asBroadcastStream method

Stream<T> asBroadcastStream(
  1. {void onListen(
    1. StreamSubscription<T> subscription
  2. void onCancel(
    1. StreamSubscription<T> subscription

Returns a multi-subscription stream that produces the same events as this.

The returned stream will subscribe to this stream when its first subscriber is added, and will stay subscribed until this stream ends, or a callback cancels the subscription.

If onListen is provided, it is called with a subscription-like object that represents the underlying subscription to this stream. It is possible to pause, resume or cancel the subscription during the call to onListen. It is not possible to change the event handlers, including using StreamSubscription.asFuture.

If onCancel is provided, it is called in a similar way to onListen when the returned stream stops having listeners. If it later gets a new listener, the onListen function is called again.

Use the callbacks, for example, for pausing the underlying subscription while having no subscribers to prevent losing events, or canceling the subscription when there are no listeners.

Cancelling is intended to be used when there are no current subscribers. If the subscription passed to onListen or onCancel is cancelled, then no further events are ever emitted by current subscriptions on the returned broadcast stream, not even a done event.


final stream =
    Stream<int>.periodic(const Duration(seconds: 1), (count) => count)

final broadcastStream = stream.asBroadcastStream(
  onCancel: (controller) {
    print('Stream paused');
  onListen: (controller) async {
    if (controller.isPaused) {
      print('Stream resumed');

final oddNumberStream = broadcastStream.where((event) => event.isOdd);
final oddNumberListener = oddNumberStream.listen(
      (event) {
    print('Odd: $event');
  onDone: () => print('Done'),

final evenNumberStream = broadcastStream.where((event) => event.isEven);
final evenNumberListener = evenNumberStream.listen((event) {
  print('Even: $event');
}, onDone: () => print('Done'));

await Future.delayed(const Duration(milliseconds: 3500)); // 3.5 second
// Outputs:
// Even: 0
// Odd: 1
// Even: 2
oddNumberListener.cancel(); // Nothing printed.
evenNumberListener.cancel(); // "Stream paused"
await Future.delayed(const Duration(seconds: 2));
print(await broadcastStream.first); // "Stream resumed"
// Outputs:
// 3


Stream<T> asBroadcastStream(
    {void onListen(StreamSubscription<T> subscription)?,
    void onCancel(StreamSubscription<T> subscription)?}) {
  return new _AsBroadcastStream<T>(this, onListen, onCancel);