object Events {
    const val LOGIN = 1
    const val LOGOUT = 2
}

abstract class BaseActivity : AppCompatActivity(), Observer {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        EventManager.register(this)
    }

    override fun onDestroy() {
        super.onDestroy()
        EventManager.unregister(this)
    }

    override fun onEvent(event: Int, vararg args: Any?) {
    }

    override fun listEvents(): IntArray {
        return IntArray(0)
    }
}

class MainActivity : BaseActivity() {
    private fun setViews(account: String?) {
        // update views
    }

    override fun listEvents(): IntArray {
        return intArrayOf(Events.LOGIN, Events.LOGOUT)
    }

    override fun onEvent(event: Int, vararg args: Any?) {
        when (event) {
            Events.LOGIN -> setViews(args[0] as String?)
            Events.LOGOUT -> setViews(null)
        }
    }
}

object AccountManager {
    fun login(account: String, password: String) {
        // process login
        EventManager.notify(Events.LOGIN, account)
    }

    fun logout() {
        // process logout
        EventManager.notify(Events.LOGOUT)
    }
}