HBox添加圆角背景

<mx:HBox xmlns:mx="http://www.adobe.com/2006/mxml" width="400" height="300"
 creationComplete="createFill()" cornerRadius="20">
     <mx:Script>
         <![CDATA[

            import flash.net.URLRequest;

            private var loader:Loader;

            private function createFill():void
            {
                loader = new Loader();
                loader.contentLoaderInfo.addEventListener(Event.COMPLETE,
completeLoad);
                loader.load(new URLRequest("../../assets/beach.jpg"));
            }

            private function completeLoad(event:Event):void
            {
                var bm:BitmapData = new BitmapData(loader.width, loader.height,
true,0x000000);
                bm.draw(this.loader);
                var m:Matrix = new Matrix();
                m.createBox(this.width/loader.width, this.height/loader.height);
                this.graphics.beginBitmapFill(bm, m, true, true);
                this.graphics.drawRoundRectComplex(0, 0, this.width, this.height,
20, 20, 20, 20);
                this.graphics.endFill();
            }

        ]]>
     </mx:Script>
 </mx:HBox>