Kotlin coroutines runBlocking async

Question

I'm new to coroutines and trying to leverage them to call a time consuming method a couple of times in less time

fun callAPI(idList: Collection<String>): List<String> {
       
        val storedIds = mutableListOf<String>()
        runBlocking {
             val ids = idList.map { data ->
                   async {timeConsumingMethod(data)}
                    }.map { it.await() }
                    storedIds.addAll(ids)

        }
        return storedIds
    }

I need all the calls to timeConsumingMethod to run in parallel, but i don't want callAPI to return until after all the timeConsumingMethods finish.

Running this i see the timeConsumingMethods are running synchronously

Can anyone give me a hand in understanding what mistake i'm missing?

How are you determining that the methods are running synchronously? — Louis Wasserman
there is logging in the timeConsumingMethod that shows the rest call it makes finishes before the next one kicks off — Adam Golden
Yea, cuz runBlocking is single threaded they'll run sequencially, why are you using runBlocking though? Any problem making the function suspend? And same suggestion as given by IR42, for a blocking process consider using Dispatchers.IO (otherwise Dispatchers.Default for CPU driven tasks). — Animesh Sahu
callAPI is called from a non-suspended method. I actually did end up using Dispatchers.Default, which worked so if @IR42 wants to make a response i'll make that the selected answer. — Adam Golden

Tenfour04 Tenfour04 · Accepted Answer · 2020-11-20T19:05:33

You need to specify a dispatcher other than runBlocking's default single thread dispatcher, or nothing can run in parallel.

Your code can also be rearranged a bit for clarity/conciseness.

fun callAPI(idList: Collection<String>): List<String> = runBlocking(Dispatchers.IO) {
    idList.map {
        async { timeConsumingMethod(it) }
    }.awaitAll()
}

If you are working with a UI that you don't want to freeze during the operation, you should consider making this a suspend function and responding to the results when they are ready, rather than using runBlocking.

Kotlin coroutines runBlocking async

1 Answers