Flex ComboBox prompt使用

<?xml version="1.0" encoding="utf-8"?> 
<!-- http://blog.flexexamples.com/2007/09/25/specifying-a-custom-label-field-on-a-flex-combobox-control/ --> 
<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml" 
        layout="vertical" 
        verticalAlign="top" 
        backgroundColor="white"> 
  <mx:Script>
  	<![CDATA[
  		import mx.controls.Alert;
  		private function clickHandle():void
  		{
  			
  			if(comboBox.selectedItem==null)
  			{
  				Alert.show("00");
  			}
  			else
  			{
  				
  			Alert.show(comboBox.selectedItem.@abbrev+"");
  			}
  		
  		}
  	]]>
  </mx:Script>
 
    <mx:XMLList id="statesXMLList"> 
        <state abbrev="AL" name="Alabama" /> 
        <state abbrev="AK" name="Alaska" /> 
        <state abbrev="AZ" name="Arizona" /> 
        <state abbrev="AR" name="Arkansas" /> 
        <state abbrev="CA" name="California" /> 
        <state abbrev="CO" name="Colorado" /> 
        <state abbrev="CT" name="Connecticut" /> 
    </mx:XMLList> 
  
    <mx:ComboBox id="comboBox" 
            prompt="Please select a State..." 
            dataProvider="{statesXMLList}" 
            labelField="@name" /> 
  	<mx:Button click="clickHandle()"/>
</mx:Application>