How to test a function that uses a callback?

Question

Suppose we have an interface for a callback function as such:

public interface Model {
    interface UserListener {
        void callback(User user);
    }

    void getUser(String username, UserListener listener);
}

And we have a method that calls a callback that was given as an argument:

public class Presenter() {
    ...

    public Presenter(Model model, View view) {
        this.model = model;
        this.view = view;
    }

    public void checkLogin() {
        String username = view.getUsername();
        String password = view.getPassword();

        model.getUser(username, user -> {
            if (user == null) {
                view.loginFailed();
            } else {
                view.loginSuccess(user.getName());
            }
        });
    }
}

I am hoping to test it along the lines of:

when(view.getUsername()).thenReturn("abc");
when(view.getPassword()).thenReturn("xyz");
Presenter presenter = new Presenter(view, model);
presenter.checkLogin();

// How do I verify that view.loginFailed() or view.loginSuccess() is called here
...

Now, to the question: Given this kind of scenario, how would one test (Mockito) the view having loginFailed() and loginSuccess() called?

Answer 1

In Mockito you can use an Answer when a call on a mock needs to do more than just return a value:

@ExtendWith(MockitoExtension.class)
public class MockEg {
    public interface Model {
        interface UserListener {
            void callback(User user);
        }
        void getUser(String username, UserListener listener);
    }

    interface User {}

    @Test
    public void aTest() {
        Model model = mock(Model.class);
        User user = mock(User.class);

        doAnswer(new Answer<Void>() {
            @Override
            public Void answer(InvocationOnMock invocation) throws Throwable {
                Model.UserListener listener = invocation.getArgument(1);
                listener.callback(user);
                return null;
            }
        }).when(model).getUser(any(), any());
        ...
    }
}