我正在尝试使用 Provider 和 Firebase 流式传输模型列表，并将字段值设置为我的应用程序中的另一个模型

我正在使用 Flutter、Provider 和 Firebase 构建一个生产力应用程序。我目前有许多流，我从 Firestore 中获取集合并将它们转换为模型列表。我遇到一个问题，当我尝试创建任务模型列表时，我返回一个空列表。在 Firestore 中，我有一组任务、状态和项目。我想将 task.project 值设置为等于项目集合中关联的项目模型。 主要.dart：

void main() {
  WidgetsFlutterBinding.ensureInitialized();
  runApp(ProductivityApp());
}

class ProductivityApp extends StatefulWidget {
  @override
  _ProductivityAppState createState() => _ProductivityAppState();
}

class _ProductivityAppState extends State<ProductivityApp> {
  bool _initialized = false;
  bool _error = false;

  void initializeFlutterFire() async {
    try {
      await Firebase.initializeApp();
      setState(() {
        _initialized = true;
      });
    } catch (e) {
      _error = true;
    }
  }

  @override
  void initState() {
    initializeFlutterFire();
    super.initState();
  }

  @override
  Widget build(BuildContext context) {
    if (_error) {
      return Center(child: Text('Something went wrong'));
    }
    if (!_initialized) {
      return Center(child: Text('Loading'));
    }
    return StreamProvider<User>.value(
      value: AuthService().user,
      child: MultiProvider(   // TODO: fix the issue for user to be logged in for the first time
        providers: [
          StreamProvider<List<Project>>.value(value: ProjectService().streamProjects()),
          StreamProvider<List<Task>>.value(value: TaskService().streamTasks(context)),
          StreamProvider<List<Status>>.value(value: StatusService().streamStatuses())
        ],
          child: MaterialApp(
            title: 'ProductivityApp',
            theme: appTheme(),
            // home: Wrapper(),
            // routes: routes,
            onGenerateRoute: generateRoute,
            initialRoute: '/',
          ),
      ),

    );
  }
}

任务流：

// Snapshot Conversion to Task Model and Stream
  Stream<List<Task>> streamTasks(BuildContext context) {
    var ref = _getTaskReference();
    return ref.snapshots().map((querySnapshot) => querySnapshot.docs
        .map((queryDocument) => Task.fromFirestore(queryDocument, context))
        .toList());
  }

任务模型：

class Task {
  String taskID = '';
  String taskName = 'Default Name';
  Project project = Project();
  Status status = Status();
  int taskTime = 0;
  DateTime dueDate = DateTime(1);
  DateTime createDate = DateTime.now();
  // List<String> subtasks = [];

  Task(
      {this.taskID,
      this.taskName,
      this.project,
      this.status,
      this.taskTime,
      this.dueDate,
      this.createDate});



  factory Task.fromFirestore(DocumentSnapshot snapshot, BuildContext context) {
    List<Project> projects = Provider.of<List<Project>>(context);
    List<Status> statuses = Provider.of<List<Status>>(context);
    Map data = snapshot.data();
    Project associatedProject = projects
        .firstWhere((project) => project.projectName == data['pojectName']);
    print(associatedProject);
    Status status = statuses
        .firstWhere((status) => status.statusName == data['statusName']);
    return Task(
        taskID: snapshot.id ?? '',
        taskName: data['taskName'].toString() ?? '',
        project: associatedProject ?? Project(),
        status: status ?? Status(),
        taskTime: data['taskTime'] ?? 0,
        dueDate:
            DateTimeFunctions().timeStampToDateTime(date: data['dueDate']) ??
                DateTime.now(),
        createDate:
            DateTimeFunctions().timeStampToDateTime(date: data['createDate']) ??
                DateTime.now());
  }
}

感谢您的任何帮助！

我终于找到了解决我的问题的方法！我使用了许多不同的资源并将它们组合在一起以获得一个工作模型。

首先，我实现了一个 Auth_widget_builder，它是根据用户信息重建的answer https://stackoverflow.com/a/59122982/14523024。这实际上解决了之前留待稍后解决的问题。通过使用这种方法，我能够根据用户在材料设计之上重建身份验证小部件，我需要访问用户集合中正确的 firestore 文档。

main.dart

@override
  Widget build(BuildContext context) {
    if (_error) {
      return Center(child: Text('Something went wrong'));
    }
    if (!_initialized) {
      return Center(child: Text('Loading'));
    }
    return Provider(
      create: (context) => AuthService(),
      child: AuthWidgetBuilder(builder: (context, userSnapshot) {
        return MaterialApp(
          title: 'Productivity App',
          theme: appTheme(),
          home: AuthWidget(userSnapshot: userSnapshot),
          // onGenerateRoute: generateRoute,
          // initialRoute: '/',
        );
      }),
    );
  }
}

其次，在 Auth_widget_builder 内部，我嵌套了两个 MultiProvider。第一个创建的提供者将生成我的服务类。然后我添加了 builder: 函数基于此answer https://stackoverflow.com/a/66872633/14523024，它返回了保存我的 StreamProvider 的第二个 MultiProvider。这是有效的，因为 builder: 函数重建了 BuildContext，这样我就能够访问其他提供者中的对象以保存为另一个对象中的字段。

auth_widget_builder.dart

/// Used to create user-dependant objects that need to be accessible by all widgets.
/// This widget should live above the [MaterialApp].
/// See [AuthWidget], a descendant widget that consumes the snapshot generated by this builder.

class AuthWidgetBuilder extends StatelessWidget {
  const AuthWidgetBuilder({Key key, @required this.builder}) : super(key: key);
  final Widget Function(BuildContext, AsyncSnapshot<UserModel>) builder;

  @override
  Widget build(BuildContext context) {
    print('AuthWidgetBuilder rebuild');
    final authService = Provider.of<AuthService>(context, listen: false);
    return StreamBuilder<UserModel>(
      stream: authService.onAuthStateChanged,
      builder: (context, snapshot) {
        print('StreamBuilder: ${snapshot.connectionState}');
        final UserModel user = snapshot.data;
        if (user != null) {
          return MultiProvider(
            providers: [
              Provider<UserModel>.value(value: user),
              Provider(create: (context) => ProjectService()),
              Provider(create: (context) => StatusService()),
              Provider(create: (context) => TaskService()),
              Provider(create: (context) => TimeService()),
            ],
            builder: (context, child) {
              return MultiProvider(
                providers: [
                  StreamProvider<List<Project>>.value(
                      value: Provider.of<ProjectService>(context)
                          .streamProjects()),
                  StreamProvider<List<Status>>.value(
                      value: Provider.of<StatusService>(context)
                          .streamStatuses()),
                  StreamProvider<List<Task>>.value(
                      value: Provider.of<TaskService>(context)
                          .streamTasks(context)),
                  StreamProvider<List<TimeEntry>>.value(
                      value: Provider.of<TimeService>(context)
                          .streamTimeEntries(context))
                ],
                child: builder(context, snapshot),
              );
            },
            child: builder(context, snapshot),
          );
        }
        return builder(context, snapshot);
      },
    );
  }
}

第三，我更新了服务类以查找适当的对象并将其传递给模型。

任务数据.dart

// Snapshot Conversion to Task Model and Stream
  Stream<List<Task>> streamTasks(BuildContext context) {
    CollectionReference ref = _getTaskReference();
    List<Project> projects;
    getProjects(context).then((projectList) => projects = projectList);
    List<Status> statuses;
    getStatuses(context).then((statusList) => statuses = statusList);
    return ref.snapshots().map((QuerySnapshot querySnapshot) =>
        querySnapshot.docs.map((QueryDocumentSnapshot queryDocument) {
          Project project = projects[projects.indexWhere((project) =>
              project.projectName ==
              queryDocument.data()['projectName'].toString())];
          Status status = statuses[statuses.indexWhere((status) =>
              status.statusName == queryDocument.data()['status'].toString())];
          return Task.fromFirestore(queryDocument, project, status);
        }).toList());
  }

  Future<List<Project>> getProjects(BuildContext context) async {
    List<Project> projects =
        await Provider.of<ProjectService>(context).streamProjects().first;
    return projects;
  }

  Future<List<Status>> getStatuses(BuildContext context) async {
    List<Status> statuses =
        await Provider.of<StatusService>(context).streamStatuses().first;
    return statuses;
  }

这个实现最好的部分是它全部发生在 Material Widget 之上，这意味着我可以使用正常范围之外的小部件（如 Navigator.push()）访问它。希望这适用于任何需要类似解决方案的人。