Riverpod，在 BuildContext 和 Provider 之外读取状态

Question

我正在努力弄清楚为什么这不起作用（与说明它应该起作用的文档相反）。

我有一个类似这样的提供者

import 'package:hooks_riverpod/hooks_riverpod.dart';
import 'package:putin_flutter_client/api/client.dart';
import 'package:putin_flutter_client/api/storage.dart';

final userProvider = StateNotifierProvider((_) => UserNotifier());

class UserNotifier extends StateNotifier<UserState> {
  UserNotifier() : super(UserState());

  set username(String username) {
    state = UserState(username: username, password: state.password, jwt: state.jwt);
    secureStorageWrite('username', username);
  }

  set password(String password) {
    state = UserState(username: state.username, password: password, jwt: state.jwt);
    secureStorageWrite('password', password);
  }

  set jwt(String jwt) {
    state = UserState(username: state.username, password: state.password, jwt: jwt);
    Client.jwt = jwt;
    secureStorageWrite('jwt', jwt);
  }

  String get jwt {
    return state.jwt;
  }

  Future<void> initState() async {
    final user = await UserState.load();
    state.username = user.username;
    state.password = user.password;
    state.jwt = user.jwt;
  }
}

class UserState {
  String username;
  String password;
  String jwt;

  UserState({
    this.username,
    this.password,
    this.jwt,
  });

  static Future<UserState> load() async {
    return UserState(
      username: await secureStorageRead('username'),
      password: await secureStorageRead('password'),
      jwt: await secureStorageRead('jwt'),
    );
  }
}

最终深入某些小部件，这样的东西会更新状态

// usilizing the setter on the provider to update the state...
user.jwt = data['token'];

现在我在代码的其他部分管理 http 客户端。这显然无法访问BuildContext 等，因此我执行以下操作以从存储状态中检索 jwt 值。

import 'package:hooks_riverpod/hooks_riverpod.dart';
import 'package:http/http.dart' as http;
import 'package:putin_flutter_client/state/user.dart';

class Client extends http.BaseClient {
  final http.Client _client = http.Client();

  Future<http.StreamedResponse> send(http.BaseRequest request) {
    // Get the container as per riverpod documentation
    final container = ProviderContainer();
    // Access the value through the getter on the provider
    final jwt = container.read(userProvider).jwt;

    request.headers['user-agent'] = 'myclient::v1.0.0';
    request.headers['Content-Type'] = 'application/json';
    if (jwt != null) {
      request.headers['X-Auth-Token'] = jwt;
    }
    return _client.send(request);
  }
}

这总是 null 并且 UserState 几乎是空的（所有成员都是 null）。

在riverpod documentation 中它说这应该可以工作

test('counter starts at 0', () {
  final container = ProviderContainer();

  StateController<int> counter = container.read(counterProvider);
  expect(counter.state, 0);
});

有人可以帮我弄清楚我上面的例子有什么问题吗？

Answer 1

正如@moulte 所指出的（非常感谢），可以通过在外部实例化并通过UncontrolledProviderScope 将其注入到小部件范围内，将提供程序作为全局变量访问，并且独立于上下文。重要的部分是记住在应用程序终止之前处置全局提供程序，否则它将永远不会真正终止。 这是一个示例代码

/// file /state/container.dart
import 'package:hooks_riverpod/hooks_riverpod.dart';
final container = ProviderContainer();

/// file /main.dart
void main() async {
  runApp(MyApp());
}

class MyApp extends StatefulWidget {
  @override
  _MyApp createState() => _MyApp();
}

class _MyApp extends State<MyApp> {

  @override
  void dispose() {
    super.dispose();
    // disposing the globally self managed container.
    container.dispose();
  }

  @override
  Widget build(BuildContext context) {
    return UncontrolledProviderScope(container: container,
      child: MaterialApp(
      // The usual widget tree
    );
  }
}

/// Somewhere in a file that is not aware of the BuildContext
/// here's how client.dart accesses the provider
import 'package:hooks_riverpod/hooks_riverpod.dart';
import 'package:http/http.dart' as http;
import 'package:putin_flutter_client/state/container.dart';
import 'package:putin_flutter_client/state/user.dart';


class Client extends http.BaseClient {
  final http.Client _client = http.Client();

  Future<http.StreamedResponse> send(http.BaseRequest request) {
    // Simply accessing the global container and calling the .read function
    var jwt = container.read(userProvider.state).jwt;
    request.headers['user-agent'] = 'putin_flutter::v1.0.0';
    request.headers['Content-Type'] = 'application/json';
    if (jwt != null) {
      request.headers['X-Auth-Token'] = jwt;
    }
    return _client.send(request);
  }
}

Answer 2

ProviderContainer() 为您的提供者创建一个新实例，它不会获得实际状态。 您需要让您的客户依赖于这样的用户状态：

final clientProvider = Provider<Client>((ref){
    return Client(ref.watch(userProvider.state))
});
class Client extends http.BaseClient {
  Client(this._userState);
  final UserState _userState;
  final http.Client _client = http.Client();

  Future<http.StreamedResponse> send(http.BaseRequest request) {
    
    final jwt = _userState.jwt;

    request.headers['user-agent'] = 'myclient::v1.0.0';
    request.headers['Content-Type'] = 'application/json';
    if (jwt != null) {
      request.headers['X-Auth-Token'] = jwt;
    }
    return _client.send(request);
  }
}

当您的用户状态发生变化时，客户端将使用新值重新实例化

如果您不想每次都通过 read 方法重新实例化：

final clientProvider = Provider<Client>((ref){
    return Client(ref.read)
});
class Client extends http.BaseClient {
  Client(this._reader);
  final Reader _reader;
  final http.Client _client = http.Client();

  Future<http.StreamedResponse> send(http.BaseRequest request) {
    
    final jwt = _reader(userProvider.state).jwt;

    request.headers['user-agent'] = 'myclient::v1.0.0';
    request.headers['Content-Type'] = 'application/json';
    if (jwt != null) {
      request.headers['X-Auth-Token'] = jwt;
    }
    return _client.send(request);
  }
}

Answer 3

ProviderContainer() 用于在 Dart 中使用 RiverPod。 Flutter 中的等价物是 ProviderScope()，但这需要通过 widget 上下文链访问，类似于 provider 包。