flex select/deselect all checkbox in datagrid

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DataGrid

Flex

Hi..
i am using below code to select/deselect all checkbox in datagrid

<mx:datagridcolumn id="testColumn" width="20" sortable="false" xmlns:mx="#unknown">
	<mx:headerrenderer>
		<fx:component xmlns:fx="#unknown">
			<mx:canvas>
			<fx:script>
				<![CDATA[
					protected function checkAll_clickHandle(event:MouseEvent):void
						{
					
						}	
					
			</fx:script>
				<s:checkbox id="checkAll" horizontalcenter="0" selected="false" click="checkAll_clickHandler(event)" xmlns:s="#unknown" />
			</mx:canvas>
		</fx:component>
	</mx:headerrenderer>	 	
	<mx:itemrenderer>
		<fx:component xmlns:fx="#unknown">
			<mx:canvas width="100%" height="100%">
			<fx:script>
				<![CDATA[
					protected function check_clickHandler(event:MouseEvent):void
						{
					data.isSelected = (event.currentTarget as CheckBox).selected;
						if(data.isSelected == false)
							{
							}
						}	
					
			</fx:script>
				<s:checkbox id="check" horizontalcenter="0" selected="{data.isSelected}" click="check_clickHandler(event)" xmlns:s="#unknown">
				</s:checkbox>
			</mx:canvas>
		</fx:component>
	</mx:itemrenderer>
</mx:datagridcolumn>

i want to unselct checkAll checkbox when i deselect any of row checkbox, i am trying to access value of checkAll to check_clickHandler(), but i am not getting its value.
how can i do this??