code: function(cb) {
          var method = cb ? 'on' : 'off';
          this[method]('change', function(model) {
            var newModel = this._updateModel(model);
            if(_.isFunction(cb)){
              cb.call(this, newModel);
            }
          }, this);
        }
docs: Determine if the model will update for every local change.
  Provide a callback function to call events after the update.