Lasagna Screens

LoginActivity.kt

class LoginActivity : AppCompatActivity(), LoginView {

    val vm: LoginViewModel by lazy {
        LoginViewModel(this, LoginService.getInstance(BuildConfig.DEBUG))
    }

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)

        setContentView(R.layout.activity_main)

        screen_header.setText("Login Screen")
        login_form_username.addTextChangedListener(AbstractTextWatcher { vm.username = it })
        login_form_password.addTextChangedListener(AbstractTextWatcher { vm.password = it })
        login_form_submit_button.setOnClickListener {
            vm.attemptLogin()
        }
    }

    override fun onLoginSuccessful() {
        startActivity(Intent(this, HomeActivity::class.java))
    }

    override fun setButtonEnabled(enabled: Boolean) {
        login_form_submit_button.isEnabled = enabled
    }
}

LoginService.kt

interface LoginService {

    fun login(username: String, password: String, onLoggedIn: () -> Unit)

    companion object {
        fun getInstance(debug: Boolean): LoginService {
            return if (debug) DebugLoginService() else ProductionLoginService(LoginApi())
        }
    }
}

class ProductionLoginService(private val loginApi: LoginApi) : LoginService {
    override fun login(username: String, password: String, onLoggedIn: () -> Unit) {}
}
class DebugLoginService : LoginService {
    override fun login(username: String, password: String, onLoggedIn: () -> Unit) {}
}

LoginView.kt

interface LoginView {
    fun onLoginSuccessful()
    fun setButtonEnabled(enabled: Boolean)
}

LoginViewModel.kt

class LoginViewModel(
    private val view: LoginView,
    private val loginService: LoginService
) {

    var username: String? = null
        set(value) {
            field = value
            onFieldsChanged()
        }

    var password: String? = null
        set(value) {
            field = value
            onFieldsChanged()
        }

    fun attemptLogin() {
        loginService.login(username!!, password!!) {
            view.onLoginSuccessful()
        }
    }

    private fun onFieldsChanged() {
        view.setButtonEnabled(username != null && password != null)
    }
}

ViewUtils.kt

class AbstractTextWatcher(
    val onTextChanged: (String)->Unit
) : TextWatcher {
    override fun afterTextChanged(s: Editable?) {}
    override fun beforeTextChanged(s: CharSequence?, start: Int, count: Int, after: Int) {}
    override fun onTextChanged(s: CharSequence?, start: Int, before: Int, count: Int) {
        onTextChanged(s.toString())
    }
}