create async function in Kotlin

Question

I am trying to create a async function in kotlin coroutine, this is what I tried by following a tutorial:

fun doWorkAsync(msg: String): Deferred<Int> = async {
    delay(500)
    println("$msg - Work done")
    return@async 42
}

But in this code the async can't be resolved by compiler, but the tutorial video shows it worked fine. Is it because the tutorial is using old Kotlin coroutines' way of doing things? Then, if so how to create async function?

are you certain that your async is the same as async in the video? — apksherlock
– apksherlock, Commented Sep 3, 2019 at 11:33
I would strongly suggest against creating async functions. Usually, there is better way to have your problem solved. Can you please describe what problem you are trying to solve with async function? — Roman Elizarov
– Roman Elizarov, Commented Sep 3, 2019 at 20:48

Andrei Tanana · Accepted Answer · 2019-09-03 11:43:05Z

20

When coroutines have experimental API it was possible to write just

async {
    // your code here
}

but in stable API you should provide a CoroutineScope where coroutine will run. You can do it in many ways:

// should be avoided usually because you cannot manage the coroutine state. For example cancel it etc
fun doWorkAsync(msg: String): Deferred<Int> = GlobalScope.async {
    delay(500)
    println("$msg - Work done")
    return@async 42
}

or

// explicitly pass scope to run in
fun doWorkAsync(scope: CoroutineScope, msg: String): Deferred<Int> = scope.async {
    delay(500)
    println("$msg - Work done")
    return@async 42
}

or

// create a new scope inside outer scope
suspend fun doWorkAsync(msg: String): Deferred<Int> = coroutineScope {
    async {
        delay(500)
        println("$msg - Work done")
        return@async 42
    }
}

or even

// extension function for your scrope to run like 'scope.doWorkAsync("Hello")'
fun CoroutineScope.doWorkAsync(msg: String): Deferred<Int> = async {
    delay(500)
    println("$msg - Work done")
    return@async 42
}

answered Sep 3, 2019 at 11:43

Andrei Tanana

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1 Comment

Emanuel Moecklin Over a year ago

Just wanted to mention that the third solution won't run multiple calls to doWorkAsync concurrently (unlike the other solutions) because it creates a new scope with coroutineScope.

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