code: function(options) {
          var mapBounds = this.getMapBounds();
          if (mapBounds !== null && ((options && options.force) ||
            (this.layer.visibility && this.layer.calculateInRange() && this.invalidBounds(mapBounds)))) {
              this.calculateBounds(mapBounds);
              this.resolution = this.layer.map.getResolution(); 
              this.triggerRead(options);
          }
      }
docs: Method: update
  Callback function called on "moveend" or "refresh" layer events.
  
  Parameters:
  options - {Object} Optional object whose properties will determine
  the behaviour of this Strategy
  
  Valid options include:
  force - {Boolean} if true, new data must be unconditionally read.
  noAbort - {Boolean} if true, do not abort previous requests.