Flex DataGrid获取、判断行数

<?xml version="1.0" encoding="utf-8"?>
<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml" layout="absolute"
    creationComplete="initApp()">
<mx:Script><![CDATA[
  import mx.collections.ArrayCollection;
  import mx.controls.Alert;
 
  [Bindable]private var _acDP:ArrayCollection;
 
 public function initApp():void
 {
   var oItem:Object;
   _acDP = new ArrayCollection();
   oItem = {Album:"The Lost Chord", Artist:"The Moody Blues", Price:"15.99" };
   _acDP.addItem(oItem);
   oItem = {Album:"Meddle", Artist:"Pink Floyd", Price:"17.99" };
   _acDP.addItem(oItem);
   oItem = {Album:"Trespass", Artist:"Genesis", Price:"18.99" };
   _acDP.addItem(oItem);
 
 }//initApp

  private function lfRowNum(oItem:Object,iCol:int):String
  {
    //var iIndex:int = ArrayCollection(dgSource.dataProvider).getItemIndex(oItem) + 1;
    var iIndex:int = _acDP.getItemIndex(oItem) + 1;
    return String(iIndex);
  }
 
]]></mx:Script>

<mx:VBox>
 <mx:DataGrid id="dgSource" dataProvider="{_acDP}"
     editable="true" >
    <mx:columns>
      <mx:Array>
        <mx:DataGridColumn headerText="Row#" labelFunction="lfRowNum"/>
        <mx:DataGridColumn headerText="Album" dataField="Album" />
        <mx:DataGridColumn headerText="Artist" dataField="Artist" />
        <mx:DataGridColumn headerText="Price" dataField="Price" />
       </mx:Array>
    </mx:columns>
  </mx:DataGrid>
</mx:VBox>
</mx:Application>

//下面方法的作用是判断当前是否为最后一行，是则执行相应操作。

if(((ctlContractMoneies.dataProvider as ArrayCollection).getItemIndex(item)+1) == ArrayCollection(ctlContractMoneies.dataProvider).length){
                    contractMoneyT = contractTotal.toString();
                }