How to mock Dio.interceptors.add?

[phamquoctrongnta]

Sorry for my English.

My real data source class:

class MovieRemoteDataSourceImpl implements MovieDataSource {
  final Dio dio;

  MovieRemoteDataSourceImpl(this.dio);

  @override
  Future<Either<Exception, List<MovieResponse>>> fetchMovies() async {
    const url = Endpoint.getTopRatedMovies;
    try {
      final addHeaderInterceptor = AddHeaderInterceptor();
      dio.interceptors.add(addHeaderInterceptor); // Error in this line when run unit test.
      final response = await dio.get(url);
      final mapFromJson = response.data['results'] as List;
      final result = mapFromJson.map((e) {
        return MovieResponse.fromJson(e);
      }).toList();
      return Right(result);
    } on Exception catch (e) {
      return Left(e);
    }
  }
}

My unit test class:

  @GenerateNiceMocks([MockSpec<Dio>()])
void main() {
  group('Fetch list of movie.', () {
    MockDio mockDioClient = MockDio();
    final dataSource = MovieRemoteDataSourceImpl(mockDioClient);
    RequestOptions requestOptions = RequestOptions();
    Response mockResponseSuccess = Response(
      requestOptions: requestOptions,
      statusCode: 200,
      data: listOfMovieMock,
    );
    test('Fetch success', () async {
      when(mockDioClient.get(any)).thenAnswer((_) async => mockResponseSuccess);
      final actual = await dataSource.fetchMovies();
      expect(actual.isRight(), true);
    });

    test('Fetch error: 404 Not found', () async {
      when(mockDioClient.get(any)).thenAnswer(
        (_) async => throw DioException(requestOptions: requestOptions),
      );
      final actual = await dataSource.fetchMovies();
      expect(actual.isLeft(), true);
    });
  });
}

Then I get an error:

FakeUsedError: 'interceptors'
No stub was found which matches the argument of this method call:
interceptors

A fake object was created for this call, in the hope that it won't be ever accessed.
Here is the stack trace where 'interceptors' was called:
...
However, member 'add' of the created fake object was accessed.
Add a stub for MockDio.interceptors using Mockito's 'when' API.

[phamquoctrongnta]

For everyone facing the same issue, just add:

final fakeInterceptors = Interceptors();
when(mockDioClient.interceptors).thenReturn(fakeInterceptors);