Flex 弹出窗口居中显示

//居中显示   
				var registerwindow:registerWindow=new registerWindow();
				var W_width:Number=this.stage.stageWidth;    
                var H_heigth:Number=this.stage.stageHeight; 
                registerwindow.x=(W_width - registerwindow.width) / 2;    
                registerwindow.y=(H_heigth - registerwindow.height) / 2; 
                PopUpManager.addPopUp(registerwindow,this);