Mocking AWS Amplify Auth APIs for Android

My Android application uses AWS Cognito and Amplify Auth SDK for authentication and I'm trying to write JUnit test cases for login/signup flows. I'm using Mockito framework to mock the classes.

I started with login, my login model looks like this

class LoginService(val auth: AuthCategory) {

 fun login(username: String, password: String): MutableLiveData<Login> {
    val liveData = MutableLiveData<Login>()
    auth.signIn(username, password,
        { result ->
            liveData.postValue(Login(result, null))
        },
        { error ->
            liveData.postValue(Login(null, error))
        }
    )
    return liveData
    }
  }

And my viewmodel calls it this way

class LoginViewModel : ViewModel() {

    val loginService = LoginService(Amplify.Auth)

    fun login(username: String, password: String): MutableLiveData<Login> {
        return loginService.login(username, password)
    }
}

And my test case looks like this

lateinit var auth: AuthCategory
lateinit var loginService: LoginService

@Before
fun onSetup() {
    auth = mock(Amplify.Auth::class.java)
    loginService = LoginService(auth)
}

@Test
fun loginTest() {
    val authSignIn: Consumer<*>? = mock(Consumer::class.java)
    val authEx: Consumer<*> = mock(Consumer::class.java)
    `when`(
        auth.signIn(
            anyString(), anyString(),
            authSignIn as Consumer<AuthSignInResult>, authEx as Consumer<AuthException>
        )
    )
    loginService.login("username", "password").observeForever {
        assertTrue(it.result?.isSignInComplete!!)
    }
}

Please help me validate this approach, I'm trying to find out a way to trigger AuthSignInResult and AuthException of Auth.signIn() method so that I would assert if signin is successful or there is an error.

I'm very new to AWS Amplify and Cognito environment, A suggestion/reference to do this in correct way would be highly appreciated. Thanks in advance.

from Mocking AWS Amplify Auth APIs for Android