I'm running into an issue when using select, and I might just be missing something very simple. Maybe someone here can spot what I'm doing wrong? I have the following code:

suspend fun select(contentMap: Map<Content, VideoContentItemHolder>) = coroutineScope {
    select<Content> {
        contentMap.forEach { content, itemHolder ->
            launch { itemHolder.onClick() }.onJoin {
                content
            }
        }
    }
}

VideoContentItemHolder.onClick() is a suspending function. Basically I am waiting for any of the itemHolder.onClick() to complete, and then returning the associated Content-object. I have attached the debugger to see that onClick() successfully completes for one of the itemHolders and I enter the onJoin-block, but the select clause never gets a result. So anyone waiting for the result is suspended indefinitely.

What do you expect as result?

forEach

returns

Unit

I'm expecting a result of Content. I'm trying to select a deferred value. Like here: https://kotlinlang.org/docs/reference/coroutines/select-expression.html#selecting-deferred-values

It is rather hard to see through it. Probably it is better to move launches from select.

👍 I'll try that

Not sure it will help. But launching coroutines in select is bad practice anyway.

Hmm, I just tried creating the Jobs outside of the select-block. Unfortunately didn't make any difference 😞

coroutineScope

is not needed here, yet I don't understand why it behaves like this.

The

coroutineScope

is suspending until after all of your item holders have been clicked. If this isn't the intended behavior, cancel all of your other jobs in the onJoin:

.onJoin {
    this@coroutineScope.cancelChildren()
    content
}

Your select clause is completing, but your

coroutineScope

won't return until all of the other launched jobs have completed. Thus preventing leaked jobs via Structured Concurrency

Makes sense, thanks @bdawg.io 🙂