code: public boolean addSubscribeListener(SubscribeListener subscribeListener) {
          Objects.requireNonNull(subscribeListener, "SubscribeListener argument must not be null");
          if (subscriptionMode != SubscriptionMode.manual) {
              previousSubscriptionMode = subscriptionMode;
              subscriptionMode = SubscriptionMode.manual;
          }
          return subscribeListeners.add(subscribeListener);
      }
docs: Add a subscribe listener, which is invoked on incoming subscription requests and if
  {@link SubscriptionMode} is set to {@link SubscriptionMode#manual}. This also sets subscription
  mode to {@link SubscriptionMode#manual}.
  
  @param subscribeListener the subscribe listener to add.
  @return <code>true</code> if the listener was not already added.
  @since 4.2