Flex dataGrid 中TAB键合计问题（急！）

实例如下（其中用到AS脚本）：
<?xml version="1.0" encoding="utf-8"?>
<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml"
                layout="absolute"
                height="700"
                verticalScrollPolicy="on"  creationComplete="init()">
  
    <mx:Script>
        <![CDATA[
            
       import tabpackage.tabkey;  
   import mx.collections.ArrayCollection;  
   import mx.events.AdvancedDataGridEvent;  
   import mx.events.FlexEvent;
   import mx.events.CollectionEvent;  
   import mx.rpc.events.ResultEvent;
      import mx.events.PropertyChangeEvent;
      import mx.events.CollectionEventKind;
      import mx.controls.Alert;
      import mx.formatters.DateFormatter;
      import mx.controls.TextInput;
 
            [Bindable]
            private var data1:ArrayCollection=new ArrayCollection([{xh: 1}, {xh: 2}, {xh: 3}, {xh: 4}, {xh: 5}]);
           
             private function sum1(event:AdvancedDataGridEvent):void{
             var obj:Object=AdvancedDataGrid(event.target).editedItemPosition;