Flutter用Mockito模拟Riverpod Provider报错，求无改真实类的测试方案

问题场景 #

在测试Riverpod的TodoList Notifier时，尝试用Mockito模拟依赖的userItemProvider，先后遇到两个报错：

1. NoSuchMethodError: Class 'MockUserItem' has no instance method '_setElement'
2. type 'Null' is not a subtype of type 'User'
   希望找到无需修改业务代码的测试方案。

依赖版本 #

# pubspec.yaml
environment:
  sdk: ^3.5.2

dependencies:
  flutter:
    sdk: flutter
  flutter_riverpod: ^2.5.1
  riverpod_annotation: ^2.3.5
  freezed_annotation: ^2.4.4

dev_dependencies:
  flutter_test:
    sdk: flutter
  riverpod_generator: ^2.4.3
  riverpod_lint: ^2.3.13
  freezed: ^2.5.7
  mockito: ^5.4.4

业务代码 #

TodoList Notifier（待测试） #

// notifier_provider_page.dart
import 'package:riverpod_annotation/riverpod_annotation.dart';
import 'package:riverpod_flutter/user_item_provider.dart';
import 'todo.dart';

part 'notifier_provider_page.g.dart';

@riverpod
class TodoList extends _$TodoList {
  @override
  List<Todo> build() {
    return [];
  }

  void add(Todo todo) {
    final user = ref.read(userItemProvider);
    if (user?.email == 'a') {
      state = [...state, todo];
    }
  }
}

UserItem Notifier（依赖） #

// user_item_provider.dart
import 'package:flutter_riverpod/flutter_riverpod.dart';
import 'package:riverpod_annotation/riverpod_annotation.dart';
import 'user.dart';

part 'user_item_provider.g.dart';

@riverpod
class UserItem extends _$UserItem {
  @override
  User build() {
    return const User(name: "a", email: "b", password: "c");
  }

  void add(User user) {
    state = user;
  }
}

初始测试代码及报错 #

测试代码 #

// notifier_provider_page_test.dart
import 'package:flutter_riverpod/flutter_riverpod.dart';
import 'package:flutter_test/flutter_test.dart';
import 'package:mockito/annotations.dart';
import 'package:mockito/mockito.dart';
import 'package:riverpod_flutter/notifier_provider_page.dart';
import 'package:riverpod_flutter/todo.dart';
import 'package:riverpod_flutter/user_item_provider.dart';

import 'notifier_provider_page_test.mocks.dart';

@GenerateNiceMocks([MockSpec<UserItem>()])
void main() {
  late ProviderContainer container;
  final MockUserItem mockUserItem = MockUserItem();

  setUp(() {
    container = ProviderContainer(overrides: [
      userItemProvider.overrideWith(() {
        return mockUserItem;
      })
    ]);
  });

  tearDown(() {
    container.dispose();
  });

  test('test', () {
    final todo = container.read(todoListProvider.notifier);

    todo.add(const Todo(
      userId: 1,
      id: 1,
      title: 'title',
    ));

    expect(container.read(todoListProvider),
        [const Todo(userId: 1, id: 1, title: 'title')]);
  });
}

首次报错 #

NoSuchMethodError: Class 'MockUserItem' has no instance method '_setElement'.
Receiver: Instance of 'MockUserItem'
Tried calling: _setElement(Instance of 'AutoDisposeNotifierProviderElement<UserItem, User>')

修改尝试后的新报错 #

在业务文件中添加自定义Mock类后，出现：

type 'Null' is not a subtype of type 'User'

无需修改业务代码的解决方案 #

方案一：直接Override Provider返回测试状态 #

因为TodoList的add方法仅依赖userItemProvider的状态值，无需Mock整个Notifier，直接通过overrideWith返回测试用的User状态即可：

// notifier_provider_page_test.dart
import 'package:flutter_riverpod/flutter_riverpod.dart';
import 'package:flutter_test/flutter_test.dart';
import 'package:riverpod_flutter/notifier_provider_page.dart';
import 'package:riverpod_flutter/todo.dart';
import 'package:riverpod_flutter/user_item_provider.dart';

void main() {
  late ProviderContainer container;

  setUp(() {
    container = ProviderContainer(overrides: [
      userItemProvider.overrideWith((ref) {
        // 返回符合测试条件的User对象
        return UserItem()..state = const User(name: "a", email: "a", password: "c");
      })
    ]);
  });

  tearDown(() {
    container.dispose();
  });

  test('当用户邮箱为"a"时，成功添加Todo', () {
    final todoNotifier = container.read(todoListProvider.notifier);
    todoNotifier.add(const Todo(userId: 1, id: 1, title: 'title'));

    expect(container.read(todoListProvider),
        [const Todo(userId: 1, id: 1, title: 'title')]);
  });

  test('当用户邮箱不为"a"时，不添加Todo', () {
    final testContainer = ProviderContainer(overrides: [
      userItemProvider.overrideWith((ref) {
        return UserItem()..state = const User(name: "a", email: "b", password: "c");
      })
    ]);

    final todoNotifier = testContainer.read(todoListProvider.notifier);
    todoNotifier.add(const Todo(userId: 1, id: 1, title: 'title'));

    expect(testContainer.read(todoListProvider), []);
    testContainer.dispose();
  });
}

方案二：测试文件内自定义Mock类（不修改业务代码） #

若需用Mockito验证方法调用，可在测试文件内创建Mock类，继承自动生成的_$UserItem并实现build方法：

// notifier_provider_page_test.dart
import 'package:flutter_riverpod/flutter_riverpod.dart';
import 'package:flutter_test/flutter_test.dart';
import 'package:mockito/mockito.dart';
import 'package:riverpod_flutter/notifier_provider_page.dart';
import 'package:riverpod_flutter/todo.dart';
import 'package:riverpod_flutter/user_item_provider.dart';

// Mock类仅存在于测试文件，不污染业务代码
class MockUserItem extends _$UserItem with Mock {
  @override
  User build() {
    // 返回非Null的测试用User，避免类型转换错误
    return const User(name: "a", email: "a", password: "c");
  }
}

void main() {
  late ProviderContainer container;
  late MockUserItem mockUserItem;

  setUp(() {
    mockUserItem = MockUserItem();
    // 可按需Mock add方法的行为
    when(mockUserItem.add(any)).thenAnswer((_) => null);
    
    container = ProviderContainer(overrides: [
      userItemProvider.overrideWith(() => mockUserItem),
    ]);
  });

  tearDown(() {
    container.dispose();
  });

  test('测试添加Todo逻辑', () {
    final todoNotifier = container.read(todoListProvider.notifier);
    todoNotifier.add(const Todo(userId: 1, id: 1, title: 'title'));

    expect(container.read(todoListProvider), [const Todo(userId: 1, id: 1, title: 'title')]);
    // 验证Mock方法调用（可选）
    // verify(mockUserItem.add(any)).neverCalled();
  });
}

错误原因解析 #

1. _setElement方法缺失：Riverpod的Notifier内部包含_setElement等私有初始化方法，Mockito自动生成的Mock类未实现这些方法，导致Provider初始化失败。
2. Null类型转换错误：自定义Mock类继承AutoDisposeNotifier但未实现build方法，默认返回Null，不符合UserItem要求的非Null返回值。

内容的提问来源于stack exchange，提问作者akkie