Flutter BlocProvider异常：上下文无StateStreamableSource

代码结构 #

AppCubit类 #

import 'package:auth_todo/models/shared/cubit/states.dart';
import 'package:flutter/material.dart';
import 'package:flutter_bloc/flutter_bloc.dart';
import 'package:sqflite/sqflite.dart';

class AppCubit extends Cubit<AppStates> {
  AppCubit() : super(AppInitialStates());
  static get(context) => BlocProvider.of(context);

  int selectedIndex = 0;

  List<Widget> screen = [
    const NewTasksScreen(),
    const ArchivedTasksScreen(),
    const DoneTasksScreen(),
  ];
  List<String> titles = [
    'New Tasks',
    'Done Tasks',
    'Archived Tasks',
  ];
  void changIndex(int index) {
    selectedIndex = index;
    emit(AppChangeBnvStates());
  }
  
}

AppStates状态类 #

abstract class AppStates {}

class AppInitialStates extends AppStates {}

class AppChangeBnvStates extends AppStates {}

class AppCreateDataBaseState extends AppStates {}

class AppInsertToDataBaseState extends AppStates {}

class AppGetFromDataBaseState extends AppStates {}

class AppChangBottomSheetState extends AppStates {}

使用BlocProvider的HomeLayout页面 #

// ignore_for_file: must_be_immutable

import 'package:auth_todo/models/shared/cubit/states.dart';
import 'package:flutter/material.dart';
import 'package:flutter_bloc/flutter_bloc.dart';
import 'package:google_nav_bar/google_nav_bar.dart';
import 'package:intl/intl.dart';
import '../models/shared/cubit/cubit.dart';
import '../widgets/components/dufalt_form_filed.dart';
import 'package:conditional_builder_null_safety/conditional_builder_null_safety.dart';

class HomeLayout extends StatelessWidget {

  TextEditingController taskTitelController = TextEditingController();
  TextEditingController taskTimeController = TextEditingController();
  TextEditingController taskDateController = TextEditingController();
  var scaffoldKey = GlobalKey<ScaffoldState>();
  var formKey = GlobalKey<FormState>();

  HomeLayout({super.key});

  @override
  Widget build(BuildContext context) {
    AppCubit cubit = AppCubit.get(context);
    return BlocProvider(
        create: (_) => AppCubit()..creatDatabase(),
        child: BlocConsumer<AppCubit, AppStates>(
          listener: (context, state) {
            if (state is AppInsertToDataBaseState) {
              Navigator.pop(context);
            }
          },
          builder: (context, state) => Scaffold(
            key: scaffoldKey,
            appBar: AppBar(
              title: Text(cubit.titles[cubit.selectedIndex]),
              centerTitle: true,
            ),
           
            body: ConditionalBuilder(
                condition: true,
                builder: (context) => cubit.screen[cubit.selectedIndex],
                fallback: (context) =>
                    const Center(child: CircularProgressIndicator())),
          ),
        ));
  }
}

异常信息 #

═══════════════════════════════════
构建HomeLayout(dirty)时抛出以下断言：
BlocProvider.of()调用传入的上下文不包含StateStreamableSource<Object?>。
从传入BlocProvider.of<StateStreamableSource<Object?>>()的上下文开始，未找到任何祖先组件。
这种情况可能是因为你使用的上下文来自BlocProvider上方的Widget。
使用的上下文为：HomeLayout(dirty)
引发错误的相关Widget是HomeLayout（main.dart:31）

解决方法 #

核心问题 #

你在HomeLayout的build方法开头就调用AppCubit.get(context)，但此时BlocProvider还未创建，当前上下文属于HomeLayout本身，没有任何祖先BlocProvider提供AppCubit实例，因此触发断言错误。

具体修复步骤 #

1. 删除build方法开头的AppCubit cubit = AppCubit.get(context);，这行代码在BlocProvider实例化前执行，上下文无法找到对应Cubit。
2. 在BlocConsumer的builder方法内部获取Cubit实例，因为BlocConsumer是BlocProvider的子组件，其上下文可以访问到已创建的Cubit。
3. 若需要在页面组件中使用Cubit属性，均通过BlocConsumer内部的上下文获取。

修复后的HomeLayout代码 #

// ignore_for_file: must_be_immutable

import 'package:auth_todo/models/shared/cubit/states.dart';
import 'package:flutter/material.dart';
import 'package:flutter_bloc/flutter_bloc.dart';
import 'package:google_nav_bar/google_nav_bar.dart';
import 'package:intl/intl.dart';
import '../models/shared/cubit/cubit.dart';
import '../widgets/components/dufalt_form_filed.dart';
import 'package:conditional_builder_null_safety/conditional_builder_null_safety.dart';

class HomeLayout extends StatelessWidget {

  TextEditingController taskTitelController = TextEditingController();
  TextEditingController taskTimeController = TextEditingController();
  TextEditingController taskDateController = TextEditingController();
  var scaffoldKey = GlobalKey<ScaffoldState>();
  var formKey = GlobalKey<FormState>();

  HomeLayout({super.key});

  @override
  Widget build(BuildContext context) {
    return BlocProvider(
        create: (_) => AppCubit()..creatDatabase(),
        child: BlocConsumer<AppCubit, AppStates>(
          listener: (context, state) {
            if (state is AppInsertToDataBaseState) {
              Navigator.pop(context);
            }
          },
          builder: (context, state) {
            // 在BlocConsumer内部获取Cubit实例
            AppCubit cubit = AppCubit.get(context);
            return Scaffold(
              key: scaffoldKey,
              appBar: AppBar(
                title: Text(cubit.titles[cubit.selectedIndex]),
                centerTitle: true,
              ),
              body: ConditionalBuilder(
                  condition: true,
                  builder: (context) => cubit.screen[cubit.selectedIndex],
                  fallback: (context) =>
                      const Center(child: CircularProgressIndicator())),
            );
          },
        ));
  }
}

额外优化建议 #

• 给AppCubit.get方法添加泛型约束，避免类型警告：

  static AppCubit get(context) => BlocProvider.of<AppCubit>(context);
  

• 确保creatDatabase方法已正确实现，否则会出现未定义方法的编译错误。

内容的提问来源于stack exchange，提问作者Taha Mohamed