StreamController.broadcast({void onListen(), void onCancel(), bool sync: false })

A controller where stream can be listened to more than once.

The Stream returned by stream is a broadcast stream. It can be listened to more than once.

A Stream should be inert until a subscriber starts listening on it (using the onListen callback to start producing events). Streams should not leak resources (like websockets) when no user ever listens on the stream.

Broadcast streams do not buffer events when there is no listener.

The controller distributes any events to all currently subscribed listeners at the time when add, addError or close is called. It is not allowed to call add, addError, or close before a previous call has returned. The controller does not have any internal queue of events, and if there are no listeners at the time the event is added, it will just be dropped, or, if it is an error, be reported as uncaught.

Each listener subscription is handled independently, and if one pauses, only the pausing listener is affected. A paused listener will buffer events internally until unpaused or canceled.

If sync is true, events may be fired directly by the stream's subscriptions during an add, addError or close call. The returned stream controller is a SynchronousStreamController, and must be used with the care and attention necessary to not break the Stream contract. See Completer.sync for some explanations on when a synchronous dispatching can be used. If in doubt, keep the controller non-sync.

If sync is false, the event will always be fired at a later time, after the code adding the event has completed. In that case, no guarantees are given with regard to when multiple listeners get the events, except that each listener will get all events in the correct order. Each subscription handles the events individually. If two events are sent on an async controller with two listeners, one of the listeners may get both events before the other listener gets any. A listener must be subscribed both when the event is initiated (that is, when add is called) and when the event is later delivered, in order to receive the event.

The onListen callback is called when the first listener is subscribed, and the onCancel is called when there are no longer any active listeners. If a listener is added again later, after the onCancel was called, the onListen will be called again.

Source 

factory StreamController.broadcast(
    {void onListen(), void onCancel(), bool sync: false}) {
  return sync
      ? new _SyncBroadcastStreamController<T>(onListen, onCancel)
      : new _AsyncBroadcastStreamController<T>(onListen, onCancel);
}