Hi folks, I have a question related to

StateFlow

and

UI recomposition

. In short, my

ViewModel

has three flows: 1.

accountFlow

, which is used to fetch the currently logged-in account from the database. 2.

timelinePositionFlow

, which is used to retrieve the last browsing position record from the currently logged-in account. 3.

timelineFlow

, which contains all posts stored in the database for the currently logged-in account. The issue I'm facing is that my app can switch between multiple accounts and browse information. Since each account has a stored timeline position, I need to use

rememberLazyListState(initialFirstVisibleItemIndex = ...)

to initialize the position of the

LazyColumn

. However, if I switch accounts and

StateFlow

emits the value of the new account, the

rememberLazyListState

won't recomposition. so that the size of the timeline of the new account may only be 20, and the

timeline Position Index

of the old account is greater than 20, which will cause a

java.lang.IndexOutOfBoundsException

error Is there a good way to solve this problem?

You can just put

rememberLazyListState

inside a

key(account) { }

.

.combine()

could help too - put the 2/3 states into 1 and filter out these half-updates that are causing you problems

What version of compose are you using?

1.6.0-alpha8

I think this is a crash from alpha08

I tested beta version, but it happened again, and 1.6.0 beta has some LazyColumn issues 🤔

Damn. Yeah, I just moved to alpha08 and released to prod and now I have hundreds of these crash reports in crashlytics. lol

Actually i submitted other LazyColumn issue to issueTacker, but I’m not sure if this problem is a about LazyColumn https://issuetracker.google.com/issues/312033389

The

checkIndexBounds

crash in alpha08 is most likely this: https://issuetracker.google.com/issues/295745063#comment57

I tried using `Albert`'s method and also tried using remember(timelinePosition) { LazyListState(...) } to manually recomposition, but it doesn't seem to solve the problem. Firstly, the firstVisibleIndex in LazyState does not update. However, I have some functionalities that rely on listening to

lazystate.firstVisibileIndex

, such as saving the position of the timeline

Are you using compose stable?

no, still alpha

try stable?

@dorche but my timelineFlow requires an active account id, how do I combine it with accountFlow?

the firstVisibleIndex in LazyState does not update.

I would assume there's some problem with your code, or maybe your timeline becomes empty temporarily.

However, I have some functionalities that rely on listening to

lazystate.firstVisibileIndex

, such as saving the position of the timeline

Just add lazy list state as a key.

The

activeAccountFlow

is already private, you should be able to do make

timelinePosition

and

timeline

flows private too, and only expose one

screenState

(for example) StateFlow to Compose to consume, which then should have all the data in a good state, without mismatches.

update: I seem to be getting close to solving this problem, but, I ran into some minor issues, detail: https://kotlinlang.slack.com/archives/CJLTWPH7S/p1700892289386529

This looks exactly like the out of sync problem I suggested.

```val timeline = activeAccountFlow

.flatMapLatest { timelineDao.getStatusListWithFlow(it.id) }```

This introduces some asynchronism and the problem is likely because timeline is updated after the other two (so the UI will first be composed with the old value of timeline and the new values of the other two, and then be composed with all the new values).

thank you @Albert Chang I just solved this problem ! now, this flow looks like:

private val activeAccountFlow = accountDao
    .getActiveAccountFlow()
    .filterNotNull()
    .distinctUntilChanged { old, new -> old.id == new.id }

  val combinedFlow = activeAccountFlow
    .flatMapLatest { activeAccount ->
      val timelineFlow = timelineDao.getStatusListWithFlow(activeAccount.id)
      timelineFlow.map {
        UiState(activeAccount.id, it, TimelinePosition(activeAccount.index, activeAccount.offset))
      }
    }
    .stateIn(
      scope = viewModelScope,
      started = SharingStarted.WhileSubscribed(),
      initialValue = null
    )

I didn't think it would work out that way 😅

albert is the GOAT