code: function() {
          
          var onScreen = false;
          if ((this.layer != null) && (this.layer.map != null)) {
              var screenBounds = this.layer.map.getExtent();
              onScreen = screenBounds.containsLonLat(this.lonlat);
          }    
          return onScreen;
      }
docs: Method: onScreen
  
  Returns:
  {Boolean} Whether or not the feature is currently visible on screen
  (based on its 'lonlat' property)