followedBy method

Stream<T> followedBy(Stream<T> next)

Emits all values and errors from next following all values and errors from this stream.

If this stream never finishes, the next stream will never get a listener.

If this stream is a broadcast stream, the result will be as well. If a single-subscription follows a broadcast stream it may be listened to and never canceled since there may be broadcast listeners added later.

If a broadcast stream follows any other stream it will miss any events or errors which occur before this stream is done. If a broadcast stream follows a single-subscription stream, pausing the stream while it is listening to the second stream will cause events to be dropped rather than buffered.

Implementation

Stream<T> followedBy(Stream<T> next) {
  var controller = isBroadcast
      ? StreamController<T>.broadcast(sync: true)
      : StreamController<T>(sync: true);

  next = isBroadcast && !next.isBroadcast ? next.asBroadcastStream() : next;

  StreamSubscription<T>? subscription;
  var currentStream = this;
  var thisDone = false;
  var secondDone = false;

  late void Function() currentDoneHandler;

  void listen() {
    subscription = currentStream.listen(controller.add,
        onError: controller.addError, onDone: () => currentDoneHandler());
  }

  void onSecondDone() {
    secondDone = true;
    controller.close();
  }

  void onThisDone() {
    thisDone = true;
    currentStream = next;
    currentDoneHandler = onSecondDone;
    listen();
  }

  currentDoneHandler = onThisDone;

  controller.onListen = () {
    assert(subscription == null);
    listen();
    if (!isBroadcast) {
      controller
        ..onPause = () {
          if (!thisDone || !next.isBroadcast) return subscription!.pause();
          subscription!.cancel();
          subscription = null;
        }
        ..onResume = () {
          if (!thisDone || !next.isBroadcast) return subscription!.resume();
          listen();
        };
    }
    controller.onCancel = () {
      if (secondDone) return null;
      var toCancel = subscription!;
      subscription = null;
      return toCancel.cancel();
    };
  };
  return controller.stream;
}