从TiledMapServiceLayer类的API可知，fullExtent属性、tileInfo属性、units属性和getTileURL（）办法是子类必必要重写的，于是最简单的TianDiTuTiledMapServiceLayer类定义如下(From the API of the tiledmapservicelayer class, you can see that the full view property, the tileinfo attribute, the unit attribute, and the gettileurl () method are subclasses that need to be rewritten, so the simplest tianditutiledmapservicelayer class is defined as follows)