Home type 'Null' is not a subtype of type 'Future<void>' when unit testing using mocktail package

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type 'Null' is not a subtype of type 'Future<void>' when unit testing using mocktail package

November 4, 2022

how can we verify whether a method inside a function is called or not?
I have here a sample code.

class MockService extends Mock implements Service {}

class MockRepository extends Mock implements Repository {}

class Repository {
  Repository({
    required this.service,
  });
  final Service service;

  Future<void> somethingFancy() async {
    await service.doSomething();
  }
}

class Service {
  Future<void> doSomething() async {}
}

void main() {
  final service = MockService();
  final repository = Repository(service: service);

  group('auth repository test', () {
    test('test name', () async {
      when(repository.somethingFancy).thenAnswer((_) => Future.value());
      await repository.somethingFancy();

      verify(service.doSomething).called(1);
    });
  });
}

if i run the test, i get this error:

type 'Null' is not a subtype of type 'Future<void>'
test/auth_repository_test.dart 20:16      MockService.doSomething
test/auth_repository_test.dart 15:19      Repository.registerUser
package:mocktail/src/mocktail.dart 210:8  when.<fn>
test/auth_repository_test.dart 29:11      main.<fn>.<fn>
===== asynchronous gap ===========================
dart:async                                _completeOnAsyncError
package:mocktail/src/mocktail.dart 210:8  when.<fn>
test/auth_repository_test.dart 29:11      main.<fn>.<fn>

if i change my repository variable to final repository = MockRepository();,
i cannot inject the Service class, so definitely the method inside service class wont be called. That’s why if I run the this test:

void main() {
  final service = MockService();
  final repository = MockRepository();

  group('auth repository test', () {
    test('test name', () async {
      when(repository.somethingFancy).thenAnswer((_) => Future.value());
      await repository.somethingFancy();

      verify(service.doSomething).called(1);
    });
  });
}

I get an error saying no matching calls

No matching calls (actually, no calls at all).
(If you called `verify(...).called(0);`, please instead use `verifyNever(...);`.)
package:test_api                           fail
package:mocktail/src/mocktail.dart 722:7   _VerifyCall._checkWith
package:mocktail/src/mocktail.dart 515:18  _makeVerify.<fn>
test/auth_repository_test.dart 32:13       main.<fn>.<fn>

What am I doing wrong here? appreciate so much help.

>Solution :

You are mokking the wrong method in your test.

You have to mock answer Service#doSomething instead of Repository#doSimethingFancy. Check your when statement.

test('test name', () async {
  when(service.doSomething()).thenAnswer((_) => Future.value());
  await repository.somethingFancy();

  verify(service.doSomething).called(1);
});