将flutter_bloc与tabview一起使用
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【中文标题】将flutter_bloc与tabview一起使用【英文标题】：Using flutter_bloc with tabview 【发布时间】：2021-02-14 03:31:36 【问题描述】：我有一个标签视图，其中包含流行的、最近的和即将到来的类别。他们对 api 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，并为每种类型的数据创建了一个主题。现在使用颤振块我想达到同样的效果。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;
【问题讨论】：
【参考方案1】：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。
我需要为每种类型创建 bloc 吗？
我向您建议的是为每个数据创建一个 BLoC（如示例），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且耦合较少，这很好。但是，如果您愿意，您仍然可以在一个 BLoC 中完成此操作。
如何从 api 获取数据，以便在切换选项卡时保持状态？
是的，只要使用相同的 BLoC/Cubit 实例，状态就会被持久化。每次构建集团（使用BlocBuilder）时，您都会获得最后一个状态。在我的示例中，我们只调用一次load() 事件，在呈现标签视图时。
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。我试图尽可能多地注释代码以帮助您理解。
另外我推荐BLOC from Zero to Hero 教程。对理解和正确使用 BLoC 库有很大帮助。
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 TabBar 中的onTap，如下所示。
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"),
],
),
注意：现在您不必在创建“Recent”和“Upcoming”cubits（非默认选项卡）时发出load() - 因为Tab 点击会处理这个问题。
BlocProvider<RecentCategoriesCubit>(
create: (context) =>
RecentCategoriesCubit(repository: repository),
),
BlocProvider<UpcomingCategoriesCubit>(
create: (context) =>
UpcomingCategoriesCubit(repository: repository),
),
【讨论】：
非常感谢。非常丰富的答案，也许是我从堆栈溢出中得到的最佳答案。我在每个选项卡小部件的 initstate 中发送事件，因此每次选项卡更改时，块都会调度加载事件。我可能需要对其进行一些调整才能使其正常工作。 不客气。我已经编辑了这篇文章，以便为您提供一个优雅的解决方案。 是的，这正是我在实现中所做的。尽管我调整了事件和状态并且我使用的是 bloc 而不是 cubit，但它按预期工作。到目前为止看起来不错。非常感谢:)以上是关于将flutter_bloc与tabview一起使用的主要内容，如果未能解决你的问题，请参考以下文章
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