Hello ! Is there a difference (except style) betwe...

# coroutines

[Nino]

Hello ! Is there a difference (except style) between the "first" and "second" way to associate the 3 flows? If yes, what is the preferred way, performance-wise ?

To me, the second way is preferrable only because it's easier to read. But maybe the first one is more performant ?

import kotlinx.coroutines.*
import kotlinx.coroutines.flow.*

fun main() = runBlocking {
    val flowA : Flow<Int> = flowOf(1, 2, 3)

    flowA.flatMapLatest { a: Int ->
        doSomeMemoryIntensiveStuffForFlowB(a).map { b: String ->
            doSomeCpuIntensiveStuff(b)
        }
    }.collect { c: Long ->
        println(c)
    }
    
    
    flowA.flatMapLatest { a: Int ->
        doSomeMemoryIntensiveStuffForFlowB(a)
    }.map { b: String ->
        doSomeCpuIntensiveStuff(b)
    }.collect { c: Long ->
        println(c)
    }
}

fun doSomeMemoryIntensiveStuffForFlowB(a: Int): Flow<String> {
    // Some Memory intensive stuff related to "a" (object creation, etc)
    var memoryGreedyList = listOf<Int>()

    repeat(10_000) { memoryGreedyList = memoryGreedyList + listOf(it) }

    return flowOf(a.toString(), a.toString(), a.toString())
}

fun doSomeCpuIntensiveStuff(b: String): Long {
    // Some CPU intensive stuff related to "b" (mapping, etc)
    repeat(10_000) { b.toDouble() * it }
    
    return b.toLong()
}

https://pl.kotl.in/sOl65cZgZ

[Nick Allen]

flatMapLatest will cancel collecting from the previously received Flow when it gets a new one. That includes the doSomeCpuIntensiveStuff call for the "first" where it's inside the flatMapLatest lambda but not the "second". So only the "first" will try to cancel doSomeCpuIntensiveStuff in response to a new item.

flatMapLatest puts items into a Channel. That means the downstream Flow operators are running in a different coroutine than the upstream (including what's in it's lambda). So for the second, doSomeMemoryIntensiveStuffForFlowB could be processing an item while doSomeCpuIntensiveStuff is still working on some previous item.

FYI: None of your "intensive" sample code is actually cancellable: https://kotlinlang.org/docs/cancellation-and-timeouts.html. But maybe that's skipped here for the purposes of simplifying the sample code.