Flutter Bloc Builder 在 state 更改后未被調用且未更新 UI

Question

我對 Flutter Bloc package 有疑問。 主要思想是從 API 獲取用戶列表，我關閉了互聯網，看看我的錯誤 state 是如何工作的，但是當 try/catch 塊捕獲錯誤時，我的 Bloc Builder 沒有重建，我的 UI 沒有改變，我有做熱重啟看看結果。

用戶塊 class：

    import 'dart:io';

import 'package:chopper/chopper.dart';
import 'package:flutter/material.dart';
import 'package:flutter_bloc/flutter_bloc.dart';
import 'package:listviewblocflutter/bloc/user/user_event.dart';
import 'package:listviewblocflutter/bloc/user/user_state.dart';
import 'package:listviewblocflutter/data/error/network_error.dart';
import 'package:listviewblocflutter/data/service/user_service.dart';
import 'package:listviewblocflutter/model/built/built_user.dart';
import 'package:built_collection/built_collection.dart';

class UserBloc extends Bloc<UserEvent, UserState> {
  UserService _userService;

  UserBloc() {
    this._userService = UserService.create();
  }

  @override
  UserState get initialState => UserInitial();

  @override
  Stream<UserState> mapEventToState(UserEvent event) async* {
    yield UserLoading();
    if (event is GetAllUsers) {
      print("getallusers");
      try {
        final listOfUsers = await _userService.getAllUsers();
        yield UsersLoaded(listOfUsers.body);
        print("users loaded (userBloc)");
      } catch (_) {
        print("got an error (userBloc)");
        yield UserLoadingError("Can't get all users!");
      }
    } else if (event is GetUserById) {
      // single user event
    }
  }
}

用戶事件 class：

import 'package:equatable/equatable.dart';

abstract class UserEvent extends Equatable {
  const UserEvent();
}

class GetAllUsers extends UserEvent {
  const GetAllUsers();

  @override
  List<Object> get props => [];
}

class GetUserById extends UserEvent {
  final int id;

  const GetUserById(this.id);

  @override
  List<Object> get props => [id];
}

用戶狀態 class：

import 'package:equatable/equatable.dart';
import 'package:flutter/cupertino.dart';
import 'package:listviewblocflutter/model/built/built_user.dart';
import 'package:built_collection/built_collection.dart';

@immutable
abstract class UserState extends Equatable {
  const UserState();
}

class UserInitial extends UserState {
  const UserInitial();

  @override
  List<Object> get props => [];
}

class UserLoading extends UserState {
  const UserLoading();

  @override
  List<Object> get props => [];
}

class UserLoaded extends UserState {
  final BuiltUser user;

  const UserLoaded(this.user);

  @override
  List<Object> get props => [user];
}

class UsersLoaded extends UserState {
  final BuiltList<BuiltUser> listOfUsers;

  const UsersLoaded(this.listOfUsers);

  @override
  List<Object> get props => [listOfUsers];
}

class UserLoadingError extends UserState {
  final String message;

  const UserLoadingError(this.message);

  @override
  List<Object> get props => [message];
}

class UserEmpty extends UserState {
  const UserEmpty();

  @override
  List<Object> get props => [];
}

我在 main.dart 文件中用 BlocProvider 包裝了我的 HomePage，以在 HomePage 中訪問我的 UserBloc：

class MyApp extends StatelessWidget {
  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      debugShowCheckedModeBanner: false,
      title: 'ListView with BLoC pattern',
      home: SafeArea(
          child: Scaffold(
              body: BlocProvider(
                  create: (context) => UserBloc(), child: HomePage()))),
    );
  }
}

試圖將我的主頁 class 設置為 StateLessWidget，但問題仍然存在，現在我的 class 是 StateFulWidget 有同樣的問題。 我在 initState 方法中得到 AllUsers ：

import 'package:flutter/cupertino.dart';
import 'package:flutter/material.dart';
import 'package:flutter_bloc/flutter_bloc.dart';
import 'package:listviewblocflutter/bloc/bloc.dart';
import 'package:listviewblocflutter/model/built/built_user.dart';
import 'package:built_collection/built_collection.dart';

class HomePage extends StatefulWidget {
  @override
  _HomePageState createState() => _HomePageState();
}

class _HomePageState extends State<HomePage> {
  @override
  void initState() {
    BlocProvider.of<UserBloc>(context).add(GetAllUsers());
    super.initState();
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text("Users"),
      ),
      body: Container(
        child: BlocListener<UserBloc, UserState>(
          listener: (context, state) {
            if (state is UserLoadingError) {
              print("showing snackbar (listener)");
              _showNetworkErrorSnackbar(context, state.message);
            }
          },
          child: BlocBuilder<UserBloc, UserState>(
            builder: (context, state) {
              print("bloc builder");
              if (state is UserLoading || state is UserInitial)
                return _showLoadingStatus();
              else if (state is UsersLoaded)
                return _buildList(state.listOfUsers);
              else if (state is UserEmpty || state is UserLoadingError) {
                print("state is empty/error (builder)");
                return _emptyListLayout();
              }
              return Center();
            },
          ),
        ),
      ),
    );
  }

  ListView _buildList(BuiltList<BuiltUser> listOfUsers) {
    return ListView.builder(
        itemCount: listOfUsers.length,
        itemBuilder: (context, index) {
          return Card(
            child: ListTile(
              leading: CircleAvatar(
                backgroundColor: Colors.redAccent,
              ),
              title: Padding(
                padding: EdgeInsets.only(left: 10),
                child: Text(
                  listOfUsers[index].username,
                  overflow: TextOverflow.ellipsis,
                ),
              ),
            ),
          );
        });
  }

  Widget _showLoadingStatus() {
    return Center(
      child: CircularProgressIndicator(),
    );
  }

  Widget _emptyListLayout() {
    return Center(
      child: Text(
        "List is empty ;(",
        textAlign: TextAlign.center,
      ),
    );
  }

  void _showNetworkErrorSnackbar(BuildContext context, String message) {
    Scaffold.of(context).showSnackBar(SnackBar(
      content: Text(message),
    ));
  }
}

我的目標是告訴用戶列表為空並在出現 UserLoadingError state 時顯示一個小吃欄，但我只看到一個卡住的 LoadingIndicator 我的“打印”調試器在日志中說：
I/flutter (14034): 塊生成器
我/顫振（14034）：getallusers
I/flutter (14034): 塊生成器
I/flutter (14034): 出現錯誤 (userBloc)
I/flutter (14034)：顯示snackbar（監聽器）

預期成績：
I/flutter (14034): 塊生成器
我/顫振（14034）：getallusers
I/flutter (14034): 塊生成器
I/flutter (14034): 出現錯誤 (userBloc)
I/flutter (14034): 顯示snackbar (listener) /// 在這條消息之后應該出現一個snackbar
I/flutter (14034): 塊生成器
I/flutter (14034): state is empty/error (builder) /// 應在此消息之后創建一個空列表布局

pubspecs.yaml 片段：
依賴項：
flutter：
sdk：flutter
庫比蒂諾圖標：^0.1.3
斬波器：^3.0.2
等價的：^1.1.1
顫振塊：^4.0.0
提供者：^4.1.1
內置值：^7.1.0

dev_dependencies：
斬波器生成器：^3.0.4
build_runner：^1.10.0
內置值生成器：^7.1.0
顫振測試：
sdk：flutter

我可以上傳我的 UserService class，在那里我使用 Chopper 從 API 獲取用戶，如果需要，請告訴我。

Answer 1

最后我發現了一個問題。 問題出在我的 BlockBuilder if/else 樹中，更准確地說：

if (state is UserLoading || state is UserInitial)
                return _showLoadingStatus();

我變成了：

if (state is UserLoading)
                return _showLoadingStatus();

現在問題消失了。