Flex AS实现自定义动画：fade效果闪烁

<?xml version="1.0" encoding="utf-8"?> 
<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml" layout="absolute" creationComplete="init()"> 
  <mx:Script> 
   <![CDATA[ 
    import mx.core.UIComponent;  // only needed if you use the second method 
    import flash.display.Sprite; 
    import flash.events.Event; 
    import flash.events.MouseEvent; 
  
    public var circle:Sprite; 
     
    public function init():void{ 
     circle = new Sprite(); 
     circle.graphics.beginFill(0x990000); 
     circle.graphics.drawCircle(50, 50, 50); 
     circle.graphics.endFill(); 
     
     this.rawChildren.addChild(circle);  // The quick and dirty method 
      
     /* OR YOU CAN DO THIS 
     var uic:UIComponent = new UIComponent(); 
     uic.addChild(circle); 
     addChild(uic); 
     */ 
      
     circle.addEventListener(MouseEvent.CLICK, startAnimation); //Event.ENTER_FRAME
    } 
    private function fadeCircle(event:Event):void{ 
        circle.alpha -= .05; 
        if (circle.alpha <= 0){