code: public synchronized LottieTask<T> addListener(LottieListener<T> listener) {
      if (result != null && result.getValue() != null) {
        listener.onResult(result.getValue());
      }
  
      successListeners.add(listener);
      return this;
    }
docs: Add a task listener. If the task has completed, the listener will be called synchronously.
  @return the task for call chaining.