有一个抽象类调用者WidgetsBindingObserver
https://docs.flutter.io/flutter/widgets/WidgetsBindingObserver-class.html
在

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

存在“状态”，可以管理为

switch(state) {
  case AppLifecycleState.resumed:
    // Handle this case
    break;
  case AppLifecycleState.inactive:
    // Handle this case
    break;
  case AppLifecycleState.paused:
    // Handle this case
    break;
  case AppLifecycleState.suspending:
    // Handle this case
    break;
}

这是一个完整的例子，演示了如何正确地处理事情，要测试这一点，按下主页按钮，并恢复应用程序，你会看到didChangeAppLifecycleState被调用。

class HomePage extends StatefulWidget {
  @override
  _HomePageState createState() => _HomePageState();
}
class _HomePageState extends State<HomePage> with WidgetsBindingObserver {
  @override
  void initState() {
    super.initState();
    // Add the observer. 
    WidgetsBinding.instance!.addObserver(this);
  }
  @override
  void dispose() {
    // Remove the observer
    WidgetsBinding.instance!.removeObserver(this);
    super.dispose();
  }
  @override
  void didChangeAppLifecycleState(AppLifecycleState state) {
    super.didChangeAppLifecycleState(state);
    // These are the callbacks
    switch (state) {
      case AppLifecycleState.resumed:
        // widget is resumed
        break;
      case AppLifecycleState.inactive:
        // widget is inactive
        break;
      case AppLifecycleState.paused:
        // widget is paused
        break;
      case AppLifecycleState.detached:
        // widget is detached
        break;
    }
  }
  @override
  Widget build(BuildContext context) => Scaffold();
}

最常见的情况是，如果您有一个动画正在运行，并且您不想在后台消耗资源，则需要执行此操作。在这种情况下，您应该使用TickerProviderStateMixin扩展State，并使用State作为AnimationController的vsync参数。Flutter只会在State可见时调用动画控制器的侦听器。
如果希望在PageRoute被其他内容遮挡时释放PageRoute中的State，可以将false的maintainState参数传递给PageRoute构造函数。如果这样做，State将在隐藏时重置自身（及其子对象），并且必须使用作为widget的构造函数参数传入的属性在initState中重新构造自身。如果不想完全重置，可以使用模型或控制器类或PageStorage来保存用户的进度信息。
下面是一个示例应用程序，演示了这些概念。

import 'package:flutter/material.dart';
void main() {
  runApp(new MaterialApp(
    onGenerateRoute: (RouteSettings settings) {
      if (settings.name == '/') {
        return new MaterialPageRoute<Null>(
          settings: settings,
          builder: (_) => new MyApp(),
          maintainState: false,
        );
      }
      return null;
    }
  ));
}
class MyApp extends StatefulWidget {
  MyAppState createState() => new MyAppState();
}
class MyAppState extends State<MyApp> with TickerProviderStateMixin {
  AnimationController _controller;
  @override
  void initState() {
    print("initState was called");
    _controller = new AnimationController(vsync: this)
      ..repeat(min: 0.0, max: 1.0, period: const Duration(seconds: 1))
      ..addListener(() {
        print('animation value ${_controller.value}');
      });
    super.initState();
  }
  @override
  void dispose() {
    print("dispose was called");
    _controller.dispose();
    super.dispose();
  }
  int _counter = 0;
  @override
  Widget build(BuildContext context) {
    return new Scaffold(
      appBar: new AppBar(
        title: new Text('home screen')
      ),
      body: new Center(
        child: new RaisedButton(
          onPressed: () {
            setState(() {
              _counter++;
            });
          },
          child: new Text('Button pressed $_counter times'),
        ),
      ),
      floatingActionButton: new FloatingActionButton(
        child: new Icon(Icons.remove_red_eye),
        onPressed: () {
          Navigator.push(context, new MaterialPageRoute(
            builder: (BuildContext context) {
              return new MySecondPage(counter: _counter);
            },
          ));
        },
      ),
    );
  }
}
class MySecondPage extends StatelessWidget {
  MySecondPage({ this.counter });
  final int counter;
  Widget build(BuildContext context) {
    return new Scaffold(
      appBar: new AppBar(
        title: new Text('Certificate of achievement'),
      ),
      body: new Column(
        crossAxisAlignment: CrossAxisAlignment.stretch,
        mainAxisAlignment: MainAxisAlignment.spaceAround,
        children: [
          new Icon(Icons.developer_mode, size: 200.0),
          new Text(
            'Congrats, you clicked $counter times.',
            style: Theme.of(context).textTheme.title,
            textAlign: TextAlign.center,
          ),
          new Text(
            'All your progress has now been lost.',
            style: Theme.of(context).textTheme.subhead,
            textAlign: TextAlign.center,
          ),
        ],
      ),
    );
  }
}

我有点晚，但来了完美的解决方案，为那些谁可能正在寻找它在未来。Navigator.push()实际上是未来。表示它有then()回调函数。所以then()将在您从第二个屏幕调用Navigator.pop()之后被调用。甚至你可以从第二个屏幕发送一些数据，并在第一个屏幕中访问数据。
举例说明：

//from Screen A
Navigator.of(context).push(MaterialPageRoute(builder:(context)=>B()))
.then((value)=>{ refresh() });
//in Screen B with data
Navigator.pop(context,[1]);
//or without data
Navigator.pop(context);

所以refresh()将在屏幕A的恢复上被调用。

我创建visibility_aware_state是为了让它的行为类似于Android的Activity.onResume()。它还考虑了弹出和推送导航。

class Example extends StatefulWidget {
  @override
  _ExampleState createState() => _ExampleState();
}
class _ExampleState extends VisibilityAwareState<Example> {
  @override
  Widget build(BuildContext context) {
    // return your widget
  }
  @override
  void onVisibilityChanged(WidgetVisibility visibility) {
    switch(visibility) {
      case WidgetVisibility.VISIBLE:
        // Like Android's Activity.onResume()
        break;
      case WidgetVisibility.INVISIBLE:
        // Like Android's Activity.onPause()
        break;
      case WidgetVisibility.GONE:
        // Like Android's Activity.onDestroy()
        break;
    }
    super.onVisibilityChanged(visibility);
  }
}

Mamnarock您的答案正确但不完整，您共享的链接不可用。
下面是完整的代码：

import 'package:flutter/material.dart';
class YourClass extends StatefulWidget {
  @override
  _YourClassState createState() => _YourClassState();
}
class _YourClassState extends State<YourClass>
    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) {
    switch (state) {
      case AppLifecycleState.resumed:
        // Handle this case
        break;
      case AppLifecycleState.inactive:
        // Handle this case
        break;
      case AppLifecycleState.paused:
        // Handle this case
        break;
      case AppLifecycleState.detached:
        // Handle this case
        break;
    }
  }
  @override
  Widget build(BuildContext context) {
    return Container();
  }
}

正如TeeTracker在评论中提到的：
这是一个应用程序级别的生命周期，这意味着当整体恢复或不活动或暂停时，而不是单个小部件。

如果您安装了Flutter Hooks（https://pub.dev/packages/flutter_hooks）：

@override
Widget build(BuildContext context) {
  useOnAppLifecycleStateChange((previous, current) => null);
}