code: function(evt) {
          var tile = evt.object;
          var queued = false;
          var layer = tile.layer;
          var url = layer.getURL(tile.bounds);
          var img = this.tileCache[url];
          if (img && img.className !== 'olTileImage') {
              // cached image no longer valid, e.g. because we're olTileReplacing
              delete this.tileCache[url];
              OpenLayers.Util.removeItem(this.tileCacheIndex, url);
              img = null;
          }
          // queue only if image with same url not cached already
          if (layer.url && (layer.async || !img)) {
              // add to queue only if not in queue already
              var tileQueue = this.tileQueue[layer.map.id];
              if (!~OpenLayers.Util.indexOf(tileQueue, tile)) {
                  tileQueue.push(tile);
              }
              queued = true;
          }
          return !queued;
      }
docs: Method: queueTileDraw
  Adds a tile to the queue that will draw it.
  
  Parameters:
  evt - {Object} Listener argument of the tile's beforedraw event