flutter - 当 AppLifeCycleState.detached 被调用时？

Question

我有:

class _PageState extends State<Page> 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('state = $state');
  }

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

AppLifeCycleState类有 4 个回调，其中 3 个

- active
- paused
- resumed

似乎有效，但 detached从来没有在任何情况下工作。

我读了 documentation但在实际场景中无法理解，任何人都可以分享相关代码，何时何地被调用？

Answer 1

正如医生所说

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

When the application is in this state, the engine is running without a view. It can either be in the progress of attaching a view when engine was first initializes, or after the view being destroyed due to a Navigator pop.

只有当您的主页小部件进入后台时，您才能在主屏幕上重现上述问题(按 Android 设备的后退按钮)

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: HomeScreen(),
    );
  }
}



class HomeScreen extends StatefulWidget {
  @override
  _HomeScreenState createState() => _HomeScreenState();
}

class _HomeScreenState extends State<HomeScreen> with WidgetsBindingObserver {

  @override
  void initState() {
    super.initState();
    WidgetsBinding.instance.addObserver(this);
  }

  @override
  didChangeAppLifecycleState(AppLifecycleState state) {
    if (AppLifecycleState.paused == state) {}
    print("Status :" + state.toString());
  }
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Book'),
      ),
      body: Center(
        child: Text('Home Screen'),
      ),
    );
  }
}

您可以在其他屏幕上生成上述内容并在分离时调用，您可以通过在小部件的任何点击事件上以编程方式关闭应用程序来完成

安卓:

SystemChannels.platform.invokeMethod('SystemNavigator.pop');

iOS:

exit(0)