该版本已更新，请移步新版本：http://www.litefeel.com/progressiveloader-update/

渐进式加载类。用法跟Loader差不多。

不同点:

1. 当加载流错误时只能获取到 "Error #2124: 加载的文件为未知类型。"
2. 侦听contentLoaderInfo的Progress事件的bytesLoaded,bytesTotal是整个文件的已加载字节和总字节.
3. contentLoaderInfo.bytesLoaded, contentLoaderInfo.bytesTotal指当前loader里的字节数和总字节.

ProgressiveLoader

package com.litefeel. 
{
    import flash.display.LoaderInfo;
    import flash.errors.IOError;
    import flash.events.Event;
    import flash.events.IOErrorEvent;
    import flash.events.ProgressEvent;
    import flash.events.SecurityErrorEvent;
    import flash.system.LoaderContext;
    import flash.display.Loader;
    import flash.net.URLRequest;
    import flash.net.URLStream;
    import flash.utils.ByteArray;

    /**
     * 渐进式加载类
     * 用法跟Loader一样,
     * <p><b>不同点:</b></p>
     * 1.当加载流错误时只能获取到 "Error #2124: 加载的文件为未知类型。" <br/>
     * 2.侦听contentLoaderInfo的Progress事件的bytesLoaded,bytesTotal是整个文件的已加载字节和总字节.<br/>
     * 3.contentLoaderInfo.bytesLoaded, contentLoaderInfo.bytesTotal指当前loader里的字节数和总字节
     * 
     * @author lite3
     */    public class ProgressiveLoader extends Loader
    {
        private var bytesLoaded:uint = 0;           // 已加载的字节数
        private var bytesToal:uint = 0;             // 总字节数

        private var dataChange:Boolean = false;     // buffer的数据是否改变    
        private var streamComplete:Boolean = false; // 文件是否加载完成

        private var context:LoaderContext;          // 

        private var buffer:ByteArray;               // 数据缓存
        private var stream:URLStream;               // 流

        /**
         * 关闭流,并清理所有侦听器
         */        override public function close():void 
        {
            // 清除流相关
            if (stream)
            {
                if (stream.connected) stream.close();
                streamRemoveEvent(stream);
            }
            // 清除conentLoaderInfo相关的事件
            if (contentLoaderInfo.hasEventListener(Event.COMPLETE))
            {
                loaderInfoRemoveEvent(super.contentLoaderInfo);
            }
            // 清除显示数据事件
            if (hasEventListener(Event.ENTER_FRAME))
            {
                removeEventListener(Event.ENTER_FRAME, showData);
            }
            buffer = null;
        }
        /**
         * 加载字节数据,不会在内部触发contentLoaderInfo相关事件
         * @param   bytes
         * @param   context
         */        override public function loadBytes(bytes:ByteArray, context:LoaderContext = null):void
        {
            close();
            super.unload();
            super.loadBytes(bytes, context);
        }

        /**
         * 加载一个url文件,并渐进显示(如果是渐进格式)
         * @param   request
         * @param   context
         */        override public function load(request:URLRequest, context:LoaderContext = null):void 
        {
            streamComplete = false;
            if (!stream) stream = new URLStream();
            if (stream.connected) stream.close();
            this.context = context;
            dataChange = false;

            buffer = new ByteArray();
            super.unload();
            addEventListener(Event.ENTER_FRAME, showData);
            loaderInfoAddEvent(super.contentLoaderInfo);
            streamAddEvent(stream);
            stream.load(request);
        }

        // 将缓存中的数据显示为图像
        private function showData(e:Event = null):void 
        {
            if (!dataChange || !stream.connected) return;

            dataChange = false;
            if (stream.bytesAvailable > 0)
            {
                stream.readBytes(buffer, buffer.length, stream.bytesAvailable);
            }
            if (buffer.length > 0)
            {
                super.unload();
                super.loadBytes(buffer, context);
            }

            // 加载完成
            if (streamComplete)
            {
                close();
                streamComplete  = false;
            }
        }
        // 修正contentLoaderInfo的ProgressEvent.PROGRESS事件的进度值
        private function loaderProgressHandler(e:ProgressEvent):void 
        {
            e.bytesLoaded = bytesLoaded;
            e.bytesTotal = bytesToal;
        }

        // 显示完成
        private function loaderCompleteHandler(e:Event):void 
        {
            if (streamComplete)
            {
                streamComplete = false;
                loaderInfoRemoveEvent(super.contentLoaderInfo);
            }else
            {
                e.stopImmediatePropagation();
            }
        }

        // 数据加载完成
        private function streamCompleteHandler(e:Event):void 
        {
            streamRemoveEvent(stream);
            // 这里不删除EnterFrame事件,最后一段总是不会显示,
            // 并且complete事件里showData也不行,
            // 所以最后延时显示一次,
            streamComplete = true;
            dataChange = true;
        }
        // 数据加载中,保存数据加载的值
        private function streamProgressHandler(e:ProgressEvent):void 
        {
            bytesLoaded = e.bytesLoaded;
            bytesToal = e.bytesTotal;
            dataChange = true;
        }
        // 数据流错误, 但是也会加载400K左右的数据,
        // 然后由contentLoaderInfo抛出IOError或者IOErrorEvent
        // 但不会是流错误,而是未知文件类型
        private function streamErrorHandler(e:IOError):void 
        {
            close();
        }

        private function streamAddEvent(stream:URLStream):void
        {
            stream.addEventListener(Event.COMPLETE, streamCompleteHandler);
            stream.addEventListener(ProgressEvent.PROGRESS, streamProgressHandler);
            stream.addEventListener(IOErrorEvent.IO_ERROR, streamErrorHandler);
        }

        private function streamRemoveEvent(stream:URLStream):void
        {
            stream.removeEventListener(Event.COMPLETE, streamCompleteHandler);
            stream.removeEventListener(ProgressEvent.PROGRESS, streamProgressHandler);
            stream.removeEventListener(IOErrorEvent.IO_ERROR, streamErrorHandler);
        }

        private function loaderInfoAddEvent(loaderInfo:LoaderInfo):void
        {
            loaderInfo.addEventListener(Event.COMPLETE, loaderCompleteHandler, false, int.MAX_VALUE);
            loaderInfo.addEventListener(ProgressEvent.PROGRESS, loaderProgressHandler, false, int.MAX_VALUE);
        }

        private function loaderInfoRemoveEvent(loaderInfo:LoaderInfo):void
        {
            loaderInfo.removeEventListener(Event.COMPLETE, loaderCompleteHandler);
            loaderInfo.removeEventListener(ProgressEvent.PROGRESS, loaderProgressHandler);
        }
    }
}

这里是旧版本，不提供下载了。