Why does verify(() => mockObject.someGetter) throw 'Used on a non-mockito object' even though the object is a Mock?

I'm currently writing unit tests for a Flutter app using mockito, and I'm running into an issue with verifying a getter call using the latest version of mockito (v5.x).

Here's my test setup:

@GenerateMocks([InternetConnectionChecker])
void main() {
  late MockInternetConnectionChecker internetConnectionChecker;
  late NetworkInfoImpl networkInfo;

  setUp(() {
    internetConnectionChecker = MockInternetConnectionChecker();
    networkInfo = NetworkInfoImpl(internetConnectionChecker);
  });

  test('should check internet connection', () async {
    // Arrange
    when(() => internetConnectionChecker.hasConnection)
        .thenAnswer((_) async => true);

    // Act
    final result = await networkInfo.isConnect;

    // Assert
    verify(() => internetConnectionChecker.hasConnection); // ❌ Throws error
    expect(result, true);
  });
}

Even though internetConnectionChecker is clearly a MockInternetConnectionChecker, the test throws this error:

Used on a non-mockito object
package:matcher                                 fail
package:mockito/src/mock.dart 1155:7            _makeVerify.<fn>
test\core\network\network_info_test.dart 30:13  main.<fn>.<fn>

However, if I write it like this, it works:

verify(internetConnectionChecker.hasConnection); // ✅ Works, but discouraged?

From what I understand, using verify(() => ...) is the recommended way to ensure that the method or getter was actually called. So why does this error happen?

Ran flutter pub run build_runner build --delete-conflicting-outputs
Verified that the correct imports are used: package:mockito/mockito.dart
Confirmed the type via print(internetConnectionChecker.runtimeType) → MockInternetConnectionChecker

Is there something special about getters in the latest versions of mockito? Or has there been a breaking change that I missed?

Solution

when requires a method call to be recorded on a Mockito mock instance. Same goes for verify.

When you write when(() => internetConnectionChecker.hasConnection) or verify(() => internetConnectionChecker.hasConnection), you are not calling a method. You are passing a lambda instead.

A lambda is not a mock (it's a lambda. yourMock is a mock, but () => yourMock is not a mock. yourMock.method() calls a method on your mock, () => yourMock.method() does not call the method on your mock. It wraps it in a lambda to be invoked at a later point in time).

Passing a lambda will not invoke the lambda and consequently not call the method on your mock.

Check the Flutter docs for example how to set up your mocks with when:

      when(
        client.get(Uri.parse('https://jsonplaceholder.typicode.com/albums/1')),
      ).thenAnswer((_) async => http.Response('Not Found', 404));

or how to verify the calls on your mocks:

cat.eatFood("chicken");
verify(cat.eatFood("fish"));