class SignInPresenter(private val api: ApiController, view: SignInView, private val state: SignInState) {

    private val view = WeakReference(view)

    fun signIn(email: String, password: String) {
        api.signIn(email, password).enqueue(object : RetrofitCallback<User>() {
            override fun onSuccess(t: User) {
                state.user = t
                view.get()?.onSignIn(t)
            }

            override fun onError(t: Throwable) {
                view.get()?.onError(t)
            }
        })
    }

    interface SignInView {
        fun onSignIn(t: User)
        fun onError(t: Throwable) // is IOException | AccountInactive | Unauthorized | ContractViolation
    }

    interface SignInState {
        var user: User?
    }
}