flutter unit testing stubbed method returns null

Question

I,m unit testing flutter class using mockito. I stubbed one method using 'when()' but it returns null. is there something wrong in my testing?. I have used freezed union to generate Result class. My Repository is an abstract class

this is my test class

class MockRepository extends Mock implements Repository {}

void main() {
  late CryptoUseCases useCases;
  late MockRepository repository;

  final Result<List<Crypto>, Exception> data = const Result.success([
    Crypto(
      id: 1,
      name: 'Bitcoin',
      symbol: 'BTC',
      priceUsd: 4000,
      availableSupply: 879787908,
      logo: '',
      volumeUsd24h: 786876,
      totalSupply: 876587,
      marketCapUsd: 698,
      maxSupply: 867987,
      rank: 1,
    ),
    Crypto(
      id: 3,
      name: 'Euthereum',
      symbol: 'ETH',
      priceUsd: 134,
      availableSupply: 879787908,
      logo: '',
      volumeUsd24h: 786876,
      totalSupply: 876587,
      marketCapUsd: 698,
      maxSupply: 867987,
      rank: 2,
    ),
  ]);
  setUp(() {
    repository = MockRepository();
    useCases = CryptoUseCases(repository);
  });

  test('getCryptos return a list of crypto', () async {
    //arrange
    when(repository.getCryptos()).thenAnswer((_) async => data);

    //act

    final result = await useCases.getCryptos();

    //assert
    expect(result, equals(data));
    verify(repository.getCryptos()).called(1);
    verifyNoMoreInteractions(repository);
  });
}

this is my class

class CryptoUseCases {
  final Repository repository;

  CryptoUseCases(this.repository);

  Future<Result<List<Crypto>, Exception>> getCryptos() async {
    final result = await repository.getCryptos();
    result.when(success: (data) {
      return Result.success(data);
    }, error: (e) {
      return Result.error(e);
    });
    return Result.error(Exception());
  }
}

i'm using freezed here. is that a problem?

this is the error

type 'Null' is not a subtype of type 'Future<Result<List<Crypto>, Exception>>'
package:crypto_app/src/domain/repositories/repository.dart 6:43  MockRepository.getCryptos
test\use_cases_test.dart 51:21                                   main.<fn>
test\use_cases_test.dart 49:46 

Answer 1

found the answer after some tries.

I believe the null error is caused due to null safe dart feature.

So to create a mock for null safe dart, we need to anotate @GenerateMocks() above the main method of test.

In my case

@GenerateMocks([Repository])
void main(){}

and use buildrunner to generate code.

after this, this code worked fine