I have a viewmodel that subscribes to logged in user ID changes. When my screen is showed up, I want...

# compose-android

[Colton Idle]

I have a viewmodel that subscribes to logged in user ID changes. When my screen is showed up, I want to log to an analytics service that my user saw a screen with the current id. That seems to work fine, but when I go to a different screen and come back to my original screen, even though the viewmodel hasn't been cleared, the composable gets kicked off again and I end up logging. I feel like I'm just missing something with how Flow's work.

In my VM I have

fun userIdFlow(): Flow<UserId> = userFlow.distinctUntilChangedBy { it?.it }.onEach { analyticLog(it) }.shareIn(viewModelScope, Lazily)

and my composable screen has

LaunchedEffect(Unit){
viewModel.userIdFlow().collect()
}

I'm probably doing something completely wrong. but im still a flow noob so appreciate pointing out any glaring issue here. thanks!

[Albert Chang]

When you use shareIn or stateIn, always make it a member instead of a function, otherwise every time you call the function a new flow will be created, which essentially makes shareIn or stateIn useless.

[Colton Idle]

Thanks. I think I'm closer to a solution. I think my issue stems from the fact athat I might have 3 viewmodels that are listening to this flow... but i want only the first one to respond to the event. I guess it's more of a queue than a flow?

[Albert Chang]

I don’t understand your issue. The onEach operator is applied before shareIn, and once the sharing starts it won’t stop (because of Lazily), so it’ll just work as you expect (i.e. onEach runs once for each value) if you make it a class member.

[Zach Klippenstein (he/him) [MOD]]

Channels are queues, Flows are not

[Colton Idle]

Yeah. I went for a channel. Sorry, the root issue is caused by the fact that we have a really bad current setup of viewmodels and basically viewmodels never clear 😭 (Ian Lake would be sad at our custom navigation setup)

so i was getting multiple invocations. Thanks for the initial pointer @Albert Chang as that unblocked me to find my actual issue.

TIL (and used) channels for the first time!