My `UiState` contains an `action` property, which ...

# compose

[Lukasz Kalnik]

My UiState contains an action property, which when set, should start an activity from the Composable. However on some devices the activity is started twice.
Code in 🧵

[Lukasz Kalnik]

data class UiState(val startActivity: Boolean)

class MyViewModel : ViewModel {

    val uiState = MutableStateFlow(UiState(startActivity = false))

    fun startActivity() {
        uiState.value = UiState(startActivity = true)
    }

    fun onActivityStarted() {
        uiState.value = UiState(startActivity = false)
    }
}

@Composable
fun MyComposable(viewModel: MyViewModel) {
    val uiState by viewModel.uiState.collectAsStateWithLifecycle()
    val activity = LocalActivity.current

    if (uiState.startActivity) {
        viewModel.onActivityStarted()
        activity.startActivity(Intent(activity, MyOtherActivity::class.java))
    }
}

On some devices the if (uiState.startActivity) block inside the composable is started twice.

[Lukasz Kalnik]

Also if I wrap the if block inside a LaunchedEffect(uiState) it doesn't help.

[Sergey Dmitriev]

I guess because we are not in control of how often recompositions happen…

> Also if I wrap the if block inside a LaunchedEffect(uiState) it doesn’t help.
What if you do LaunchedEffect(uiState.startActivity)?

[Lukasz Kalnik]

Shouldn't that be effectively the same? It's a data class.

[Lukasz Kalnik]

I have added the LaunchedEffect(uiState.startActivity) and it seems the problem comes not so often, but still.
This must be simply a race condition. I wonder how a pattern like this should be properly implemented in Compose.

[Sergey Dmitriev]

Shouldn’t that be effectively the same? It’s a data class.

It should, you are right

This must be simply a race condition.

I doubt it, it is guaranteed that LaunchedEffect runs only once for the given arguments.
Is it possible that the ViewModel is initialized twice or something like that?

[Lukasz Kalnik]

Should not be, the ViewModel logic looks sound. And strange that it seems to happen only on some devices.

[Lukasz Kalnik]

E.g. not on Pixel

[Lukasz Kalnik]

And with LaunchedEffect it happens on these devices quite rarely, sometimes 1 in 10 times.

[Lukasz Kalnik]

Maybe after all I have a race condition in the ViewModel

[Sergey Dmitriev]

Hard to tell what’s exactly going wrong here, perhaps you can add some logs to see what exactly happens? Check LaunchedEffect argument etc.

Anyway, I wouldn’t recommend putting action into UiState, feels like it’s not a good place for it and we shouldn’t change the state based on action.

Instead we can have another field in the ViewModel

class MyViewModel : ViewModel() {

  sealed interface Action {
    object StartAnotherActivity : Action
  }

  val actions = Channel<Action>() // or SharedFlow

  fun onButtonClick() {
    actions.trySend(Action.StartAnotherActivity)
  }
}

And then in compose

@Composable
fun MyComposable(viewModel: MyViewModel) {
  val activity = LocalActivity.current

  LaunchedEffect(Unit) {
    viewModel.actions.collect { action ->
      when (action) {
        Action.StartAnotherActivity -> {
          activity.startActivity(Intent(activity, MyOtherActivity::class.java))
        }
      }
    }
  }
}

[Lukasz Kalnik]

Yes, we used to do this with Channel which then through receiveAsFlow() was converted to a hot Flow.
However Google advises against it, as hot flows don't care about the consumers and e.g. if the Activity gets destroyed after the event has been emitted, but before it got consumed, the event can get lost.

[Lukasz Kalnik]

https://medium.com/androiddevelopers/viewmodel-one-off-event-antipatterns-16a1da869b95

[Sergey Dmitriev]

This is the case with SharedFlow only and with use of RENDEZVOUS Channel you can make sure that the even is consumed properly if we add

fun onButtonClick() {
  viewModelScope.launch {
    actions.send(Action) // this will suspend until received by the consumer
  }
}

Or am I getting something wrong?

[Lukasz Kalnik]

This is actually the current recommendation of handling navigation through uiState, I will try that
https://developer.android.com/topic/architecture/ui-layer/events#navigation-events-destination-back-stack

[Lukasz Kalnik]

No, the point is, even with a RENDEZVOUS channel the event can get lost "in between". I.e. after being emitted but before it's consumed.

[Lukasz Kalnik]

I'm not sure myself how/when it can happen, I imagine when the Activity gets destroyed in between etc.

[Sergey Dmitriev]

I see, I was under impression that it will suspend until there is a new receiver connected to the Channel 🤔

[Lukasz Kalnik]

It's a hot flow

[Lukasz Kalnik]

It doesn't care about collectors

[Lukasz Kalnik]

BTW you cannot collect a Channel directly, that's why we used to use the receiveAsFlow() so that it emits as a Flow

[Lukasz Kalnik]

But I added some more logging to the LaunchedEffect and looks like the ViewModel triggers the startActivity as true again after first navigation. So there is definitely some error in my code.

[Sergey Dmitriev]

BTW can’t we collect the channel via

for (action in actions) {
  // Handle Action         
}

?

This way actions.send() will suspend indefinitely until we handle it on the receiving side?

[Lukasz Kalnik]

Like I said, I'm not sure why the events from the channel can get lost, but I got it already confirmed from another expert on Coroutines as well that it's possible.

[Sergey Dmitriev]

Alright alright, I’m just not quite enjoying the recommended way 🙂

[Lukasz Kalnik]

See also Manuel Vivo's comment to the linked article

Hi! It doesn't guarantee the processing of the event because of the prompt cancellation guarantee changes that landed in Coroutines 1.4. More info here:

https://github.com/Kotlin/kotlinx.coroutines/issues/2886

It could happen that the Channel sends the event and the collector just goes to the background (missing the event) pretty much instantly.

[Lukasz Kalnik]

Alright alright, I’m just not quite enjoying the recommended way

Yes, it's more hassle. Also we actually didn't have any customer complaints, so I guess this is rather an "err on the safe side" approach

[Lukasz Kalnik]

Also with navigation usually the user can trigger the action again (if it's through a button press)

[Lukasz Kalnik]

The cause of the problem was that we trigger the navigation on a barcode scan. The barcode scanner delivered the scanned code twice (in each processed frame) and that's what triggered the navigation twice in the ViewModel (we didn't have proper thread safe locking in the method that was processing the barcode).

[Sergey Dmitriev]

Glad you found the root cause!
And I’ve been rethinking quite a lot regarding ViewModel actions yesterday 😄

[Lukasz Kalnik]

Actually, if you read Roman Elizarov's comment in the Github issue, it should be enough to emit/collect the events from a Channel on Dispatchers.Main.immediate to guarantee consumption. It makes the whole Manuel Vivo's article a bit misleading.

[Lukasz Kalnik]

Namely viewModelScope and lifecycleScope use Dispatchers.Main.immediate already.

[Lukasz Kalnik]

https://github.com/Kotlin/kotlinx.coroutines/issues/2886#issuecomment-901188295

[Sergey Dmitriev]

Roman Elizarov’s comment

Yes that is my conclusion so far, because I really don’t want to model Actions as State. Even though there are some valid arguments advocating for such approach

[Sergey Dmitriev]

I wonder why this topic didn’t get enough traction to proceed with something like ExactlyOnceEventBus, I guess folks are still somewhat satisfied with the workarounds blob shrug

[Lukasz Kalnik]

I think using `viewModelScope`/`lifecycleScope` is not actually a workaround, but a proper solution in Android world.

[Lukasz Kalnik]

Also the issue got a little blown up by Manuel Vivo, I have the impression.

[Sergey Dmitriev]

> using `viewModelScope`/`lifecycleScope` is not actually a workaround, but a proper solution in Android world.
Yeah, this is why I’ve never encountered an issue and until yesterday was under impression that Channel is the right tool for the job 🙂

[Lukasz Kalnik]

Yes, we also never got any complaints while using Channel...