flex生成遮罩层

<?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" minWidth="955" minHeight="600">
	<s:layout>
		<s:BasicLayout/>
	</s:layout>
<fx:Style>
	@namespace s "library://ns.adobe.com/flex/spark";
	@namespace mx "library://ns.adobe.com/flex/mx";
	.maskCover{
		backgroundAlpha: 0.4;
		backgroundColor: #ECE9D8;
		borderStyle: solid;
		borderThickness: 3;
		borderColor: #58B299;
	}
</fx:Style>
	<fx:Script>
		<![CDATA[
			import mx.containers.Canvas;
			private var maskCover:Canvas;
			protected function button1_clickHandler(event:MouseEvent):void
			{
		/* 		maskCover = new Canvas();
				maskCover.styleName = "maskCover";
				maskCover.visible = false;
				//this.rawChildren.addChild(maskCover);
				this.setElementIndex(maskCover,0);
				onSomeEvent(); */
				
				this.setElementIndex(bc,0);
				bc.visible = true;
				bc.styleName = "maskCover";
				bc.width = this.width;
				bc.height = this.height;
				bc.move(0,0);
			}
			
			private function onSomeEvent():void {
				maskCover.visible = true;
				maskCover.width = this.width;
				maskCover.height = this.height;
				maskCover.move(0, 0);
			}
		]]>
	</fx:Script>

	<fx:Declarations>
		<!-- 将非可视元素（例如服务、值对象）放在此处 -->
	</fx:Declarations>
	<s:Button x="293" y="268" label="按钮" click="button1_clickHandler(event)"/>
	<s:BorderContainer id="bc" x="434" y="80" width="146" height="103" visible="false">
	</s:BorderContainer>
</s:Application>