按钮控制DataGrid的Column升序或降序

想要对DataGrid进行排序，不知道Flex有没有这个方法

<?xml version="1.0" encoding="utf-8"?>
<s:Application xmlns:fx="http://ns.adobe.com/mxml/2009" 
			   xmlns:s="library://ns.adobe.com/flex/spark" 
			   xmlns:mx="library://ns.adobe.com/flex/mx" 
			   xmlns:component="pizazz.flex4.component.*"
			   minWidth="955" minHeight="600">
	<s:layout>
		<s:VerticalLayout />
	</s:layout>
	<fx:Script>
		<![CDATA[
			import mx.events.DataGridEvent;
			
			private const _arr:Array = [	
				{"str": "B", "num": 34},
				{"str": "A", "num": 256},
				{"str": "D", "num": 97},
				{"str": "C", "num": 18},
				{"str": "E", "num": 0}
			];
			
			private function onClick(columnIndex:int = -1, 
									 sortDescending:Boolean = true):void{
				var _column:DataGridColumn = grid.columns[columnIndex];
				_column.sortDescending = sortDescending;
				grid.dispatchEvent(new DataGridEvent(
					DataGridEvent.HEADER_RELEASE, false, false, columnIndex));
			}
		]]>
	</fx:Script>
	<mx:DataGrid id="grid" dataProvider="{_arr}" width="320" height="240">
		<mx:columns>
			<mx:DataGridColumn dataField="str" headerText="String" />
			<mx:DataGridColumn dataField="num" headerText="Number" />
		</mx:columns>
	</mx:DataGrid>
	<s:Button label="String排序" width="100" click="onClick(0, false)" />
	<s:Button label="Number排序" width="100" click="onClick(1)" />
</s:Application>

视图：