code: @Override
      protected boolean isTileStale(Tile tile, TileBitmap bitmap) {
          return this.mapDataStore.getDataTimestamp(tile) > bitmap.getTimestamp();
      }
docs: Whether the tile is stale and should be refreshed.
  <p/>
  This method is called from {@link #draw(org.mapsforge.core.model.BoundingBox, byte, org.mapsforge.core.graphics.Canvas, org.mapsforge.core.model.Point)} to determine whether the tile needs to
  be refreshed.
  <p/>
  A tile is considered stale if the timestamp of the layer's {@link #mapDataStore} is more recent than the
  {@code bitmap}'s {@link org.mapsforge.core.graphics.TileBitmap#getTimestamp()}.
  <p/>
  When a tile has become stale, the layer will first display the tile referenced by {@code bitmap} and attempt to
  obtain a fresh copy in the background. When a fresh copy becomes available, the layer will replace is and update
  the cache. If a fresh copy cannot be obtained for whatever reason, the stale tile will continue to be used until
  another {@code #draw(BoundingBox, byte, Canvas, Point)} operation requests it again.
  
  @param tile   A tile.
  @param bitmap The bitmap for {@code tile} currently held in the layer's cache.