[ex.image]

Какой Rectangle? Под рисованием в данном посте имхо такой код имееться ввиду (здесь уже с масочкой):

package
{
	import flash.display.Shape;
	
	import mx.core.IUIComponent;
	import mx.core.UIComponent;
	
	public class Sample extends UIComponent implements IUIComponent
	{
		private var shapeMask:Shape
		
		public function Sample()
		{
			shapeMask = new Shape()
			addChild(shapeMask)
			
			mask = shapeMask
			
			graphics.beginFill(0xFF0000);
			graphics.drawRect(0, 0, 500, 500);
		}
		override protected function updateDisplayList(unscaledWidth:Number, unscaledHeight:Number):void
		{
			super.updateDisplayList(unscaledWidth, unscaledHeight)
			
			shapeMask.graphics.clear()
			shapeMask.graphics.beginFill(0x00FF00);
			shapeMask.graphics.drawRect(0, 0, unscaledWidth, unscaledHeight);
		}
	}
}

проверить можно этим

<?xml version="1.0" encoding="utf-8"?>
<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml" layout="absolute" xmlns:local="*" enterFrame="update()">
	<mx:Script>
		<![CDATA[
			private function update():void
			{
				sample.width = mouseX
				sample.height = mouseY
			}
		]]>
	</mx:Script>
	<local:Sample id="sample"/>
</mx:Application>