flex弹出窗口居中

	var popWin:ListTrace=ListTrace(PopUpManager.createPopUp(this, ListTrace, true));
	//居中显示
	var W_width:Number=this.stage.stageWidth;
	var H_heigth:Number=this.stage.stageHeight;
	popWin.x=(W_width - popWin.width) / 2;
	popWin.y=(H_heigth - popWin.height) / 2;