Kotlin Stateflow get one-but-last value - kotlin

Question

In my viewModel i have a StateFlow<Destination>, with those values:
enum class Destination {
FIRST_SCREEN,
SECOND_SCREEN,
THIRD_SCREEN,
FOURTH_SCREEN,
}
Somewhere in my activity i use this flow for navigation:
viewModel.destinations[...]
.collect { destination ->
fragmentManager.navigateTo(
when(destination){
FIRST_SCREEN -> FirstScreenFragment(),
SECOND_SCREEN -> SecondScreenFragment(),
THIRD_SCREEN -> ThirdScreenFragment(),
FOURTH_SCREEN -> FourthScreenFragment(),
}
)
}
...where navigateTo() is very simple in the moment
fun FragmentManager.navigateTo(fragment: Fragment) {
beginTransaction()
.replace(R.id.contentFragment, fragment)
.commit()
}
I would now like to add transitions ~ like in a viewPager.
The animations themselves are a piece of cake, but i need to know in which direction to animate:
Am i moving "forward" or "backward", which boils down to:
Is my current destination < or > my new destination?
i could use the fragment-backstack (but i and a few others hate it)
i could simply use a variable in my activity storing the last screen we navigated to, but that feels hacky
i could try to use flows for that, but i have no real idea how to do that. Any suggestions?
An optimal usage would look like:
viewModel.destinations[...]
.rememberHistory()
.collect { currentDestination, lastDestination ->
}

Answer 1

For those wondering how i solved this:
I used runningFold
data class DestinationHistory(val previous: Destination?, val current: Destination)
val destinationHistory: Flow<DestinationHistory> = destination.runningFold(
initial = DestinationHistory(previous = null, current = Destination.FIRST_SCREEN)
) { history, newDestination ->
DestinationHistory(
previous = history.current,
current = newDestination
)
}
Effectively this maps the destination-flow to a "pair" of previous & current destination.