flex，datagrid多列排序

<?xml version="1.0"?>
<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml"
				initialize="initDP();"
				width="550"
				height="400">
	<mx:Script>
		<![CDATA[
			import mx.events.DataGridEvent;
			import mx.collections.*;

			// Declare storage variables and initialize the simple variables.
			// The data provider collection.
			private var myDPColl:ArrayCollection;
			// The Sort object used to sort the collection.
			[Bindable]
			private var sortA:Sort;
			// The sort fields used to determine the sort.
			private var sortByInStock:SortField;
			private var sortByArtist:SortField;
			private var sortByAlbum:SortField;
			private var sortByPrice:SortField;
			// The data source that populates the collection.
			private var myDP:Array=[{Artist: 'Pavement', Album: 'Slanted and Enchanted', Price: 11.99, InStock: true}, {Artist: 'Pavement', Album: 'Crooked Rain, Crooked Rain', Price: 10.99, InStock: false}, {Artist: 'Pavement', Album: 'Wowee Zowee', Price: 12.99, InStock: true}, {Artist: 'Asphalt', Album: 'Brighten the Corners', Price: 11.99, InStock: false}, {Artist: 'Asphalt', Album: 'Terror Twilight', Price: 11.99, InStock: true}, {Artist: 'Asphalt', Album: 'Buildings Meet the Sky', Price: 14.99, InStock: true}, {Artist: 'Other', Album: 'Other', Price: 5.99, InStock: true}];


			//Initialize the DataGrid control with sorted data.
			private function initDP():void
			{
				//Create an ArrayCollection backed by the myDP array of data.
				myDPColl=new ArrayCollection(myDP);
				//Create a Sort object to sort the ArrrayCollection.
				sortA=new Sort();
				//Initialize SortField objects for all valid sort fields:
				// A true second parameter specifies a case-insensitive sort.
				// A true third parameter specifies descending sort order.
				// A true fourth parameter specifies a numeric sort.
				sortByInStock=new SortField("InStock", true, true);
				sortByArtist=new SortField("Artist", true);
				sortByAlbum=new SortField("Album", true);
				sortByPrice=new SortField("Price", true, false, true);
				// Sort the grid using the InStock, Artist, and Album fields.
				sortA.fields=[sortByInStock, sortByArtist, sortByAlbum];
				myDPColl.sort=sortA;
				// Refresh the collection view to show the sort.
				myDPColl.refresh();
				// Initial display of sort fields
				tSort0.text="First Sort Field: InStock";
				tSort1.text="Second Sort Field: Artist";
				tSort2.text="Third Sort Field: Album";


				// Set the ArrayCollection as the DataGrid data provider.
				myGrid.dataProvider=myDPColl;
				// Set the DataGrid row count to the array length, 
				// plus one for the header.
				myGrid.rowCount=myDPColl.length + 1;
			}

			// Re-sort the DataGrid control when the user clicks a header.
			private function headRelEvt(event:DataGridEvent):void
			{
				// The new third priority was the old second priority.
				sortA.fields[2]=sortA.fields[1];
				tSort2.text="Third Sort Field: " + sortA.fields[2].name;
				// The new second priority was the old first priority.
				sortA.fields[1]=sortA.fields[0];
				tSort1.text="Second Sort Field: " + sortA.fields[1].name;
				// The clicked column determines the new first priority.
				if (event.columnIndex == 0)
				{
					sortByArtist.descending=!sortByArtist.descending;
					sortA.fields[0]=sortByArtist;
				}
				else if (event.columnIndex == 1)
				{
					sortByAlbum.descending=!sortByAlbum.descending;
					sortA.fields[0]=sortByAlbum;
				}
				else if (event.columnIndex == 2)
				{
					sortByPrice.descending=!sortByPrice.descending;
					sortA.fields[0]=sortByPrice;
				}
				else
				{
					sortA.fields[0]=sortByInStock;
				}
				tSort0.text="First Sort Field: " + sortA.fields[0].name;
				// Apply the updated sort fields and re-sort.
				myDPColl.sort=sortA;
				// Refresh the collection to show the sort in the grid.
				myDPColl.refresh();
				// Prevent the DataGrid from doing a default column sort.
				event.preventDefault();
			}
		]]>

	</mx:Script>


	<!-- The Data Grid control.
		 By default the grid and its columns can be sorted by clicking.
		 The headerRelease event handler overrides the default sort
		 behavior. -->
	<mx:DataGrid id="myGrid"
				 width="100%"
				 headerRelease="headRelEvt(event);">
		<mx:columns>
			<mx:DataGridColumn minWidth="120"
							   dataField="Artist"/>
			<mx:DataGridColumn minWidth="200"
							   dataField="Album"/>
			<mx:DataGridColumn width="75"
							   dataField="Price"/>
			<mx:DataGridColumn width="75"
							   dataField="InStock"
							   headerText="In Stock"/>
		</mx:columns>
	</mx:DataGrid>
	<mx:VBox>
		<mx:Label id="tSort0"
				  text="First Sort Field: "/>
		<mx:Label id="tSort1"
				  text="Second Sort Field: "/>
		<mx:Label id="tSort2"
				  text="Third Sort Field: "/>
	</mx:VBox>
</mx:Application>