lifecycle.kt

@Composable fun LifecycleObservable.observeLifecycle() {
  val lifecycleOwner = LocalLifecycleOwner.current
  
  LaunchedEffect(lifecycleOwner) {
    var createdJob: Job? = null
    var startedJob: Job? = null
    var resumedJob: Job? = null
    lifecycleOwner.addObserver {
      fun onCreated() = createdToDestroy().launchIn(this@LaunchedEffect).let { createdJob = it }
      fun onStarted() = startedToStop().launchIn(this@LaunchedEffect).let { startedJob = it }
      fun onResumed() = resumedToPaused().launchIn(this@LaunchedEffect).let { resumedJob = it }
      
      fun onPaused() = resumedJob?.takeIf { it.isActive }?.cancel()
      fun onStopped() = startedJob?.takeIf { it.isActive }?.cancel()
      fun onDestroyed() = createdJob?.takeIf { it.isActive }?.cancel()
    }
  }
}

interface LifecycleObservable {
   fun createdToDestroy(): Flow<*> = emptyFlow()
   fun startedToStop(): Flow<*> = emptyFlow()
   fun resumedToPaused(): Flow<*> = emptyFlow()
}

class FooViewModel(val chatRepo: ChatRepo): ViewModel(), LifecycleObservable {
   private val newMessageToast = mutableStateOf("")
   private val newMessages = mutableStateOf(emptyList<Message>())
   
   override fun resumedToPaused() = observeNewChatMessage() 
   
   fun observeChatMessage() = chatRepo.observeNewMessage()
      .flowOn(IO)
      .onEach { newMessage -> 
        newMessages.value = newMessages.value + newMessage
        newMessageToast.value = "received new message : {newMessage.content}"
      }
      .flowOn(Main)
}

@Composable
fun FooScreen(viewModel: FooViewModel) {
  viewModel.observeLifecycle()
  val toast = remeber { viewModel.newMessageToast }
  val newMessages = remember { viewModel.newMessages }
  
  // TODO show toast and messages
}