Mocking Dependencies with Mockito in Flutter

In Flutter development, mocking Dependencies is a crucial technique for isolating unit tests and ensuring that your codebase remains stable and efficient with the cross language library Mockito. Any developer with long enough experience has faced challenges of testing and maintaining complex software systems. In this article, we’ll explore the usage of Mockito in Flutter, a popular library for mocking dependencies.

What is Mockito?

Mockito is an open-source library that provides a simple and intuitive way to create mock objects for testing purposes. A mock object is an instance of a class that mimics the behavior of a real object, allowing you to test your code in isolation without relying on external dependencies.

Why Use Mockito in Flutter?

In Flutter, you often encounter complex dependencies that make it difficult to write unit tests. By using Mockito, you can create mock objects for these dependencies, allowing you to test your code in isolation and ensuring that it behaves correctly even when the real dependencies are not available.

Getting Started with Mockito in Flutter

To use Mockito in your Flutter project, you’ll need to add the following dependencies to your pubspec.yaml file:

dependencies:
  mockito: ^5.4.5

Once you’ve added the dependencies, you can start using Mockito in your tests.

Basic Mocking with Mockito

Let’s consider a simple example where we have a UserRepository class that depends on an external API to fetch user data. We want to test the UserRepository class in isolation, without relying on the external API.

// user_repository.dart
class UserRepository {
  final ApiClient _apiClient;

  UserRepository(this._apiClient);

  Future<User> getUser() async {
    final response = await _apiClient.fetchUser();
    return User.fromJson(response);
  }
}

To test the UserRepository class, we can create a mock object for the ApiClient dependency using Mockito:

// user_repository_test.dart
import 'package:flutter_test/flutter_test.dart';
import 'package:mockito/mockito.dart';
import 'package:user_repository/user_repository.dart';

class MockApiClient extends Mock implements ApiClient {}

void main() {
  test('should fetch user data from API', () async {
    // Create a mock object for ApiClient
    final mockApiClient = MockApiClient();

    // Configure the mock object to return a dummy response
    when(mockApiClient.fetchUser()).thenAnswer((_) async => {'id': 1, 'name': 'John Doe'});

    // Create a UserRepository instance with the mock object
    final userRepository = UserRepository(mockApiClient);

    // Call the getUser method and verify that it returns a User instance
    final user = await userRepository.getUser();
    expect(user, isA<User>());
  });
}

In this example, we create a mock object for the ApiClient dependency using Mockito’s Mock class. We then configure the mock object to return a dummy response when its fetchUser method is called. Finally, we create a UserRepository instance with the mock object and call its getUser method to verify that it returns a User instance.

Best Practices for Using Mockito in Flutter

When using Mockito in your Flutter project, keep the following best practices in mind:

• Use mock objects to isolate dependencies: Mockito is designed to help you test your code in isolation, without relying on external dependencies. Use mock objects to isolate dependencies and ensure that your tests are reliable.
• Keep mock objects simple: Avoid complex mock objects with multiple behaviors. Instead, focus on creating simple mock objects that mimic the behavior of a single dependency.
• Use when() to configure mock objects: When configuring mock objects, use Mockito’s when() method to specify the behavior of the dependency. This makes it easier to test your code and ensures that your tests are reliable.
• Avoid using real dependencies in tests: In general, it’s a good idea to avoid using real dependencies in your tests. Instead, use mock objects to isolate dependencies and ensure that your tests are reliable.

Conclusion

Mocking dependencies with Mockito is a powerful technique for testing and maintaining complex software systems in Flutter. By using mock objects to isolate dependencies, you can ensure that your codebase remains stable and efficient, even when the real dependencies are not available. Remember to keep mock objects simple, use when() to configure mock objects, and avoid using real dependencies in tests. With these best practices in mind, you’ll be well on your way to writing reliable and efficient tests for your Flutter project.