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),
),