If you no longer need an event listener, remove it by calling removeEventListener(), or memory problems could result. Objects with registered event listeners are not automatically removed from memory because the garbage collector does not remove objects that still have references.

Objects with registered event listeners怎么理解
对于
objA.addEventListener(objB.func);
是指objA还是objB？

如果是objB，
objB=null;
//理解为objB不会被gc清理，因为objB的func是objA的注册侦听

如果是objA
objA=null;
//理解为objA不会被gc清理，因为objA注册了侦听器

objA.addEventListener(objB.func);
  |                                     |
  |---->Object                   |---->event listener

Objects with registered event listeners------> means the objects that have (registered) event listeners.

[ 本帖最后由 goldview 于 2008-7-5 14:52 编辑 ]

关于Event,看AS2时的原代码,

复制内容到剪贴板

代码:

//****************************************************************************
//Copyright (C) 2003 Macromedia, Inc. All Rights Reserved.
//The following is Sample Code and is subject to all restrictions on
//such code as contained in the End User License Agreement accompanying
//this product.
//****************************************************************************
/**
* base class for event listening and dispatching
*
* @helpid 3295
* @tiptext
*/
class mx.events.EventDispatcher
{
    // make a instance of ourself so we can add methods to other objects
    static var _fEventDispatcher:EventDispatcher = undefined;
    // these events do not get called via backdoor because of name collisions with other methods
    static var exceptions:Object = {move: 1, draw: 1, load:1};
    // internal function for removing listeners
    static function _removeEventListener(queue:Object, event:String, handler):Void
    {
        if (queue != undefined)
        {
            var l:Number = queue.length;
            var i:Number;
            for (i = 0; i < l; i++)
            {
                var o = queue[i];
                if (o == handler) {
                    queue.splice(i, 1);
                    return;
                }
            }
        }
    }
/**
* add listening and dispatching methods to an object
* @param object the object to receive the methods
*/
    static function initialize(object:Object):Void
    {
        if (_fEventDispatcher == undefined)
        {
            _fEventDispatcher = new EventDispatcher;
        }
        object.addEventListener = _fEventDispatcher.addEventListener;
        object.removeEventListener = _fEventDispatcher.removeEventListener;
        object.dispatchEvent = _fEventDispatcher.dispatchEvent;
        object.dispatchQueue = _fEventDispatcher.dispatchQueue;
    }
    // internal function for dispatching events
    function dispatchQueue(queueObj:Object, eventObj:Object):Void
    {
        var queueName:String = "__q_" + eventObj.type;
        var queue:Array = queueObj[queueName];
        if (queue != undefined)
        {
            var i:String;
            // loop it as an object so it resists people removing listeners during dispatching
            for (i in queue)
            {
                var o = queue[i];
                var oType:String = typeof(o);
                // a handler can be a function, object, or movieclip
                if (oType == "object" || oType == "movieclip")
                {
                    // this is a backdoor implementation that
                    // is not compliant with the standard
                       if (o.handleEvent != undefined)
                       {
                           // this is the DOM3 way
                           o.handleEvent(eventObj);
                       }
                    if (o[eventObj.type] != undefined)
                       {
                        if (EventDispatcher.exceptions[eventObj.type] == undefined)
                        {    
                            // this is a backdoor implementation that
                            // is not compliant with the standard
                            o[eventObj.type](eventObj);
                        }
                       }
                }
                else // it is a function
                {
                    o.apply(queueObj, [eventObj]);
                }
            }
        }
    }
/**
* dispatch the event to all listeners
* @param eventObj an Event or one of its subclasses describing the event
*/
    function dispatchEvent(eventObj:Object):Void
    {
        if (eventObj.target == undefined)
            eventObj.target = this;
        this[eventObj.type + "Handler"](eventObj);
        // Dispatch to objects that are registered as listeners for
        // this object.
        this.dispatchQueue(this, eventObj);
    }
/**
* add a listener for a particular event
* @param event the name of the event ("click", "change", etc)
* @param the function or object that should be called
*/
    function addEventListener(event:String, handler):Void
    {
        var queueName:String = "__q_" + event;
        if (this[queueName] == undefined)
        {
            this[queueName] = new Array();
        }
        _global.ASSetPropFlags(this, queueName,1);
        EventDispatcher._removeEventListener(this[queueName], event, handler);
        this[queueName].push(handler);
    }
/**
* remove a listener for a particular event
* @param event the name of the event ("click", "change", etc)
* @param the function or object that should be called
*/
    function removeEventListener(event:String, handler):Void
    {
        var queueName:String = "__q_" + event;
        EventDispatcher._removeEventListener(this[queueName], event, handler);
    }
}