I'm running into an issue when using select, and I...

# coroutines

[madsbf]

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.

[altavir]

What do you expect as result? forEach returns Unit

[madsbf]

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

[madsbf]

I think the select-block is expecting Unit, so the forEach should be fine as long as I return a Content-object inside the onJoin-block. But maybe I'm misunderstanding something there?

[altavir]

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

[madsbf]

👍 I'll try that

[altavir]

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

[madsbf]

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

[madsbf]

Seems to be my lack of understanding of Scope. My current implementation waits for all Jobs to complete, not just one of them. If I replace launch with GlobalScope.launch f.x. it works. I'll have to look more into that to figure out how I should be handling this.

[altavir]

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

[bdawg.io]

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 {
    [e-mail]()
    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

[madsbf]

Makes sense, thanks @bdawg.io 🙂