code: function() {
          if(this.features) {
              for(var i=0; i<this.features.length; ++i) {
                  this.features[i].destroy();
              }
          }
          this.features = null;
          this.num = null;
      }
docs: Method: clearCache
  Clear out the cached features.  This destroys features, assuming
  nothing else has a reference.