How do you properly test each individual distinct layer of a Clean Architecture based Flutter app, including the domain, data, and presentation layers, using genuinely appropriate distinct testing strategies for each one?

Testing a Clean Architecture based app typically involves using several genuinely distinct, appropriate testing strategies for each individual separate layer, with the domain layer's use cases being thoroughly unit tested using simple mock repository implementations, since that core layer contains only pure genuine business logic with absolutely no external dependencies at all, the data layer's specific repository implementations being tested against either a properly mocked network client or a dedicated test database, and the presentation layer being tested using Flutter's own dedicated widget testing tools to verify the user interface correctly and appropriately responds to different specific states the ViewModel might actually produce.

test('PlaceOrderUseCase successfully calls the repository', () async {
  final mockRepo = MockOrderRepository();
  final useCase = PlaceOrderUseCase(mockRepo);
  await useCase.execute(testCart);
  verify(mockRepo.createOrder(testCart)).called(1);
});