Dart streams

Skill almasumdev/awesome-flutter-agent-skills/.github/skills/concurrency_and_networking/dart-streams

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npx -y skills add almasumdev/awesome-flutter-agent-skills --skill dart-streams

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Best practices for Dart Streams, StreamController, subscriptions, and combining streams with rxdart. Use this when implementing reactive data flows or subscribing to events.

SKILL.md

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Dart Streams Best Practices

Instructions

Streams represent sequences of async events. Use them for reactive data (DB change feeds, websockets, user input events) — not for one-shot results (use Future).

1. Single vs Broadcast

Single-subscription (default): exactly one listener. Good for request/response-style sequences.
Broadcast: multiple listeners, events replayed only to active listeners. Use .asBroadcastStream() or StreamController<T>.broadcast().

2. Always Cancel Subscriptions

Leaking subscriptions causes memory leaks and duplicate work.

class _MyScreenState extends State<MyScreen> {
  StreamSubscription<Event>? _sub;

  @override
  void initState() {
    super.initState();
    _sub = bus.events.listen(_handle);
  }

  @override
  void dispose() {
    _sub?.cancel();
    super.dispose();
  }
}

In widgets, prefer StreamBuilder — it cancels automatically on dispose.

3. StreamController Patterns

class SearchController {
  final _queries = StreamController<String>.broadcast();
  Stream<String> get queries => _queries.stream;
  void submit(String q) => _queries.add(q);
  Future<void> dispose() => _queries.close();
}

Always close() the controller when done.
Expose Stream<T>, not StreamController<T>, to consumers.

4. Transforming Streams

repo.watchArticles()
    .where((list) => list.isNotEmpty)
    .map((list) => list.first)
    .distinct()
    .listen(_handle);

For richer operators (debounce, throttle, combineLatest, switchMap), use rxdart:

queries
    .debounceTime(const Duration(milliseconds: 300))
    .distinct()
    .switchMap((q) => repo.search(q).asStream())
    .listen(_renderResults);

5. async*/yield

Use generator functions for custom streams:

Stream<int> countdown(int from) async* {
  for (var i = from; i > 0; i--) {
    yield i;
    await Future<void>.delayed(const Duration(seconds: 1));
  }
}

6. Error Handling

.listen(onData, onError: ..., cancelOnError: false) — decide whether an error should terminate the subscription.
In async* generators, throw to propagate an error to the listener.
In StreamBuilder, always handle snapshot.hasError.

7. Back-Pressure

Dart streams have no built-in back-pressure for broadcast streams. If a producer is faster than the consumer:

Debounce, throttle, or sample (rxdart).
Buffer with a bounded queue and drop or merge.

8. Testing

Use the test package with expectLater(stream, emitsInOrder([...])) and StreamMatchers (emits, emitsDone, emitsError).

9. Checklist

Every listen has a matching cancel (or uses StreamBuilder).
Every StreamController is close()d.
Public API exposes Stream<T>, not the controller.
Errors on the stream are handled, not ignored.
distinct() used where duplicate events cause unnecessary work.