Kotlin - How To Collect X Values From a Flow? - kotlin

Question

Let's say I have a flow that is constantly sending updated like the following:
locationFlow = StateFlow<Location?>(null)
I have a use-case where after a particular event occurs, I want to collect X values from the flow and continue, so something like what I have below. I know that collect is a terminal operator, so I don't think the logic I have below works, but how could I do this in this case? I'd like to collect X items, save them, and then send them to another function for processing/handling.
fun onEventOccurred() {
launch {
val locations = mutableListOf<Location?>()
locationFlow.collect {
//collect only X locations
locations.add(it)
}
saveLocations(locations)
}
}
Is there a pre-existing Kotlin function for something like this? I'd like to collect from the flow X times, save the items to a list, and pass that list to another function.

Answer 1

It doesn't matter that collect is terminal. The upstream StateFlow will keep behaving normally because StateFlows don't care what their collectors are doing. you can use the take function to get a specific number of items, and you can use toList() (another terminal function) to concisely copy them into a list once they're all ready.
fun onEventOccurred() {
launch {
saveLocations(locationFlow.take(5).toList())
}
}

Answer 2

If I understood correctly your use case, you want to:
discard elements until a specific one is sent – actually, after re-reading your question I don't think this is the case.. I'm leaving it in the example just FYI
when that happens, you want to collect X items for further processing
Assuming that's correct, you can use a combination of dropWhile and take, like so:
fun main() = runBlocking {
val messages = flow {
repeat(10) {
println(it)
delay(500)
emit(it)
}
}
messages
.dropWhile { it < 5 }
.take(3)
.collect { println(it) } // prints 5, 6, 7
}
You can even have more complex logic, i.e. discard any number that's less than 5, and then take the first 10 even numbers:
fun main() = runBlocking {
val messages = flow {
repeat(100) {
delay(500)
emit(it)
}
}
messages
.dropWhile { it < 5 }
.filter { it % 2 == 0}
.take(10)
.collect { println(it) } // prints even numbers, 6 to 24
}