How to test Function passed as an argument in Flutter? - flutter

Question

How to test Function passed as an argument in Flutter?
code:
Future<User> execute({
required String username,
required String password,
required void Function(AuthFailure fail) onFailure,
required void Function(User user) onSuccess,
}) async {
if (username.isNonValid || password.isNonValid) {
onFailure(const AuthFailure.wrongCredentials()); // I want to test this line
return const User.anonymous();
}
...
}
test:
test('use case - failure execution for incorrect credentials', () async {
// GIVEN
// WHEN
final user = await useCase.execute(
username: "noname",
password: "password",
onFailure: (fail) {},
onSuccess: (user) {},
);
// THEN
// TODO how to verify onFailure call inside useCase?
expect(user, const User.anonymous());
});
Or maybe testing this way is not the idiomatic way, because the test becomes more white-box instead black-box? Should I perceive passing functions as arguments to use cases as anti-pattern? I can change it then. The proposition is to return sth like Either from useCase.execute():
Future<Either<Failure, Success>> execute({
required String username,
required String password,
}) async {
if (username.isEmpty || password.isEmpty) {
// return wrapper around AuthFailure.wrongCredentials()) of Either left subtype (Either has two subtypes)
}
...
}
This way I only verify return type, and all the lines are covered this way. It's gonna work, but I feel better with the simplest, not the smartest solution.
PS I use Mocktail for mocking, but using Mockito in solution is also warmly welcomed.

Answer 1

If you just want to verify that the callback is triggered, I personally would just make your callback set a flag and then test that flag afterward, which I think is straightforward, simple, and easy to understand with no magic:
test('use case - failure execution for incorrect credentials', () async {
var failureCalled = false;
final user = await useCase.execute(
username: "noname",
password: "password",
onFailure: (fail) => failureCalled = true,
onSuccess: (user) {},
);
expect(user, const User.anonymous());
expect(failureCalled, true);
});
But if you really want to use Mocks, you will need some Mock object to use and to call instance methods on that in callbacks. With Mockito you could do:
test('use case - failure execution for incorrect credentials', () async {
dynamic mock = Mock();
final user = await useCase.execute(
username: "noname",
password: "password",
onFailure: (fail) => mock.fail(fail),
onSuccess: (user) {},
);
expect(user, const User.anonymous());
verify(mock.fail(any)).called(1);
});
Some things to note:
To avoid declaring a class with the expected instance methods and then code-generating stubs, create a raw Mock instance but declare it as dynamic to disable static type-checking. This will then take advantage the Mock.noSuchMethod implementation.
You can't use onFailure: mock.fail directly since the Mock has no generated stubs, and mock.fail will just be null instead of a Function.
I am not experienced with Mocktail, but I imagine that you could do something similar.