How do you properly use the mockito package to create a genuine mock object specifically for testing a class that genuinely depends on an external dependency, such as a network client or a database repository?

The mockito package lets you generate a genuine mock implementation of a given interface or class, letting you precisely configure exactly what that particular mock should genuinely return when a given specific method is actually called, and then subsequently verify exactly how many times, and with exactly what specific arguments, that particular mocked method was genuinely actually called, letting you thoroughly test a class's own genuine business logic in complete isolation without needing an actual real implementation of whatever external dependency it genuinely relies on.

class MockApiClient extends Mock implements ApiClient {}

test('fetchUser returns parsed user', () async {
  final mockClient = MockApiClient();
  when(mockClient.get('/user')).thenAnswer((_) async => {'name': 'Alex'});
  final service = UserService(mockClient);
  expect((await service.fetchUser()).name, 'Alex');
});