code: function _forEachRenderable(callback) {
          if (this._nodesById) {
              for (var key in this._nodesById) {
                  callback(this._nodesById[key]);
              }
          }
          else {
              var sequence = this._viewSequence.getHead();
              while (sequence) {
                  var renderable = sequence.get();
                  if (renderable) {
                      callback(renderable);
                  }
                  sequence = sequence.getNext();
              }
          }
      }
docs: Helper function to enumerate all the renderables in the datasource