avoid_unassigned_stream_subscriptions

v0.4.0 Warning Resource Management

Flags Stream.listen() calls whose return value (a StreamSubscription) is not assigned to a variable, returned, or passed as an argument. Without storing the subscription, you have no way to cancel it later, which leads to memory leaks and unexpected behavior.

Why use this rule

A StreamSubscription that is never stored cannot be cancelled. The listener keeps running indefinitely, holding references to the callback closure and everything it captures. This is especially problematic in StatefulWidgets where the stream may outlive the widget, causing setState() calls on a disposed State.

See also: Dart - Streams | StreamSubscription

Don’t

void example() {
  final stream = Stream.fromIterable([1, 2, 3]);

  // Subscription not assigned -- cannot cancel later
  stream.listen((event) {
    print(event);
  });
}

Do

void example() {
  final stream = Stream.fromIterable([1, 2, 3]);

  // Assigned to a variable -- can be cancelled later
  final subscription = stream.listen((event) {
    print(event);
  });
  subscription.cancel();
}

// Returning the subscription is also fine:
StreamSubscription<int> listen(Stream<int> stream) {
  return stream.listen((event) => print(event));
}

// Passing as an argument is also fine:
void track(List<StreamSubscription> subs, Stream<int> stream) {
  subs.add(stream.listen((event) => print(event)));
}

Configuration

To disable this rule:

plugins:
  many_lints:
    diagnostics:
      avoid_unassigned_stream_subscriptions: false