code: function(layer, callback) {
      layer = layer == undefined ? 0: layer;
      var self = this;
      this.getTiles(function(urls) {
        if(!urls) {
          callback(null);
          return;
        }
        if(callback) {
          callback(self._tileJSONfromTiles(layer, urls));
        }
      });
    }
docs: get tile json for layer