code: function() {
      var projection = this.projection;
      
      this.datasources.forEach(function(datasource) {
        datasource.load && datasource.load(function(error) {
          datasource.project && datasource.project(projection);
        });
      });
    }
docs: Triggers all the map's datasources to prepare themselves. Usually this
  connecting to a database, loading and processing a file, etc.
  Calling this method is completely optional, but allows you to speed up
  rendering of the first tile.