由于渲染器render是重复使用的，所以render里可以通过dataChange事件来检查是否需要更新

<?xml version="1.0" encoding="utf-8"?>
<s:ItemRenderer xmlns:fx="http://ns.adobe.com/mxml/2009" 
				xmlns:s="library://ns.adobe.com/flex/spark" 
				xmlns:mx="library://ns.adobe.com/flex/mx" 
				autoDrawBackground="true"
				dataChange="dataChangeHandler(event)">
	<fx:Script>
		<![CDATA[
			import mx.events.FlexEvent;
			
			protected function dataChangeHandler(event:FlexEvent):void
			{
				if(data.email ==""){
					//........
				}else{
					//........
				}
			}
		]]>
	</fx:Script>
	<s:Label text="{data.text}"/>
</s:ItemRenderer>