Hi, maybe this is a trivial, but I’m trying to cre...

# flow

[Geert]

Hi, maybe this is a trivial, but I’m trying to create a new Flow from a StateFlow that is based on the current and the previous state. Eg. a StateFlow<Int> with state 2 and then 5 and then 1 would cause the flow to emit 3 and then -4.

[Geert]

I’m currently working in the lines of:

val sf: StateFlow<Int> = MutableStateFlow(0)
var state: Int = sf.value
val someFlow = sf.map {
    val diff = it - state
    state = it
    diff
}

But I feel that there could be some better solution.

[Peter Farlow]

Are you looking for runningReduce? https://kotlinlang.org/api/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines.flow/running-reduce.html

[Peter Farlow]

or runningFold? https://kotlinlang.org/api/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines.flow/running-fold.html

[Geert]

Wow, I did not consider using a fold, thanks!

[Geert]

Lovely:

val stateFlow: StateFlow<Int> = MutableStateFlow(0)
stateFlow
    .runningFold(0 to 0) { (_, last), next -> last to next }
    .map { (last, current) -> last - current }