Flutter报错：type '_Type' is not a subtype of type 'BuildContext'求解

DevOps 小满

2026-8-20

Flutter中添加TasksCubit后触发type '_Type' is not a subtype of type 'BuildContext'错误的解决方法

错误概述

添加TasksCubit类后，运行Flutter项目触发类型错误：type '_Type' is not a subtype of type 'BuildContext'，无法定位错误触发点，以下是问题分析与解决步骤。

错误堆栈

═══════ Exception caught by widgets library ═══════════════════════════════════
The following _TypeError was thrown building MyApp(dirty):
type '_Type' is not a subtype of type 'BuildContext'

The relevant error-causing widget was
MyApp
package:my_first_app/main.dart:15
When the exception was thrown, this was the stack
#0      TasksCubit.get
package:my_first_app/…/mycubitclasses/taskscubit_class.dart:14
#1      new HomeLayout
package:my_first_app/layout/homelayout.dart:29
#2      MyApp.build
package:my_first_app/main.dart:24
#3      StatelessElement.build
package:flutter/…/widgets/framework.dart:4876
#4      ComponentElement.performRebuild
package:flutter/…/widgets/framework.dart:4806
#5      Element.rebuild
package:flutter/…/widgets/framework.dart:4529
#6      ComponentElement._firstBuild
package:flutter/…/widgets/framework.dart:4787
#7      ComponentElement.mount
package:flutter/…/widgets/framework.dart:4781
#8      Element.inflateWidget
package:flutter/…/widgets/framework.dart:3817
#9      Element.updateChild
package:flutter/…/widgets/framework.dart:3551
#10     RenderObjectToWidgetElement._rebuild
package:flutter/…/widgets/binding.dart:1202
#11     RenderObjectToWidgetElement.mount
package:flutter/…/widgets/binding.dart:1171
#12     RenderObjectToWidgetAdapter.attachToRenderTree.<anonymous closure>
package:flutter/…/widgets/binding.dart:1119
#13     BuildOwner.buildScope
package:flutter/…/widgets/framework.dart:2597
#14     RenderObjectToWidgetAdapter.attachToRenderTree
package:flutter/…/widgets/binding.dart:1118
#15     WidgetsBinding.attachRootWidget
package:flutter/…/widgets/binding.dart:953
#16     WidgetsBinding.scheduleAttachRootWidget.<anonymous closure>

相关代码

main.dart

import 'package:bloc/bloc.dart';
import 'package:flutter/material.dart';
import 'package:my_first_app/layout/homelayout.dart';
import 'package:my_first_app/moduls/login/login.dart';
import 'package:my_first_app/moduls/messanger/messangerscreen.dart';
import 'package:my_first_app/moduls/counter/counter_screen.dart';
import 'package:my_first_app/moduls/messanger/mydesign.dart';
import 'moduls/homescreen/HomeScreen.dart';
import 'moduls/counter/counter_screen.dart';
import 'moduls/bmiscreen/ibm_calculator.dart';
import 'shared/bloc_observ.dart';

void main() {
  //  Bloc.observer = MyBlocObserver();
 return runApp(MyApp());
}
//=========================================================
class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) 
  {  
    return MaterialApp( 
      debugShowCheckedModeBanner: false  ,
      home: HomeLayout(),
    );
  }
}

HomeLayout页面代码

// ignore_for_file: prefer_const_constructors

import 'dart:ffi';
import 'package:conditional_builder_null_safety/conditional_builder_null_safety.dart';
import 'package:flutter/material.dart';
import 'package:flutter_bloc/flutter_bloc.dart';
import 'package:intl/intl.dart';
import 'package:my_first_app/moduls/archive_task_screen/archive_task.dart';
import 'package:my_first_app/moduls/done_task_screen/done_task.dart';
import 'package:my_first_app/moduls/new_task_screen/new_task.dart';
import 'dart:async';
import 'package:flutter/widgets.dart';
import 'package:my_first_app/shared/componants/componants.dart';
import 'package:my_first_app/shared/mycubitclasses/taskscubit_class.dart';
import 'package:my_first_app/shared/mycubitclasses/taskstates.dart';
import 'package:path/path.dart';
import 'package:sqflite/sqflite.dart';
import '../shared/componants/constants.dart';
class HomeLayout extends StatelessWidget 
{
  //======================variables===============================
  
  var scaffoldkey = GlobalKey<ScaffoldState>();
  var formkey = GlobalKey<FormState>();
  var titleController = TextEditingController();
  var timeController = TextEditingController();
  var dateController = TextEditingController();
 
  TasksCubit cubitt = TasksCubit.get(context);
  // scaffold contain in this screen :
  // 1. appBar  2. FAB floatingActionButton 3. body 4. bottomNavigationBar
//=================================================================
  //=================================================================
  @override
  Widget build(BuildContext context) 
  {
    return BlocProvider (
      
      create:  (context) {
        return TasksCubit()..creatDataBase();
      },
      child: Scaffold(
        key: scaffoldkey,
    //========================app bar =================================
        appBar: AppBar(
          title: Text(
            cubitt.titls[cubitt.currentindex],
          ),
        ),
    //===========================body ======================================
     
        body:  BlocBuilder<TasksCubit,TasksStates>(
          builder:  (BuildContext context, TasksStates state)
          {
            if (state is InsertDataBase)
         {
          Navigator.pop(context);
         }
         return cubitt.screens[cubitt.currentindex];
         
          }),
    //===========================FAB======================================
        floatingActionButton: FloatingActionButton(
          onPressed: () 
          {
            if (cubitt. isBottomSheetShown) 
            {
             cubitt.InserttTODataBase(
                titel: titleController.text,
                date: dateController.text,
                time: timeController.text,
                status: 'new',
              ).then((value) {
                print( '*************************** Insert succesfully  **************************');
               
                cubitt.ChangeBottomState(false,Icons.edit);
              });
            }
            else // if bottomsheet closed
            {
              scaffoldkey.currentState!.showBottomSheet(
                    (context) => Container(
                      padding: EdgeInsets.all(20.0),
                      color: Colors.grey[200],
                      child: Column(
                        mainAxisSize: MainAxisSize.min,
                        children:
                         [
                          DefoultFormField(
                            controller: titleController,
                            label: 'input title',
                            prefix: Icons.title,
                            type: TextInputType.text,
                            validate: ((String value) {}),
                            // validate: (String? value){
                            //   if(value!.isEmpty)
                            //   {
                            //     return' must title not empty ';
                            //   }
                            //   return null ;
                            // },
                          ),
                          SizedBox(
                            height: 5.0,
                          ),
                          //////////////////////////////////////////////////////////////////////
                          DefoultFormField(
                            controller: timeController,
                            label: 'input time',
                            prefix: Icons.watch,
                            type: TextInputType.datetime,
                            validate: ((String value) {}),
                            //  validate: (String? value){
                            //     if(value!.isEmpty)
                            //     {
                            //       return' must time not empty ';
                            //     }
                            //    // return null ;
                            //   },
                            ontap: () {
                              showTimePicker(
                                context: context,
                                initialTime: TimeOfDay.now(),
                              ).then((value) {
                                return timeController.text =
                                    value!.format(context);
                              });
                            },
                          ),
                          ///////////////////////////// formfied  date /////////////////////////////////////////
                          SizedBox(
                            height: 5.0,
                          ),
                          DefoultFormField(
                            controller: dateController,
                            label: 'input date',
                            prefix: Icons.calendar_today,
                            type: TextInputType.datetime,
                            validate: ((String value) {}),
                            // validate: (String value){
                            //   if(value.isEmpty)
                            //   return' must date not empty ';
                            //   return null ;
                            // },
                            ontap: () {
                              showDatePicker(
                                context: context,
                                initialDate: DateTime.now(),
                                firstDate: DateTime.now(),
                                lastDate: DateTime.parse('2023-05-05'),
                              ).then((value) {
                                return dateController.text =
                                    DateFormat.yMMMd().format(value!);
                              });
                            },
                          ),
    
                          //////////////////////////////////////////////////////////////////////
                        ],
                      ),
                    ),
                    // if i closed the bottomSheet in my hand not in FAB will give error so i did this :
                  )
                  .closed .then((value)
                   {
                    
                  cubitt.ChangeBottomState(false,Icons.edit);
              }); // closed func
    
              cubitt.ChangeBottomState(true,Icons.add);
            }
          },
          child: Icon(
         cubitt. fabicon,
          ),
        ),
    //========================  BottomNavigationBar =========================================
        bottomNavigationBar: BottomNavigationBar(
          type: BottomNavigationBarType.fixed,
          elevation: 50.0,
          currentIndex: cubitt.currentindex,
          onTap: (index) {
            cubitt.ChangeIndex(index);
          },
          backgroundColor: Colors.blue[200],
          items:
           
              [
            BottomNavigationBarItem(
              icon: Icon(
                Icons.menu,
              ),
              label: 'NEW TASK',
            ),
            BottomNavigationBarItem(
              icon: Icon(
                Icons.done_all,
              ),
              label: 'DONE TASK',
            ),
            BottomNavigationBarItem(
              icon: Icon(
                Icons.archive,
              ),
              label: 'ARCHIVE TASK',
            ),
          ],
        ),
      ),
    );
  }
//============================ finish scaffold =======================================

  


}

TasksCubit类代码

import 'package:flutter/material.dart';
import 'package:flutter_bloc/flutter_bloc.dart';
import 'package:my_first_app/moduls/new_task_screen/new_task.dart';
import 'package:my_first_app/shared/mycubitclasses/taskstates.dart';
import 'package:sqflite/sqflite.dart';
import '../../moduls/archive_task_screen/archive_task.dart';
import '../../moduls/done_task_screen/done_task.dart';

class TasksCubit extends Cubit<TasksStates>
{
  
 TasksCubit():super ( InitailTasksState());
static TasksCubit get(context)=>BlocProvider.of(context);
//////////////////////////////////////////////////////////////////////
int currentindex = 0;
 List<Widget> screens = [
    NewTask(),
    DoneTask(),
    ArchiveTask(),
  ];
  List<String> titls = [
    'new task',
    'done task ',
    'archive task',
  ];
  void ChangeIndex (index)
  {
    currentindex =index;
    emit(changBottomNavBar());
  }
  var isBottomSheetShown = false;
  IconData fabicon = Icons.add;

void ChangeBottomState ( bool isShow ,IconData icon )
{
  isBottomSheetShown=isShow;
  fabicon=icon;
  emit(changBottomSheet());

}



  // **************************** DataBase ************************************
  late Database Mydatabase;
List<Map> tasks =[];
  void creatDataBase()  {
     openDatabase(
      'todo.db',
      version: 1,
      onCreate: (db, version) {
        print(
            '************************** daataBase Created  ****************************');
        db            .execute(
                'CREATE TABLE tasks (id INTEGER PRIMARY KEY,titel TEXT,date TEXT,time TEXT,status TEXT)')
            .then((value) {
          print(
              '************************** table creaated  ****************************');
        }).catchError((err) {
          print(
              '************************** error in create table  ****************************${err.toString()}');
        });
      },
      onOpen: (db) {
        // ignore: avoid_print
        print(
            '************************** daataBase opened  ****************************');
            GetDataFromDataBase(db).then((value) 
            {
            tasks=value;
            emit(GetFromDataBase());
               print(
            '************************** your table next ****************************');
              print(tasks);
               print(
            '************************** your table above ****************************');
            }
            ).catchError((err){
               print(
            '************************** error when get data  ${err.toString()} ****************************');
            });
      },
    ).then((value) {
      Mydatabase=value;
      emit(CreatDataBase());
    });
  }
//============================================================================================
//========================== insert data to dataBase =========================================

   InserttTODataBase(
      {
      required String titel,
      required String date,
      required String time,
      required String status
      }) async 
      {
    await Mydatabase.transaction((txn)
     {
      return txn.rawInsert('INSERT INTO tasks (titel,date,time,status) VALUES (&quot;$titel&quot;,&quot;$date&quot;,&quot;$time&quot;,&quot;$status&quot;)')
          .then((value) 
       {
        print('************************** inserted successfulyt to  id ${value}  ****************************');
        emit(InsertDataBase());
         GetDataFromDataBase(Mydatabase).then((value) 
            {
            tasks=value;
            emit(GetFromDataBase());
               print( '************************** your table next ****************************');
              print(tasks);
               print('************************** your table above ****************************');
            });

      }).catchError((err) 
      {
        print('************************** error when inserted to record  ****************************${err.toString()}');
      });

    });
  }

 /////////////////////////// get data from dataBase////////////////////////////////////
 //============================================================================================

Future<List<Map>> GetDataFromDataBase (database) async
{
 return await database.rawQuery('SELECT * FROM tasks');

}

}

问题定位

从错误堆栈和代码可以看出，错误根源在HomeLayout类的成员变量定义处：

TasksCubit cubitt = TasksCubit.get(context);

在StatelessWidget的类成员级别，context并不是可用的BuildContext实例——此时Widget还未被构建，没有上下文对象，这里的context实际引用的是BuildContext这个类型（Type对象），而非具体的上下文实例。调用TasksCubit.get(context)时，将Type类型传入了需要BuildContext的方法，导致类型转换错误。

另外，即使这里的context是有效实例，当前代码中BlocProvider是在build方法内创建的，类成员初始化时BlocProvider还未存在，也无法通过context获取到Cubit实例。

解决步骤

步骤1：移除错误的类成员变量

删除HomeLayout类中的这一行：

TasksCubit cubitt = TasksCubit.get(context);

步骤2：在BuildContext有效范围内获取Cubit实例

在build方法内部或者BlocBuilder的回调中，通过有效BuildContext获取Cubit：

方法一：在build方法开头获取实例，后续复用：

@override
Widget build(BuildContext context) 
{
  return BlocProvider (
    create:  (context) {
      return TasksCubit()..creatDataBase();
    },
    child: Builder(builder: (context) {
      final cubitt = TasksCubit.get(context);
      return Scaffold(
        // ... 后续代码中直接使用cubitt即可
        appBar: AppBar(
          title: Text(cubitt.titls[cubitt.currentindex]),
        ),
        // ... 其他部分代码同理替换
      );
    }),
  );
}

方法二：在BlocBuilder的builder回调中获取：

body: BlocBuilder<TasksCubit,TasksStates>(
  builder:  (BuildContext context, TasksStates state)
  {
    final cubitt = TasksCubit.get(context);
    if (state is InsertDataBase)
    {
      Navigator.pop(context);
    }
    return cubitt.screens[cubitt.currentindex];
  }),

对于AppBar、FAB等其他需要使用Cubit的地方，可以用Builder包裹，或者将BlocProvider移到HomeLayout的父级（比如main.dart的MaterialApp内部）。

步骤3：修复SQL语句中的转义错误

TasksCubit的InserttTODataBase方法

Flutter报错：type '_Type' is not a subtype of type 'BuildContext'求解

错误概述 #

错误堆栈 #

相关代码 #

main.dart #

HomeLayout页面代码 #

TasksCubit类代码 #

问题定位 #

解决步骤 #

步骤1：移除错误的类成员变量 #

步骤2：在BuildContext有效范围内获取Cubit实例 #

步骤3：修复SQL语句中的转义错误 #