class LoginContracts {
  interface View {
    fun goToHomeScreen(user: User)
    fun showError(message: String)
  }

  interface Presenter {
    fun onDestroy()
    fun onLoginButtonPressed(username: String, password: String)
  }

  interface Interactor {
    fun login(username: String, password: String)
  }

  interface InteractorOutput {
    fun onLoginSuccess(user: User)
    fun onLoginError(message: String)
  }
}

class LoginActivity: LoginContracts.View{

  var presenter: LoginContracts.Presenter? = LoginPresenter(this)

  //other fields

  override fun onCreate() {
    //...
    loginButton.setOnClickListener { onLoginButtonClicked() }
    //...
  }

  override fun onDestroy() {
    presenter?.onDestroy()
    presenter = null
    super.onDestroy()
  }

  private fun onLoginButtonClicked() {
    presenter?.onLoginButtonClicked(usernameEditText.text, passwordEditText.text)
  }

  fun goToHomeScreen(user: User) {
    val intent = Intent(view, HomeActivity::class.java)
    intent.putExtra(Constants.IntentExtras.USER, user)
    startActivity(intent)
  }

  fun showError(message: String) {
    //shows the error on a dialog
  }
}

class LoginPresenter(var view: LoginContract.View?): LoginContract.Presenter, LoginContract.InteractorOutput {
    var interactor: LoginContract.Interactor? = LoginInteractor(this)

    fun onDestroy() {
      view = null
      interactor = null
    }

    fun onLoginButtonPressed(username: String, password: String) {
      interactor?.login(username, password)
    }

    fun onLoginSuccess(user: User) {
      view?.goToNextScreen(user)
    }

    fun onLoginError(message: String) {
      view?.showError(message)
    }
}

class LoginInteractor(var output: LoginContract.InteractorOutput?): LoginContract.Interactor {
  fun login(username: String, password: String) {
    LoginApiManager.login(username, password)
                ?.subscribeOn(Schedulers.newThread())
                ?.observeOn(AndroidSchedulers.mainThread())
                ?.subscribe({
                          //does something with the user, like saving it or the token
                          output?.onLoginSuccess(it)
                          },
                        { output?.onLoginError(it.message ?: "Error!") })
  }
}