I was having some problems with trying to run a mo...

# squarelibraries

[Stylianos Gakis]

I was having some problems with trying to run a molecule presenter on iOS with an EXC_BAD_ACCESS error. Perhaps similarly to this https://github.com/cashapp/molecule/issues/547
I am using DisplayLinkClock from molecule in the context of my coroutineScope running this presenter.
I ended up having to use Dispatchers.Main + DisplayLinkClock as the CoroutineContext which is running the presenter to have the app not crash, but I am not sure if this is what I should be doing since then the entire persenter would be running on the main thread if I understand this correctly?

Anyone who has tackled this in the past has any ideas here perhaps?

[Stylianos Gakis]

Well, on Android I suppose I am using AndroidUiDispatcher.Main on my presenters already. So from my understanding, doing Dispatchers.Main should be the equivalent on iOS, so this should be fine

[curioustechizen]

Looking at my usage of Molecule in a KMP app (Android + iOS), I see that my implementation matches yours. Adding some snippets in a moment

[curioustechizen]

expect abstract class MoleculeViewModel() : ViewModel {
    val moleculeScope: CoroutineScope
}

// iOS implementation
actual abstract class MoleculeViewModel : ViewModel() {
    actual val moleculeScope = CoroutineScope(
        viewModelScope.coroutineContext +
            Dispatchers.Main + // <- This prevents hangs on iOS. Without this, presenters collecting flows will happen immediately without a hop
            DisplayLinkClock
    )
}

// Android implementation
actual abstract class MoleculeViewModel actual constructor() : ViewModel() {
    actual val moleculeScope: CoroutineScope = CoroutineScope(viewModelScope.coroutineContext + AndroidUiDispatcher.Main)
}

[Stylianos Gakis]

// <- This prevents hangs on iOS. Without this, presenters collecting flows will happen immediately without a hop

Surprised it even ran in the first place tbh because I am just getting a crash immediately before anything would be emitted from the presenter when I am missing it.

But thanks a lot for sharing your snippets, then it sounds like I probably got the right idea then. Thanks a lot!

[curioustechizen]

It is possible that the "hang" was actually the thing that Xcode does when an app crashes: pausing at the "breakpoint" in the stacktrace.

[Stylianos Gakis]

Ah yeah, then it must've been the same behavior we experienced

[Ryan Payne]

Thanks for the snippet @curioustechizen. I asked this over in #C3PQML5NU (https://kotlinlang.slack.com/archives/C3PQML5NU/p1775263858145339) and @Chrimaeon sent me here. thank you color

Based on some advice from ole Claude I was able to consolidate the view model into commonMain and switch the moleculeRecompositionMode and moleculeCoroutineContext per platform.

Untitled

[Stylianos Gakis]

Why would you not want RecompositionMode.ContextClock for the iOS target? Isn't the point of DisplayLink that it provides the necessary information inside the coroutine context so that you will actually get recompositions following that rather than just use the immediate alternative?

[Ryan Payne]

Good point. I believe I do want RecompositionMode.ContextClock for both ios and android.

[Stylianos Gakis]

What did you end up doing after all? 😄

[Ryan Payne]

I went with RecompositionMode.ContextClock on both Android and iOS. 🙂 It worked great 👍

[curioustechizen]

I'm seeing some "refreshing" issues on iOS that might be related to ContextClock but I cannot tell for sure.
My usecase is
1. I have a screen that upon entering starts collecting a flow. In the presenter this is in the present method as LaunchedEffect(Unit) { startCollectingMyFlow() }
2. This screen has a button that when pressed, opens a bottom sheet (this is a navigation event)
3. When the bottom sheet is closed, looks like the flow doesn't "resume" collection immediately. I see some missed emissions
I don't see this problem on Android so I'm guessing it might have to do with the recomposition mode or the molecule coroutine context.
Edit: The problem exists on Android too. So this has nothing to do with molecule scope or recomposition mode.

[curioustechizen]

Does this ring a bell to anyone here?

[curioustechizen]

Also, is there a reason for the moleculeCoroutineContext on iOS to use Dispatchers.Main and not Dispatchers.Main.immediate?

[Stylianos Gakis]

Regarding 1. When a bottom sheet opens the screen in the background should still be in a non-destroyed state, so how come your presenter is not just working normally? What makes it stop presenting?

Regarding .immediate I don't actually know what the difference is in iOS, but the answer may be that we need to use that there too blob shrug

[curioustechizen]

Yes I just don't understand what's up with this flow. Looks like something more fundamental. Will dig deeper (it might even have to do with the flow emission and not the collection).

[curioustechizen]

Update: This was an operator error. The bug was in the flow emission. Once I fixed that everything on the presenter end Just Works ™️

[Ryan Payne]

Update for me: I've decided to not use Molecule to expose state. I've found a simpler approach.

More info: https://kotlinlang.slack.com/archives/C5HT9AL7Q/p1780244622720639?thread_ts=1775936064.005949&cid=C5HT9AL7Q