如何在Flutter中获取当前上下文？

Question

我正在使用底部导航栏。在某个事件触发器上，我需要在当前屏幕上显示一些警报。

这就是我实现底部导航栏的方式。我有四个标签。在这里，当第4个选项卡的图标_isHomeDetected为true且用户单击该图标（即在索引3上）时，我需要更改该图标，无论用户位于哪个选项卡中，我都必须显示一条警报消息。我该如何做？

class LandingScreen extends StatefulWidget {
  static Widget bottomNavigationBar;
  ..
}

class _LandingScreenState extends State<LandingScreen> {
  ...
  StreamSubscription<String> _subscription;
  bool _isHomeDetected = false;

  @override
  void initState() {
    ...
    _subscription = server.messages.listen(onData, onError: onError);
  }
  onData(message) {
    setState(() {
      _isHomeDetected = json.decode(message)["isHomeBeacon"];
    });
  }
  ...
  @override
  Widget build(BuildContext context) {
    LandingScreen.bottomNavigationBar = new BottomNavigationBar(
        ....
    );
    return new Scaffold(
      body: _currentPage,
      bottomNavigationBar: LandingScreen.bottomNavigationBar,
    );
  }

  _navigateToScreens(int currentIndex) {
    List screens = [
      ..
    ];
    setState((){

      if (!_isHomeDetected || currentIndex != 3){
        _currentPage = screens[currentIndex];
      } else {
        Utils.showCupertinoAlert(
            context: context, content: "You wanna log out???");
      }
    });
  }
}

Answer 1

声明一个全局密钥

final globalNavigatorKey = GlobalKey<NavigatorState>();

void main() async {
  WidgetsFlutterBinding.ensureInitialized();
  runApp(MyApp());
}

然后将此全局密钥分配给您的MaterialApp

return MaterialApp(
          title: 'Flutter Demo',
          navigatorKey: globalNavigatorKey,
          theme: ThemeData(primarySwatch: Colors.blue,),
          home: MyHomePage(),
);

然后无论你想在哪里调用 example ：-

const snackBar = SnackBar(content: Text('Yay! A SnackBar!'));

ScaffoldMessenger.of(globalNavigatorKey.currentContext).showSnackBar(snackBar);

Answer 2

在任何有状态小部件的构建方法中，您将能够获取构建上下文。如果您做不到这一点，下面来自 flutter 团队的示例使用GlobalKey<ScaffoldState>to hace 访问来显示 SnackBar 警报。

https://github.com/flutter/flutter/blob/master/examples/flutter_gallery/lib/demo/cupertino/cupertino_alert_demo.dart