Tree每一项的tooltip不显示,求助

[pywepe]

连dataTipFunction函数都没调用

<mx:TabNavigator width="100%" height="100%" id="tabContainer">
<s:NavigatorContent label="好友列表" width="100%" height="100%" id="rosterList" >
<mx:Tree x="0" y="0" width="100%" height="100%" id="rosterTree"
labelField="@label" showRoot="false"
itemRenderer="com.ys.webchat.ui.RosterItemRenderer"
iconFunction="getIcon"
itemDoubleClick="rosterTree_itemDoubleClickHandler(event)"
doubleClickEnabled="true"
creationComplete="rosterTree_creationCompleteHandler(event)"
showDataTips="true"
dataTipFunction="rosterTree_dataTipFunction"
>
</mx:Tree>
</s:NavigatorContent>
</mx:TabNavigator>

protected function rosterTree_dataTipFunction(item:Object):String{
trace("fk");
return "fuck";
}

当光标移动到树的结点上果,rosterTree_dataTipFunction根本就没调用

这是什么问题,网上都说是这么添加tooltip的啊,这怎么不行呢?
谢谢