flex上把xml内容存成xml文件！

<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml"
    layout="absolute" width="900" height="700" fontSize="12"
    creationComplete="init()" xmlns:s="library://ns.adobe.com/flex/spark"> 
<mx:Script> 
  <![CDATA[ 
   import mx.collections.ArrayCollection; 
   import mx.controls.Alert;
   [Bindable]private var shijxml:XML; 
  
   private function init():void{ 
    shijxml = arrToXml(exams);
   } 
   [Bindable] 
   public var exams:ArrayCollection = new ArrayCollection( 
    [{sjtitle:"数据结构第一章", kemu:"数据结构",shijID:"sj000" ,num:"30"},  
     {sjtitle:"数据结构第二章", kemu:"数据结构",shijID:"sj001" ,num:"30"},  
     {sjtitle:"组成原理第一章", kemu:"组成原理",shijID:"zc000" ,num:"50"},                     
     {sjtitle:"组成原理第二章", kemu:"组成原理",shijID:"zc001" ,num:"30"} ]); 
   private function arrToXml(arr:ArrayCollection):XML{  //形成xml内容
    var root:XML = new XML("<root/>"); 
    for(var i:int = 0;i<arr.length;i++){ 
     var node:XML = new XML("<node/>"); 
     node.sjtitle = arr[i].sjtitle; 
     node.kemu= arr[i].kemu; 
     node.shijID= arr[i].shijID; 
     node.num= arr[i].num; 
     root.appendChild(node); 
    } 
    return root; 
   }  
   protected function confirm_clickHandler(event:MouseEvent):void
   {
    var file:FileReference=new FileReference();
    trace(shijxml);
    file.save(shijxml,"test.xml");
   }
  ]]> 
</mx:Script> 
<s:VGroup width="100%" height="100%">
  <mx:Panel width="100%" height="40%" > 
   <mx:DataGrid height="100%" width="50%" dataProvider="{exams}"> 
   </mx:DataGrid> 
  </mx:Panel> 
  <mx:Button id="confirm" click="confirm_clickHandler(event)" label="生成xml文件"/>
</s:VGroup>
</mx:Application>