flex 动态生成树

<?xml version="1.0" encoding="utf-8"?>
<mx:Application creationComplete="createAndShow()" xmlns:mx="http://www.adobe.com/2006/mxml" layout="absolute" width="400" height="300" backgroundColor="0xFFFFFF">
 
<mx:Script>
<![CDATA[
import mx.collections.ArrayCollection;
import mx.events.ListEvent;
import mx.controls.Alert;
[Bindable]
 private var subNodes:ArrayCollection = new 
             ArrayCollection([{label:"华盛顿"}]); 
             private function createAndShow():void{  
  subNodes.addItem({label: "纽约"});
  subNodes.addItem({label: "城市1"});
  tree.invalidateList();
  for(var i:int = 0; i<subNodes.length; i++) {  
//Alert.show(subNodes.getItemAt(i).label);  
} 
  tree.dataProvider=treeData;  
 
 }
             private var treeData:ArrayCollection=new ArrayCollection([{label:"中国",
children:[{label:"北京",children:[{label:"东城区"},{label:"西城区"}]},{label:"上海"}]},
{label:"美国",children:subNodes},
{label:"日本"}
]);