Flutter AppLifecycleState.resumed does not detect app open (Android onStart)

Question

According to the flutter AppLifecycleState documentation:

resumed → The application is visible and responding to user input

But It seems not to detect app opening.

I have implemented the AppLifecycleState in the main.dart:

SystemChannels.lifecycle.setMessageHandler((msg) {
    switch (msg) {
      case 'AppLifecycleState.paused':
        {
          function...
        }
        break;
      case 'AppLifecycleState.resumed':
        {
          function...
        }
        break;
    }
    return Future.value();
  });

How to check whether an application becomes in Foreground in Flutter (based on resume and app open)?

Answer 1

I don't know why the built in lifecycle management does not provide a app-open event, but this is how we do it.

We define our own enum that replicates the AppLifecycleState where we add opened:

/// see [AppLifecycleState] but added [opened]
enum AppState {
  opened, // <--
  resumed,
  paused,
  inactive,
  detached,
}

Then we define a widget that uses initialState to trigger the opened event and use the WidgetsBindingObserver mixing to detect the other changes.

class AppLifecycleTracker extends StatefulWidget {
  final Widget child;
  final void Function(AppState state) didChangeAppState;

  const AppLifecycleTracker({
    Key? key,
    required this.didChangeAppState,
    required this.child,
  }) : super(key: key);

  @override
  State<AppLifecycleTracker> createState() => _AppLifecycleTrackerState();
}

class _AppLifecycleTrackerState extends State<AppLifecycleTracker>
    with WidgetsBindingObserver {
  @override
  void initState() {
    super.initState();
    widget.didChangeAppState(AppState.opened);
    WidgetsBinding.instance.addObserver(this);
  }

  @override
  void dispose() {
    WidgetsBinding.instance.removeObserver(this);
    super.dispose();
  }

  @override
  void didChangeAppLifecycleState(AppLifecycleState state) {
    AppState s;
    switch (state) {
      case AppLifecycleState.resumed:
        s = AppState.resumed;
        break;
      case AppLifecycleState.inactive:
        s = AppState.inactive;
        break;
      case AppLifecycleState.paused:
        s = AppState.paused;
        break;
      case AppLifecycleState.detached:
        s = AppState.detached;
        break;
    }
    widget.didChangeAppState(state);
  }

  @override
  Widget build(BuildContext context) {
    return widget.child;
  }
}

Then be sure to add it at the very top of the widget tree to prevent it from being re-rendered (otherwise it would trigger the opened event again:

runApp(
  AppLifecycleTracker(
    didChangeAppState: (state) => log(state.name),
    child: ...,
  )
);