flutter - 在 tabview 中使用 flutter_bloc

Question

我有一个 tabview，我有流行的、最近的和即将到来的类别。他们在 api 上都有相同的响应。我正在尝试使用 flutter_bloc 从 api 获取数据。以前我使用 rxdart 主题，我为每种类型的数据制作了一个主题。现在使用 flutter 块我想达到同样的目的。我想要做的是在选项卡之间切换。以前我使用 behaviorsubject 将数据保存到下一个事件，但现在我想转换为 bloc 模式。如何使用 flutter_bloc 获得相同类型的结果？或者我需要为每种类型创建块？最后，如何从 api 获取数据，以便在切换选项卡时保持状态？我的 Rxdart 实现:

class DataBloc {
  final DataRepo _repository = DataRepo();
  final BehaviorSubject<Data> _recent = BehaviorSubject<Data>();
  final BehaviorSubject<Data> _popular = BehaviorSubject<Data>();
  final BehaviorSubject<Data> _upcoming = BehaviorSubject<Data>();
  
getData(String type) async {
    
    Data response = await _repository.getData(type);
    if (type == "recent") {
      _recent.sink.add(response);
    } else if (type == "upcoming") {
      _upcoming.sink.add(response);
    } else {
      _popular.sink.add(response);
    }
  }

  dispose() {
    _recent?.close();
    _popular?.close();
    _upcoming?.close();
  }

  BehaviorSubject<Data> get recent => _recent;
  BehaviorSubject<Data> get popular => _popular;
  BehaviorSubject<Data> get upcoming => _upcoming;
}

Answer 1

对于您的问题，肯定没有单一的解决方案。我会回答你的问题，我会给你一个完整的实现/示例，以便于理解。

I need to create bloc for each type?

我建议您为每个数据创建一个 BLoC(如示例中所示)，因为它会简化 BLoC 的逻辑(特别是如果您不想一次加载所有数据)并且应用程序将结构化并且更少耦合哪个好。但是，如果您愿意，您仍然可以在一个 BLoC 中完成此操作。

How can I fetch data from api such that when tab is switched, state is persisted?

是 ，一旦它使用相同的 BLoC/Cubit 实例，状态就会持久化。每次构建块时(使用 BlocBuilder )，您都会获得最后一个状态。在我的例子中，我们调用 load()只有渲染选项卡 View 时才执行一次。

import 'package:bloc/bloc.dart';
import 'package:flutter/material.dart';
import 'package:flutter_bloc/flutter_bloc.dart';

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  ///
  /// The repository that is shared among all BLOCs
  ///
  final Repository repository = Repository();

  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    // For example purpose I will expose them as global cubits
    return MultiBlocProvider(
        providers: [
          BlocProvider<PopularCategoriesCubit>(
            create: (context) =>
                // Create the cubit and also call the LOAD event right away.
                //
                // NOTE #4. The cubit is created only when is requested (by
                // BlocBuilder, BlocListener, etc). This is why when you move to
                // a FRESH NEW tab you see the loading state.
                //
                PopularCategoriesCubit(repository: repository)..load(),
          ),
          BlocProvider<RecentCategoriesCubit>(
            create: (context) =>
                RecentCategoriesCubit(repository: repository)..load(),
          ),
          BlocProvider<UpcomingCategoriesCubit>(
            create: (context) =>
                UpcomingCategoriesCubit(repository: repository)..load(),
          ),
        ],
        child: MaterialApp(
          title: 'Flutter Demo',
          theme: ThemeData(
            primarySwatch: Colors.blue,
            visualDensity: VisualDensity.adaptivePlatformDensity,
          ),
          home: MyHomePage(),
        ));
  }
}

class MyHomePage extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return DefaultTabController(
      length: 3,
      child: Scaffold(
        appBar: AppBar(
          title: Text("Bloc Tabs"),
          bottom: TabBar(
            tabs: [
              Tab(text: "Popular"),
              Tab(text: "Recent"),
              Tab(text: "Upcoming"),
            ],
          ),
        ),
        body: TabBarView(
          children: [
            // POPULAR TAB
            BlocBuilder<PopularCategoriesCubit, GenericState>(
              builder: (context, state) {
                if (state.isFailed)
                  return Text("Failed to fetch popular categories.");

                if (state.isLoading)
                  return Text("Loading popular categories...");

                return ListView(
                  children: [
                    for (var category in state.categories) Text(category)
                  ],
                );
              },
            ),

            // RECENT TAB
            BlocBuilder<RecentCategoriesCubit, GenericState>(
              builder: (context, state) {
                if (state.isFailed)
                  return Text("Failed to fetch recent categories.");

                if (state.isLoading)
                  return Text("Loading recent categories...");

                return ListView(
                  children: [
                    for (var category in state.categories) Text(category)
                  ],
                );
              },
            ),

            // UPCOMMING TAB
            BlocBuilder<UpcomingCategoriesCubit, GenericState>(
              builder: (context, state) {
                if (state.isFailed)
                  return Text("Failed to fetch upcoming categories.");

                if (state.isLoading)
                  return Text("Loading upcoming categories...");

                return ListView(
                  children: [
                    for (var category in state.categories) Text(category)
                  ],
                );
              },
            ),
          ],
        ),
        // This trailing comma makes auto-formatting nicer for build methods.
      ),
    );
  }
}

// =============================================================================

///
/// Repository Mock
///
class Repository {
  ///
  /// Retreive data by type.
  ///
  /// NOTE #1. Is better to use enum instead of String.
  ///
  Future<List<String>> getData(String type) async {
    // Emulate netword delay
    return Future<List<String>>.delayed(Duration(seconds: 2)).then((_) {
      switch (type) {
        case "popular":
          return [
            "Popular 1",
            "Popular 2",
            "Popular 3",
            "Popular 5",
            "Popular 6"
          ];

        case "recent":
          return [
            "Recent 1",
            "Recent 2",
            "Recent 3",
          ];

        case "upcoming":
        default:
          return [
            "Upcomming 1",
            "Upcomming 2",
          ];
      }
    });
  }
}

///
/// This is a generic state used for all categories types
///
/// NOTE #2. Use Equatable. Also if you feel you can break this GenericState in
/// multiple classes as CategoriesLoadedState, CategoriesLoadingState,
/// CategoriesFailedState ...
///
class GenericState {
  ///
  /// Categories data
  ///
  final List<String> categories;

  ///
  /// Tells the data is loading or not
  ///
  final bool isLoading;

  ///
  /// Tells whether the state has errors or not
  ///
  final bool isFailed;

  GenericState(
      {this.categories, this.isLoading = false, this.isFailed = false});
}

///
/// Popular categories Cubit
///
class PopularCategoriesCubit extends Cubit<GenericState> {
  ///
  /// Repository dependency
  ///
  final Repository repository;

  ///
  /// Cubit constructor. Send a loading state as default.
  ///
  PopularCategoriesCubit({@required this.repository})
      : super(GenericState(isLoading: true));

  // ==================================
  // EVENTS
  // ==================================

  ///
  /// Load data from repository
  ///
  void load() async {
    //#log
    print("[EVENT] Popular Categories :: Load");

    // Every time when try to load data from repository put the application
    // in a loading state
    emit(GenericState(isLoading: true));

    try {
      // Wait for data from repository
      List categories = await this.repository.getData("popular");

      // Send a success state
      emit(GenericState(categories: categories, isFailed: false));
    } catch (e) {
      // For debugging
      print(e);
      // For example purpose we do not have a message
      emit(GenericState(isFailed: true));
    }
  }
}

///
/// Recent categories Cubit
///
class RecentCategoriesCubit extends Cubit<GenericState> {
  ///
  /// Repository dependency
  ///
  final Repository repository;

  ///
  /// Cubit constructor. Send a loading state as default.
  ///
  RecentCategoriesCubit({@required this.repository})
      : super(GenericState(isLoading: true));

  // ==================================
  // EVENTS
  // ==================================

  ///
  /// Load data from repository
  ///
  void load() async {
    //#log
    print("[EVENT] Recent Categories :: Load");

    // Every time when try to load data from repository put the application
    // in a loading state
    emit(GenericState(isLoading: true));

    try {
      // Wait for data from repository
      List categories = await this.repository.getData("recent");

      // Send a success state
      emit(GenericState(categories: categories, isFailed: false));
    } catch (e) {
      // For debugging
      print(e);
      // For example purpose we do not have a message
      emit(GenericState(isFailed: true));
    }
  }
}

///
/// Upcoming categories Cubit
///
class UpcomingCategoriesCubit extends Cubit<GenericState> {
  ///
  /// Repository dependency
  ///
  final Repository repository;

  ///
  /// Cubit constructor. Send a loading state as default.
  ///
  UpcomingCategoriesCubit({@required this.repository})
      : super(GenericState(isLoading: true));

  // ==================================
  // EVENTS
  // ==================================

  ///
  /// Load data from repository
  ///
  void load() async {
    //#log
    print("[EVENT] Upcoming Categories :: Load");

    // Every time when try to load data from repository put the application
    // in a loading state
    emit(GenericState(isLoading: true));

    try {
      // Wait for data from repository
      List categories = await this.repository.getData("upcoming");

      // Send a success state
      emit(GenericState(categories: categories, isFailed: false));
    } catch (e) {
      // For debugging
      print(e);
      // For example purpose we do not have a message
      emit(GenericState(isFailed: true));
    }
  }
}

只需将代码复制并粘贴到 main.dart 中即可查看结果。我尝试尽可能多地注释代码以帮助您理解。
我也推荐 BLOC from Zero to Hero 教程。它将对您理解 BLoC 库并正确使用它有很大帮助。

更新 1
每次更改选项卡时重新加载数据
要在每次更改选项卡时重新加载数据，您可以使用 onTap从 TabBar 如下。

TabBar(
  onTap: (tabIndex) {
    switch (tabIndex) {
      // Popular
      case 0:
        BlocProvider.of<PopularCategoriesCubit>(context).load();
        break;

      // Recent
      case 1:
        BlocProvider.of<RecentCategoriesCubit>(context).load();
        break;

      // Upcoming
      case 2:
        BlocProvider.of<UpcomingCategoriesCubit>(context).load();
        break;
    }
  },
  tabs: [
    Tab(text: "Popular"),
    Tab(text: "Recent"),
    Tab(text: "Upcoming"),
  ],
),

注:现在您不必发出 load()当创建最近的和即将到来的肘时(非默认选项卡) - 因为 Tab tap 会处理这些。

BlocProvider<RecentCategoriesCubit>(
  create: (context) =>
      RecentCategoriesCubit(repository: repository),
),
BlocProvider<UpcomingCategoriesCubit>(
  create: (context) =>
      UpcomingCategoriesCubit(repository: repository),
),