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);