadModule(url:String = null, bytes:ByteArray = null):void
		{
			if (url != null)
				_url = url;
 
			if (_url == null)
			{
				//trace("loadModule() - null url");
				return;
			}
 
			if (map[_url])
			{
				//trace("loadModule() - already created the child");
				return;
			}
 
			if (module)
			{
				//trace("loadModule() - load already initiated");
				return;
			}
 
			dispatchEvent(new FlexEvent(FlexEvent.LOADING));
 
			module = ModuleManager.getModule(_url);
 
			module.addEventListener(ModuleEvent.PROGRESS, moduleProgressHandler);
			module.addEventListener(ModuleEvent.SETUP, moduleSetupHandler);
			module.addEventListener(ModuleEvent.READY, moduleReadyHandler);
			module.addEventListener(ModuleEvent.ERROR, moduleErrorHandler);
			module.addEventListener(ModuleEvent.UNLOAD, moduleUnloadHandler);
 
			module.load(applicationDomain, null, bytes);
		}
 
		override public function unloadModule():void
		{
			if (child && contains(child))
			{
				removeChild(child);
				child = null;
			}
 
			if (module)
			{
				module.removeEventListener(ModuleEvent.PROGRESS, moduleProgressHandler);
				module.removeEventListener(ModuleEvent.SETUP, moduleSetupHandler);
				module.removeEventListener(ModuleEvent.READY, moduleReadyHandler);
				module.removeEventListener(ModuleEvent.ERROR, moduleErrorHandler);
 
				module.unload();
				module.removeEventListener(ModuleEvent.UNLOAD, moduleUnloadHandler);
				module = null;
			}
 
			if(map[_url])
			{
				delete map[_url];
			}
		}
 
		private function moduleProgressHandler(event:ModuleEvent):void
		{
			dispatchEvent(event);
		}
 
		private function moduleSetupHandler(event:ModuleEvent):void
		{
			// Not ready for creation yet, but can call factory.info().
 
			dispatchEvent(event);
		}
 
		private function moduleReadyHandler(event:ModuleEvent):void
		{
			child = module.factory.create() as DisplayObject;
			dispatchEvent(event);
 
			if (child)
			{
				var p:DisplayObjectContainer = parent;
				// p.removeChild(this);
				addChild(child);
 
				map[url] = child;//ModuleManager.getModule(_url);
				//trace(map + ":" + url + ":" + map[url]);
			}
		}
 
		private function moduleErrorHandler(event:ModuleEvent):void
		{
			unloadModule();
			dispatchEvent(event);
		}
 
		private function moduleUnloadHandler(event:ModuleEvent):void
		{
			dispatchEvent(event);
		}
	}
}

The usage of this class is exactly the same as the regular mx.modules.ModuleLoader class. I hope this helps!

Download this file: Download

**DISCLAIMER**
I know a bit about Flex modules from hours of obsessing over them, but i do not know everything. From my tests of the efficacy of the above code & explanation, i found that it reduces memory usage and increases the general usability of my Flex project; i could be very wrong on this topic, and if i am – please tell me. Maybe i’m just an idiot, but it seems to work…