error: Invalid constant value. while setting global key for scaffoldMessengerKey

Question

我想在没有 Scaffold 的情况下使用 snackbar 。发现很多类似的问题都在做同样的事情，但由于某种原因它对我不起作用。

    Future<void> main() async {
      await dotenv.load(fileName: '.env');
      runApp(const HomeScreens());
    }
      class HomeScreens extends StatefulWidget {
  const HomeScreens({Key? key}) : super(key: key);

  @override
  _HomeScreensState createState() => _HomeScreensState();
}

class _HomeScreensState extends State<HomeScreens> {
  late final GlobalKey<ScaffoldMessengerState>? scaffoldMessengerKey;
  @override
  Widget build(BuildContext context) {
    return ChangeNotifierProvider(
     create:(BuildContext context) => MyCurrentTrack(),
      child: const MaterialApp(
        scaffoldMessengerKey: scaffoldMessengerKey, // <= this
        home: LoginScreen(),
        debugShowCheckedModeBanner: false,
      ),
    );
  }
}

也试过这个，但总是出现同样的错误

class _HomeScreensState extends State<HomeScreens> {
  final GlobalKey<ScaffoldMessengerState> rootScaffoldMessengerKey = GlobalKey<ScaffoldMessengerState>();

  @override
  Widget build(BuildContext context) {
    return ChangeNotifierProvider(
     create:(BuildContext context) => MyCurrentTrack(),
      child: const MaterialApp(
        scaffoldMessengerKey: rootScaffoldMessengerKey, // <= this
        home: LoginScreen(),
        debugShowCheckedModeBanner: false,
      ),
    );
  }
}

Answer 1

您的 material 应用不再是常量，因为它采用变量作为参数。 const 值必须是不变的并且在运行时定义。您的 material 应用程序不能再保持不变，您需要删除 const 关键字：

  @override
  Widget build(BuildContext context) {
    return ChangeNotifierProvider(
     create:(BuildContext context) => MyCurrentTrack(),
      // child: const MaterialApp(
      child: MaterialApp( // no const keyword
        scaffoldMessengerKey: rootScaffoldMessengerKey,
        home: LoginScreen(),
        debugShowCheckedModeBanner: false,
      ),
    );
  }

error: Invalid constant value. while setting global key for scaffoldMessengerKey

error: Invalid constant value. while setting global key for scaffoldMessengerKey

flutter

dart

scaffold