code: function(event) {
          if (!this.caching && this.filter) {
              var features = event.features;
              event.features = [];
              var feature;
              for (var i=0, ii=features.length; i<ii; ++i) {
                  feature = features[i];
                  if (this.filter.evaluate(feature)) {
                      event.features.push(feature);
                  } else {
                      this.cache.push(feature);
                  }
              }
          }
      }
docs: Method: handleAdd