docs: public void fetch(@Nullable Callback callback) {
      long started = System.nanoTime();
  
      if (deferred) {
        throw new IllegalStateException("Fit cannot be used with fetch.");
      }
      if (data.hasImage()) {
        // Fetch requests have lower priority by default.
        if (!data.hasPriority()) {
          data.priority(Priority.LOW);
        }
  
        Request request = createRequest(started);
  
        if (shouldReadFromMemoryCache(request.memoryPolicy)) {
          Bitmap bitmap = picasso.quickMemoryCacheCheck(request.key);
          if (bitmap != null) {
            if (picasso.loggingEnabled) {
              log(OWNER_MAIN, VERB_COMPLETED, request.plainId(), "from " + MEMORY);
            }
            if (callback != null) {
              callback.onSuccess();
            }
            return;
          }
        }
  
        Action action = new FetchAction(picasso, request, callback);
        picasso.submit(action);
      }
    }