Flutter detect killing off the app

See also https://flutter.io/flutter-for-android/#how-do-i-listen-to-android-activity-lifecycle-events

You can listen for inactive, paused, and detached.
This might be a bit too early but usually, it’s better to do some cleanup a bit too early and too often than not at all:

WidgetsBinding.instance.addObserver(LifecycleEventHandler(
    detachedCallBack: () async => widget.appController.persistState(),
    resumeCallBack: () async {
      _log.finest('resume...');
    }));

class LifecycleEventHandler extends WidgetsBindingObserver {
  LifecycleEventHandler({this.resumeCallBack, this.detachedCallBack});

  final FutureVoidCallback resumeCallBack;
  final FutureVoidCallback detachedCallBack;

//  @override
//  Future<bool> didPopRoute()

//  @override
//  void didHaveMemoryPressure()

  @override
  Future<void> didChangeAppLifecycleState(AppLifecycleState state) async {
    switch (state) {
      case AppLifecycleState.inactive:
      case AppLifecycleState.paused:
      case AppLifecycleState.detached:
        await detachedCallBack();
        break;
      case AppLifecycleState.resumed:
        await resumeCallBack();
        break;
    }
    _log.finest('''
=============================================================
               $state
=============================================================
''');
  }

//  @override
//  void didChangeLocale(Locale locale)

//  @override
//  void didChangeTextScaleFactor()

//  @override
//  void didChangeMetrics();

//  @override
//  Future<bool> didPushRoute(String route)
}

Edit

With this pull request on 4th November 2019, the enum AppLifecycleState.suspending was renamed to AppLifecycleState.detached. If you are using Flutter with a version prior to 1.12, you must still use AppLifecycleState.suspending.