How to handle app lifecycle with Flutter (on Android and iOS)?

There is a method called when the system put the app in the background or return the app to foreground named didChangeAppLifecycleState.

Example with widgets:

  class _AppLifecycleReactorState extends State<AppLifecycleReactor> with WidgetsBindingObserver {
  @override
  void initState() {
    super.initState();
    WidgetsBinding.instance.addObserver(this);
  }

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

  AppLifecycleState _notification;

  @override
  void didChangeAppLifecycleState(AppLifecycleState state) {
    setState(() { _notification = state; });
  }

  @override
  Widget build(BuildContext context) {
    return new Text('Last notification: $_notification');
  }
}

Also there are CONSTANTS to know the states that an application can be in, eg:

1. inactive
2. paused
3. resumed
4. suspending

The usage of these constants would be the value of the constant e.g:

const AppLifecycleState(state)

Run the following code, press the home button and then reopen the app to see it working. There are 4 AppLifecycleState:

resumed: The application is visible and responding to user input.

inactive: The application is in an inactive state and is not receiving user input.

paused: The application is not currently visible to the user, not responding to user input, and running in the background.

detached: The application is still hosted on a flutter engine but is detached from any host views.

Null safe code:

class MyPage extends StatefulWidget {
  @override
  _MyPageState createState() => _MyPageState();
}

class _MyPageState extends State<MyPage> with WidgetsBindingObserver {
  @override
  void initState() {
    super.initState();
    WidgetsBinding.instance!.addObserver(this);
  }

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

  @override
  void didChangeAppLifecycleState(AppLifecycleState state) {
    super.didChangeAppLifecycleState(state);
    print('Current state = $state');
  }

  @override
  Widget build(BuildContext context) => Scaffold();
}