Esri为我们提供了TiledMapServiceLayer类，这个类正是所有瓦片地图服务都必须继承的类，像ArcGISTiledMapServiceLayer, OpenStreetMapLayer, VETiledLayer等esri提供的瓦片类都是继承了该类的，所以我们要扩展该TiledMapServiceLayer类(Esri provides an TiledMapServiceLayer class for us, this class is all tile map service