flex 4 TabNavigator 切换触发事件

</pre><pre name="code" class="html"><?xml version="1.0" encoding="utf-8"?>
<s:Application  xmlns:fx="http://ns.adobe.com/mxml/2009" 
xmlns:s="library://ns.adobe.com/flex/spark" 
xmlns:mx="library://ns.adobe.com/flex/mx" minWidth="955" minHeight="600" creationComplete="init()">
<fx:Script>
<![CDATA[
import mx.controls.TabBar;
import mx.controls.Button;
import mx.controls.Alert; 
import mx.events.CloseEvent;
import mx.managers.PopUpManager;

private var iTabIndex:int;
private var iToTabIndex:int;
private var _updateAlert:Alert; 

public function init():void
{
iTabIndex = 0;
iToTabIndex = 0;
var btn:Button;
for(var i:int=0;i<tab.getChildren().length;i++)
{
btn = tab.getTabAt(i);
btn.addEventListener(MouseEvent.MOUSE_DOWN,tabClickHandler,false,111);
}
}
private function tabClickHandler(event:MouseEvent):void
{
iToTabIndex = event.currentTarget.parent.getChildIndex(event.currentTarget);
//终止事件
event.stopImmediatePropagation();
//给出提示message
if(iToTabIndex !=iTabIndex)
{
showGoBack();
}
}
// alert
public function showGoBack():void
{
Alert.buttonWidth = 100;
Alert.yesLabel    = "yes";
Alert.noLabel     = "no";
var updateLabel:String =  "要切换tab吗？";

this._updateAlert = Alert.show(updateLabel,"",Alert.YES | Alert.NO);
this._updateAlert.addEventListener(CloseEvent.CLOSE,closeShowGoBack); 
}
private function closeShowGoBack(e:CloseEvent):void 
{ 
switch (e.detail) 
{ 
case Alert.YES:             
iTabIndex    = iToTabIndex;
tab.selectedIndex   = iTabIndex;
break; 
case Alert.NO: 
PopUpManager.removePopUp(this._updateAlert); 
break; 
}   
}

public function change(e:Event):void
{
Alert.show("CHANGE " + tab.selectedIndex.toString());

}
//注释;creationPolicy="all",把tab的所有child全部初始化


]]>
</fx:Script>
<mx:TabNavigator creationPolicy="all" id="tab" x="21" y="28" change="change(event)" width="200" height="200" >
<mx:Canvas id="a" label="aa" width="100%" height="100%">
<mx:TextInput x="10" y="10" width="178"/>
</mx:Canvas>
<mx:Canvas id="b" label="bb" width="100%" height="100%">
<mx:Button id="abtn" x="10" y="50" label="Button"/>
</mx:Canvas>
</mx:TabNavigator>
</s:Application>