flex按钮控制文本标签内容

<?xml version="1.0" encoding="utf-8"?>
<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml" layout="absolute">
<mx:Array id="coffeeArray">
</mx:Array>
<mx:Script>
  <![CDATA[
    import mx.controls.Alert;
    public function addToCart():void
    {
      txt.text="OK";
    }
  ]]>
</mx:Script>
  <mx:Label id="txt" text="hello" height="100" x="32" y="19"/>
  <mx:Button label="Add to Cart" click="addToCart()" x="149" y="97"/>
</mx:Application>