code: function(evt) {
          if (this.active && window.localStorage &&
                  evt.tile instanceof OpenLayers.Tile.Image) {
              var tile = evt.tile,
                  url = tile.url;
              // deal with modified tile urls when both CacheWrite and CacheRead
              // are active
              if (!tile.layer.crossOriginKeyword && OpenLayers.ProxyHost &&
                      url.indexOf(OpenLayers.ProxyHost) === 0) {
                  url = OpenLayers.Control.CacheWrite.urlMap[url];        
              }
              var dataURI = window.localStorage.getItem("olCache_" + url);
              if (dataURI) {
                  tile.url = dataURI;
                  if (evt.type === "tileerror") {
                      tile.setImgSrc(dataURI);
                  }
              }
          }
      }
docs: Method: fetch
  Listener to the <fetchEvent> event. Replaces a tile's url with a data
  URI from the cache.
  
  Parameters:
  evt - {Object} Event object with a tile property.