code: public void addListener(IsSimplePromiseResponseHandler<T> listener) {
      if (handlers == null) {
        handlers = new LinkedList<>();
      }
  
      handlers.add(listener);
  
      if (response != null || exception != null) {
        listener.onResponse(this);
      }
    }
docs: Add a promise to do when Response comes in
  
  @param listener to add