01_RxBinder.kt

interface RxBinder {
  fun bindUntil(event: LifecycleEvent)
  fun apply(event: LifecycleEvent)
}

class RxBinderImpl {
  private val subject = BehaviorSubject.<LifecycleEvent>create()
  private val disposables = mapOf<LifecycleEvent, MutableList<() -> Disposable>>()
  
  override fun bindUntil(event: LifecycleEvent, disposable: () -> Disposable) {
    if (subject.value >= event) {
      (disposables[event] ?: run { disposables[event] = mutableList(); disposables[event]; })
        .let { it += disposable }
    } else {
      disposable.dispose()
    }
  }
  override fun apply(event: LifecycleEvent) {
    disposables[event]?.foreach { it.dispose() }
  }
  
}

02_RxActivity.kt

class RxActivity {
  val rxBinder : RxBinder = RxBinderImpl()
  fun onCreate() {
    rxBinder.apply(LifecycleEvent.ON_CREATED)
  }
  fun onStart() {
    rxBinder.apply(LifecycleEvent.ON_STARTED)
  }
  fun onResume() {
    rxBinder.apply(LifecycleEvent.ON_RESUMED)
  }
  fun onPause() {
    rxBinder.apply(LifecycleEvent.ON_PAUSED)
  }
  fun onStop() {
    rxBinder.apply(LifecycleEvent.ON_STOPPED)
  }
  fun onDestroy() {
    rxBinder.apply(LifecycleEvent.ON_DESTROYED)
  }
  
}

03_MainActivity.kt

class MainActivity : RxActivity {
  private lateinit var viewModel : MainViewModel
  
  fun onCreate() {
    viewModel = MainViewModel(rxBinder)
  }
}

04_MainViewModel.kt

class MainViewModel(private val rxBinder: RxBinder) {
  
  private val repository by lazy { TemporaryMsgRepository() }
  private val msg = ObservableField<String>()
  
  init {
    rxBinder.bindUntil(LifecycleEvent.ON_CREATED) {
      Disposables.fromAction { msg.set(respository.getLastMessage()) }
    }
    
    rxBinder.bindUntil(LifecycleEvent.ON_DESTROYED) {
      Disposables.fromAction { respository.saveLastMessage(msg) }
    }
  }
}