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 must inherit the class, such as ArcGISTiledMapServiceLayer, OpenStreetMapLayer, VETiledLayer and ESRI provide tile classes are inherited the class, so we want to extend the TiledMapServiceLayer clas)