silverlight如何使用 Downloader 物件非同步下載各式資源

if (!window.demoVideo21)
    window.demoVideo21 = {};

demoVideo21.Scene = function() 
{
}

demoVideo21.Scene.prototype =
{
    handleLoad: function(plugIn, userContext, rootElement) 
    {
        this.plugIn = plugIn;
        
        // Sample button event hookup: Find the button and then attach event handlers
        this.button = rootElement.children.getItem(0);    
        
        this.button.addEventListener("MouseEnter", Silverlight.createDelegate(this, this.handleMouseEnter));
        this.button.addEventListener("MouseLeftButtonDown", Silverlight.createDelegate(this, this.handleMouseDown));
        this.button.addEventListener("MouseLeftButtonUp", Silverlight.createDelegate(this, this.handleMouseUp));
        this.button.addEventListener("MouseLeave", Silverlight.createDelegate(this, this.handleMouseLeave));
    },
    
    // Sample event handlers
    handleMouseEnter: function(sender, eventArgs) 
    {
        // The following code shows how to find an element by name and call a method on it.
        var mouseEnterAnimation = sender.findName("mouseEnter");
        mouseEnterAnimation.begin(); 
    },
    
    handleMouseDown: function(sender, eventArgs) 
    {
        var mouseDownAnimation = sender.findName("mouseDown");
        mouseDownAnimation.begin(); 
    },
    
    handleMouseUp: function(sender, eventArgs) 
    {
        var mouseUpAnimation = sender.findName("mouseUp");
        mouseUpAnimation.begin(); 
        
        // Put clicked logic here
        var plugIn=sender.getHost();
        
        var dl=plugIn.createObject("Downloader");
        dl.addEventListener("Completed",onCompleted);
        dl.addEventListener("DownloadProgressChanged",onDownloadProgressChanged);
        dl.addEventListener("DownloadFailed",onDownloadFailed);
        
        //下载请求
        dl.open("get","test.zip");
        //开始下载
        dl.send();
    },
    
    handleMouseLeave: function(sender, eventArgs) 
    {
        var mouseLeaveAnimation = sender.findName("mouseLeave");
        mouseLeaveAnimation.begin(); 
    }
}
function onCompleted(sender, EvengArgs)
{
    var music=sender.findName("Music");
    music.setSource(sender, "summer.Wma");
    
    var movie=sender.findName("Movie");
    movie.setSource(sender, "SampleVideo.wmv");
    
    var img=sender.findName("Image");
    img.setSource(sender, "downloader.jpg");
    
    var strJs=sender.GetResponseText("js.js");
    eval(strJs);
    
    var xaml=sender.GetResponseText("asdf.xaml");
    var obj=sender.getHost().content.createFromXaml(xaml,true);
    var rootCanvas=sender.findName("rootCanvas");
    rootCanvas.children.add(obj);
}
function onDownloadProgressChanged(sender, EvengArgs)
{
    var txtProgress=sender.findName("txtProgress");
    var p=Math.floor(sender.downloadProgress*100);
    txtProgress.text=p+" %";
}
function onDownloadFailed(sender, EvengArgs)
{
    alert("falied"+"\n code"+sender.status+"\n text:"+sender.statusText);
}