always-remove-listener

added in: 1.0.0 warning

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 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 those 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
    _thirdListener.addListener(listener); // LINT
    _disposedListener.addListener(listener);

    widget.someListener.addListener(listener); // LINT

    widget.anotherListener
      ..addListener(listener)
      ..addListener(listener)
      ..addListener(() {}); // LINT
  }

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

    widget.anotherListener.addListener(listener); // LINT

    _someListener.addListener(listener); // LINT

    _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() {
    // ...
  }
}