hi.. i probably just need a refresher.. but how would i manage launching 1 coroutinesfor each entry ...

# coroutines

[Nikky]

hi.. i probably just need a refresher.. but how would i manage launching 1 coroutinesfor each entry in a list coming fro ma stateflow? and relaunching them as the stateflow changes ?

[kevin.cianfarini]

val flow: StateFlow<List<...>> = ...
flow.collectLatest { list ->
  coroutineScope {
    list.forEach { item -> 
      launch { item.doSomething() }
    }
  }
}

[Nikky]

thanks

[ross_a]

This will collect each state flow item and wait for each launched task to complete before accepting the next which may mean they are conflated
stateFlow.collect { it.forEach { launch { task() } } }

[Nikky]

i was missing the bit about collectLatest cancelling the previous one

[ross_a]

ah sorry I missed that 🙂

[kevin.cianfarini]

stateFlow.collect will also conflate because state flow is always conflated for slow collectors

[ross_a]

yep - you'd need to buffer

[Nikky]

my previous code looked like so using just a List<>

decks.forEach {
    it
        .doThing()
        .launchIn(flowScope)
}

[ross_a]

I misunderstood the relaunching, I didn't realise you'd want to cancel on a new emission, I just flagged the conflation as a warning

[Nikky]

so.. if both collect and collectlatest conflate.. how would i cancel jobs launched inside of it properly before launching them again ?

[kevin.cianfarini]

collectLatest will conflate less than collect.

[kevin.cianfarini]

The former does not exert backpressure, the latter does

[kevin.cianfarini]

You’d run into conflation with collectLatest when you change the state of a state flow multiple times before it has a chance to emit those changes to flow collectors

[kevin.cianfarini]

Eg.

val sf = MutableStateFlow(0)
sf.value = sf.value + 1
sf.value = sf.value + 1
sf.value = sf.value + 1

In this scenario, you’re not guaranteed to get delivered 0, 1, 2, 3 because those updates might happen quicker than coroutine dispatch can occur. This happens regardless of the mechanism by which you collect this state flow.

[Nikky]

this is unlikely to be a issue.. i will try this solution for now https://kotlinlang.slack.com/archives/C1CFAFJSK/p1718985324364759?thread_ts=1718985008.564809&amp;cid=C1CFAFJSK