code: function(mapBounds) {
          if(!mapBounds) {
              mapBounds = this.getMapBounds();
          }
          var invalid = !this.bounds || !this.bounds.containsBounds(mapBounds);
          if(!invalid && this.resFactor) {
              var ratio = this.resolution / this.layer.map.getResolution();
              invalid = (ratio >= this.resFactor || ratio <= (1 / this.resFactor));
          }
          return invalid;
      }
docs: Method: invalidBounds
  Determine whether the previously requested set of features is invalid.
  This occurs when the new map bounds do not contain the previously
  requested bounds.  In addition, if <resFactor> is set, it will be
  considered.
  
  Parameters:
  mapBounds - {<OpenLayers.Bounds>} the current map extent, will be
  retrieved from the map object if not provided
  
  Returns:
  {Boolean}