code: public DataSource<Void> prefetchToDiskCache(
        ImageRequest imageRequest,
        Object callerContext) {
      return prefetchToDiskCache(imageRequest, callerContext, Priority.MEDIUM);
    }
docs: Submits a request for prefetching to the disk cache with a default priority.
  
  <p> Beware that if your network fetcher doesn't support priorities prefetch requests may slow
  down images which are immediately required on screen.
  
  @param imageRequest the request to submit
  @return a DataSource that can safely be ignored.