avoid_unassigned_stream_subscriptions

v0.4.0WarningConfigurableResource 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 | Dart lint: cancel_subscriptions

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)));
}

Turning this rule off

This rule is in the core preset, so it is on with preset: core, preset: recommended or preset: opinionated.

To turn it off:

many_lints.yaml

rules:
  avoid_unassigned_stream_subscriptions: false

To keep the rule on but skip certain paths, use per-rule exclude.

Options

analysis_options.yaml

analysis_options.yaml

many_lints:
  rules:
    avoid_unassigned_stream_subscriptions:
      ignored_instances: [eventBus]

many_lints.yaml

many_lints.yaml

rules:
  avoid_unassigned_stream_subscriptions:
    ignored_instances: [eventBus]

Option	Type	Default	Description
ignored_instances	list of strings	[]	Receiver expressions whose subscriptions are torn down centrally

Related rules

• always_remove_listener — Ensure every addListener() has a matching removeListener() in dispose().
• avoid_late_final_reassignment — Flag a late final field assigned twice on one path.
• avoid_unremovable_callbacks_in_listeners — Don’t pass an inline closure to addListener.
• dispose_fields — Ensure State fields with disposal methods are cleaned up in dispose().