code: public static void callOnCancellationRequested(
        @Nullable List<ProducerContextCallbacks> callbacks) {
      if (callbacks == null) {
        return;
      }
      for (ProducerContextCallbacks callback : callbacks) {
        callback.onCancellationRequested();
      }
    }
docs: Calls {@code onCancellationRequested} on each element of the list. Does nothing if list == null