docs: private long next() {
          while(true) {
              final long index;
              synchronized (mTileAreas) {
                  if(!mTileIndices.hasNext()) {
                      return -1;
                  }
                  index = mTileIndices.next();
              }
              final Drawable drawable = mCache.getMapTile(index);
              if (drawable == null) {
                  return index;
              }
          }
      }