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always-remove-listener

added in: 1.6.0
warning
Free+
preset: recommended

Warns when an event listener is added but never removed.

StatefulWidget classes sometimes add event listeners but fail to remove them. This results in memory leaks if the ValueListenable lifecycle is significantly longer than the lifecycle of the widget.

Every listener added manually needs to be removed typically in the dispose method. If listeners are added in didUpdateWidget or updateDependencies then they also need to be removed from these methods as otherwise widgets end up with multiple listeners.

Example​

❌ Bad:

class ShinyWidget {
  final someListener = Listener();
  final anotherListener = Listener();

  const ShinyWidget();
}

class _ShinyWidgetState extends State {
  final _someListener = Listener();
  final _anotherListener = Listener();
  final _thirdListener = Listener();
  final _disposedListener = Listener();

  const _ShinyWidgetState();

  @override
  void initState() {
    super.initState();

    _someListener.addListener(listener);
    _anotherListener.addListener(listener); // LINT: This listener is not removed, which can lead to a memory leak.
    _thirdListener.addListener(listener); // LINT: This listener is not removed, which can lead to a memory leak.
    _disposedListener.addListener(listener);

    widget.someListener.addListener(listener); // LINT: This listener is not removed, which can lead to a memory leak.

    widget.anotherListener
      ..addListener(listener)
      ..addListener(listener)
      ..addListener(() {}); // LINT: This listener is not removed, which can lead to a memory leak.
  }

  @override
  void didUpdateWidget(ShinyWidget oldWidget) {
    widget.someListener.addListener(listener);
    oldWidget.someListener.removeListener(listener);

    widget.anotherListener.addListener(listener); // LINT: This listener is not removed, which can lead to a memory leak.

    _someListener.addListener(listener); // LINT: This listener is not removed, which can lead to a memory leak.

    _anotherListener.removeListener(listener);
    _anotherListener.addListener(listener);
  }

  void dispose() {
    _someListener.removeListener(listener);
    _thirdListener.removeListener(wrongListener);

    _disposedListener.dispose();

    widget.anotherListener.removeListener(listener);

    super.dispose();
  }

  void listener() {
    // ...
  }

  void wrongListener() {
    // ...
  }
}

✅ Good:

class ShinyWidget {
  final someListener = Listener();
  final anotherListener = Listener();

  const ShinyWidget();
}

class _ShinyWidgetState extends State {
  final _someListener = Listener();
  final _disposedListener = Listener();

  const _ShinyWidgetState();

  @override
  void initState() {
    super.initState();

    _someListener.addListener(listener);

    _disposedListener.addListener(listener);

    widget.anotherListener.addListener(listener);
  }

  @override
  void didUpdateWidget(ShinyWidget oldWidget) {
    widget.someListener.addListener(listener);
    oldWidget.someListener.removeListener(listener);

    widget.anotherListener.removeListener(listener);
    widget.anotherListener.addListener(listener);

    _someListener.removeListener(listener);
    _someListener.addListener(listener);
  }

  void dispose() {
    _someListener.removeListener(listener);

    _disposedListener.dispose();

    widget.anotherListener.removeListener(listener);

    super.dispose();
  }

  void listener() {
    // ...
  }
}