未处理的异常： 错误： 无法<User>在此 MyApp 小组件上方找到正确的提供程序

Question

我正在构建一个用于注册和登录的示例应用程序，第一个页面显示了使用firebase的移动身份验证，在成功身份验证后，我从firebase获取uid，并尝试与我的云firestore集合('user'(的documenteID匹配。如果它给我null，那么我想显示SignupScreen((，否则，显示HomeScreen((。

所以这是我的简单逻辑，但我不知道发生了这种类型的错误，

我也尝试了一些不同的代码，那就是

ChangeNotifierProvider(
      create: (BuildContext context) => User(),
    ),

但它仍然显示出错误。

这是我的密码。

main.dart

import 'package:firebase_auth/firebase_auth.dart';
import 'package:flutter/material.dart';
import 'package:phone_auth_example/homePage.dart';
import 'package:phone_auth_example/signup_screen.dart';
import 'package:provider/provider.dart';
import './signUpPage.dart';
import './user.dart';
void main() {
  runApp(MyApp());
}
class MyApp extends StatefulWidget {
  @override
  _MyAppState createState() => _MyAppState();
}
class _MyAppState extends State<MyApp> {
  @override
  Widget build(BuildContext context) {
    final FirebaseAuth auth = FirebaseAuth.instance;
    var checkUser = false;
    void isNewUser() async {
      final FirebaseUser user = await auth.currentUser();
      final uid = user.uid;
      Provider.of<User>(context, listen: false).isUserNew(uid);
      checkUser = Provider.of<User>(context, listen: false).checkUser;
    }
    isNewUser();
    return MultiProvider(
        providers: [
          ChangeNotifierProvider.value(
            value: User(),
          ),
        ],
        child: MaterialApp(
          debugShowCheckedModeBanner: false,
          home: StreamBuilder(
            // ignore: deprecated_member_use
            stream: FirebaseAuth.instance.onAuthStateChanged,
            builder: (ctx, userSnapshot) {
              if (userSnapshot.hasData) {
                if (checkUser) {
                  return SignupScreen();
                } else {
                  return HomePage();
                }
              } else if (userSnapshot.hasError) {
                return CircularProgressIndicator();
              }
              return LoginPage();
            },
          ),
          routes: {
            SignupScreen.routeName: (ctx) => SignupScreen(),
          },
        ),);
  }
}

user.dart

import 'package:cloud_firestore/cloud_firestore.dart';
import 'package:flutter/cupertino.dart';
enum UserType {
  Member,
  HigherAuthority,
  SecurityGuard,
}
enum UserStatus {
  Owner,
  Renter,
}
enum OccupancyStatus {
  CurrentlyResiding,
  EmptyHouse,
}
class User with ChangeNotifier {
  final String id;
  final UserType userType;
  final String mobileNumber;
  final String name;
  final String email;
  final String password;
  final String houseNumberId;
  final UserStatus userStatus;
  final OccupancyStatus occupancyStatus;
  User({
    @required this.id,
    @required this.userType,
    @required this.mobileNumber,
    @required this.name,
    @required this.email,
    @required this.password,
    @required this.houseNumberId,
    @required this.userStatus,
    @required this.occupancyStatus,
  });
  bool checkUser;
  Future<void> isUserNew(String userId) async {
    // Get data from firestore
    DocumentSnapshot user =
        await Firestore.instance.collection('user').document(userId).get();
    //print(variable.data == null);
    // Check whether userId is in collection or not
    checkUser = (user.data == null) ? true : false;
  }
}

首先，它打开了LoginPage((，它工作得很好，然后它直接转到HomePage((而不是signupPage((，无论uid是否匹配，所以这是一个问题，它显示的错误是。。。

错误

I/Choreographer(10505): Skipped 246 frames!  The application may be doing too much work on its main thread.
E/flutter (10505): [ERROR:flutter/lib/ui/ui_dart_state.cc(177)] Unhandled Exception: Error: Could not find the correct Provider<User> above this MyApp Widget
E/flutter (10505):
E/flutter (10505): This likely happens because you used a `BuildContext` that does not include the provider
E/flutter (10505): of your choice. There are a few common scenarios:
E/flutter (10505):
E/flutter (10505): - The provider you are trying to read is in a different route.
E/flutter (10505):
E/flutter (10505):   Providers are "scoped". So if you insert of provider inside a route, then
E/flutter (10505):   other routes will not be able to access that provider.
E/flutter (10505):
E/flutter (10505): - You used a `BuildContext` that is an ancestor of the provider you are trying to read.
E/flutter (10505):
E/flutter (10505):   Make sure that MyApp is under your MultiProvider/Provider<User>.
E/flutter (10505):   This usually happen when you are creating a provider and trying to read it immediately.
E/flutter (10505):
E/flutter (10505):   For example, instead of:
E/flutter (10505):
E/flutter (10505):   ```
E/flutter (10505):   Widget build(BuildContext context) {
E/flutter (10505):     return Provider<Example>(
E/flutter (10505):       create: (_) => Example(),
E/flutter (10505):       // Will throw a ProviderNotFoundError, because `context` is associated
E/flutter (10505):       // to the widget that is the parent of `Provider<Example>`
E/flutter (10505):       child: Text(context.watch<Example>()),
E/flutter (10505):     ),
E/flutter (10505):   }
E/flutter (10505):   ```
E/flutter (10505):
E/flutter (10505):   consider using `builder` like so:
E/flutter (10505):
E/flutter (10505):   ```
E/flutter (10505):   Widget build(BuildContext context) {
E/flutter (10505):     return Provider<Example>(
E/flutter (10505):       create: (_) => Example(),
E/flutter (10505):       // we use `builder` to obtain a new `BuildContext` that has access to the provider
E/flutter (10505):       builder: (context) {
E/flutter (10505):         // No longer throws
E/flutter (10505):         return Text(context.watch<Example>()),
E/flutter (10505):       }
E/flutter (10505):     ),
E/flutter (10505):   }
E/flutter (10505):   ```
E/flutter (10505):
E/flutter (10505): If none of these solutions work, consider asking for help on StackOverflow:

Answer 1

您的问题是，您正试图调用Provider.of<User>(context, listen: false).checkUser，但上下文中尚未给定提供程序(因为您使用树下的MultiProvider(。

您应该向上移动MultiProvider。以下是一个实现示例：

import 'package:firebase_auth/firebase_auth.dart';
import 'package:flutter/material.dart';
import 'package:phone_auth_example/homePage.dart';
import 'package:phone_auth_example/signup_screen.dart';
import 'package:provider/provider.dart';
import './signUpPage.dart';
import './user.dart';
void main() {
  runApp(MyApp());
}
class MyApp extends StatefulWidget {
  @override
  _MyAppState createState() => _MyAppState();
}
class _MyAppState extends State<MyApp> {
  @override
  Widget build(BuildContext context) {
    @override
    Widget build(BuildContext context) {
      return MultiProvider(
        providers: [
          ChangeNotifierProvider<User>(
            create: (context) => User(),
          ),
        ],
        builder: (BuildContext context, Widget child) {
          final FirebaseAuth auth = FirebaseAuth.instance;
          var checkUser = false;
          void isNewUser() async {
            final FirebaseUser user = await auth.currentUser();
            final uid = user.uid;
            Provider.of<User>(context, listen: false).isUserNew(uid);
            checkUser = Provider.of<User>(context, listen: false).checkUser;
          }
          isNewUser();
          return child;
        },
        child: MaterialApp(
          debugShowCheckedModeBanner: false,
          home: StreamBuilder(
            // ignore: deprecated_member_use
            stream: FirebaseAuth.instance.onAuthStateChanged,
            builder: (ctx, userSnapshot) {
              if (userSnapshot.hasData) {
                if (checkUser) {
                  return SignupScreen();
                } else {
                  return HomePage();
                }
              } else if (userSnapshot.hasError) {
                return CircularProgressIndicator();
              }
              return LoginPage();
            },
          ),
          routes: {
            SignupScreen.routeName: (ctx) => SignupScreen(),
          },
        ),
      );
    }
  }
}

此外，还有2种ChangeNotifier的误用：

1 User类应扩展ChangeNotifier。

更换

class User with ChangeNotifier

带

class User extends ChangeNotifier

2如果您已经创建了ChangeNotificationer的实例(在您的案例中是User(，则应该使用ChangeNotificationerProvider.value：这里是对用例的一个很好的解释。既然你没有，你应该像这样使用ChangeNotificationProvider：

ChangeNotifierProvider<User>(
      create: (context) => User(),
      child: ...
)