Testing Network Requests in Flutter: A Guide to Mocking Dio

In the world of Flutter development, handling and testing network requests is a crucial skill. While Dio is an excellent HTTP client for making API calls, testing these network interactions can be challenging. Today, let’s explore how to effectively mock Dio requests for unit testing in Flutter.

Understanding the Importance of Mocking

When writing unit tests, we want them to be fast, reliable, and independent of external services. Making actual network calls during tests can lead to flaky tests and slower execution times. This is where mocking comes in handy.

Setting Up Your Testing Environment

First, let’s set up our testing environment with the necessary dependencies. Add these to your pubspec.yaml:

dev_dependencies:
  mockito: ^5.4.0
  http_mock_adapter: ^0.4.4

Creating Mock Responses

Let’s look at a practical example of how to mock Dio requests. We’ll create a simple user service and test it:

class UserService {
  final Dio dio;

  UserService(this.dio);

  Future<User> getUser(int id) async {
    final response = await dio.get('/users/$id');
    return User.fromJson(response.data);
  }
}

Here’s how we can test this service:

void main() {
  late Dio dio;
  late UserService userService;
  late DioAdapter dioAdapter;

  setUp(() {
    dio = Dio();
    dioAdapter = DioAdapter(dio: dio);
    dio.httpClientAdapter = dioAdapter;
    userService = UserService(dio);
  });

  test('should return user when API call is successful', () async {
    final mockData = {
      'id': 1,
      'name': 'John Doe',
      'email': 'john@example.com'
    };

    dioAdapter.onGet(
      '/users/1',
      (server) => server.reply(200, mockData),
    );

    final user = await userService.getUser(1);

    expect(user.name, equals('John Doe'));
    expect(user.email, equals('john@example.com'));
  });
}

Best Practices for Mocking Dio

1. Always mock error scenarios
2. Test different HTTP methods
3. Verify request headers and parameters
4. Test timeout scenarios
5. Mock interceptors when needed

Here’s an example of testing an error scenario:

test('should throw exception when API call fails', () async {
  dioAdapter.onGet(
    '/users/1',
    (server) => server.throws(
      404,
      DioError(
        requestOptions: RequestOptions(path: '/users/1'),
        response: Response(
          statusCode: 404,
          requestOptions: RequestOptions(path: '/users/1'),
        ),
      ),
    ),
  );

  expect(
    () => userService.getUser(1),
    throwsA(isA<DioError>()),
  );
});

Conclusion

Mocking Dio requests is essential for writing reliable unit tests in Flutter. By following these patterns and best practices, you can ensure your network-related code is thoroughly tested and maintainable.