flutter :使用 Provider 时状态在热重载时丢失

Question

我正在使用 Provider 来管理我的应用程序的状态。以下是我的实现方式。

催眠.dart

class _HypnoseAppState extends State<HypnoseApp> {
  @override
  Widget build(BuildContext context) {
    UserService userService = UserService();
    AudioUtilService audioUtilService = AudioUtilService();

    return MultiProvider(
      providers: [
        ChangeNotifierProvider<UserService>.value(
          value: userService,
        ),
        ChangeNotifierProvider<AudioUtilService>.value(
          value: audioUtilService,
        )
      ],
      child: MaterialApp(
          debugShowCheckedModeBanner: false,
          title: Globals.title,
          theme: ThemeData(primarySwatch: Colors.cyan),
          darkTheme: ThemeData.dark(),
          initialRoute: '/',
          routes: {
            '/': (BuildContext context) => WelcomeScreen(userService),
            '/home': (BuildContext context) => HomePageSwitcher(),
            '/audiocreate': (BuildContext context) => AudioCreateScreen()
          }),
    );
  }
}

home_switcher.dart

class HomePageSwitcher extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Consumer<UserService>(
      builder: (BuildContext context, UserService userService, Widget child) {
        return Scaffold(
            appBar: AppBar(),
            drawer: Drawer(
              child: Column(
                children: <Widget>[
                  UserAccountsDrawerHeader(
                    accountEmail: Text(userService.loggedInUser.email),
                    accountName: Text(userService.loggedInUser.name),
                    currentAccountPicture:
                        Image.network(userService.loggedInUser.avatar),
                  )
                ],
              ),
            ),
            body: Center(
              child: RaisedButton(
                child: Text('Sign out'),
                onPressed: () async {
                  await userService.signOut();
                  Navigator.pushNamed(context, '/');
                },
              ),
            ));
      },
    );
  }
}

user_service.dart

class UserService extends ChangeNotifier {
  // Get auth instances
  final GoogleSignIn _googleSignIn = GoogleSignIn();
  final FirebaseAuth _auth = FirebaseAuth.instance;

  // Store reference of user collection
  final CollectionReference userDb = Firestore.instance.collection('user');

  // Master instance of logged in user
  User _loggedInUser;

  // Getter to access loggedInUser
  User get loggedInUser {
    return _loggedInUser;
  }

  PublishSubject<AuthState> _authStateSubject = PublishSubject();

.... other code

现在这里的问题是，每次我热重载时，在主页上，我开始得到 NoSuchMethodError，因为它说电子邮件、名称等属性在 null 上被调用，我认为这意味着状态是丢失的。我怎样才能克服同样的问题？我做错了什么吗？

Answer 1

您不应使用 ChangeNotifierProvider.value。而是使用默认构造函数:

ChangeNotifierProvider(
  builder: (_) => UserService(),
)

否则，您的构建方法是不纯的，您将遇到 How to deal with unwanted widget build? 中描述的问题