[]拖动地图的时候怎么设置地图不会超出屏幕啊，例如官网的滚动地图

private function mouseMove(e:*=null):void

		{

			//移动地图视口

			tiledMap.moveViewPort(mX - (Laya.stage.mouseX - mLastMouseX), mY - (Laya.stage.mouseY - mLastMouseY));

		}

var starX:Number=mX - (Laya.stage.mouseX - mLastMouseX);

var starY:Number=mY - (Laya.stage.mouseY - mLastMouseY);

if(starX<=0)

{

     starX=0;

}

if(starY<=0)

{

    starY=0

}

if(starX>=400)

{

   starX=400

}

if（starY>=400）

{

   starY=400;

}

tiledMap.moveViewPort(starX,starY));