class RepositoryImpl(private val auth: FirebaseAuth) : Repository {

    override fun signIn(email: String,password: String) {
        auth.signWithInEmailAndPassword(email, password)
    }
}

// domain
interface AuthRepository {
     fun signIn()
}

class MyApplication : Application() {
    lateinit var viewModel: MainViewModel

    override fun onCreate(){
        val auth = FirebaseAuth.getInstance()
        val repository = RepositoryImpl(auth)
        val interactor = InteractorImpl(repository)
        viewModel = MainViewModel(interactor)
    }
}

class MainActivity {

    override fun onCreate() {
        val myApplication = application as MyApplication

        val viewModel = myApplication.viewModel
    }
}