Broadcast streams

Learn multiple listeners with broadcast streams.

Broadcast streams

A standard Stream is single-subscription: a second listen throws. A broadcast stream allows multiple listeners.

To broadcast a stream, call .asBroadcastStream() or open it with StreamController.broadcast(). Every listener receives the same events.

Usage: fan out an event source (e.g. a timer, a WebSocket) to multiple parts of the app. However, stateful operators (buffer, debounce) may behave differently on broadcast streams.

asBroadcastStream

stream.asBroadcastStream() converts a single-subscription stream into a broadcast one. You can listen to the result as many times as you like.

Example: var b = Stream.fromIterable([1, 2, 3]).asBroadcastStream();. Both b.listen(print) and b.listen(print) receive 1, 2, 3.

Note: once asBroadcastStream is called, events are dispatched to listeners; a late listener misses earlier ones.

Exercise Listen to a broadcast stream

Make main async. Add var s = Stream.fromIterable([1, 2, 3]).asBroadcastStream();. Add await s.forEach(print);.

Show example solution

void main() async {
  var s = Stream.fromIterable([1, 2, 3]).asBroadcastStream();
  await s.forEach(print);
}

Test yourself

Which is a property of broadcast streams?
- Allows multiple listeners
- A second listener throws
- Accepts only a single listener
- Works only with Future
Show answer

Allows multiple listeners — a broadcast stream allows multiple listeners.

Convert to broadcast

var b = s.____();

Show answer

var b = s.asBroadcastStream();

Match stream types

Single-subscription

Single listener
Broadcast

Many listeners
asBroadcastStream

Conversion
StreamController.broadcast

Many-listener source

Next lesson: Stream transformer

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