I want to downlaod list of assets in parallel, is ...

# coroutines

[ursus]

I want to downlaod list of assets in parallel, is this correct?

private suspend fun prefetchStories(stories: List<Story>) {
    coroutineScope {
        val fetches = mutableListOf<Deferred<Unit>>()
        for (story in stories) {
            fetches += async {
                prefetchStory(story)
            }
        }
        fetches.awaitAll()
    }
}

Unsure about the coroutineScope. Most example don't use it but I presume that means they're using GlobalScope, right?

[Joffrey]

coroutineScope is the recommended way to go when you're inside a suspend function and want to parallelize some work, and wait for everything before returning.

Since you don't need results, there is no point in using async, you can simply use launch instead.
Also, coroutineScope will automatically wait for child coroutines to finish so you don't need to explicitly wait for them:

private suspend fun prefetchStories(stories: List<Story>) {
    coroutineScope {
        stories.forEach { story ->
            launch {
                prefetchStory(story)
            }
        }
    }
}

[uli]

No need for `fetches`and awaitAll. Neither for async.

private suspend fun prefetchStories(stories: List<Story>) {
    coroutineScope {
        for (story in stories) {
            launch {
                prefetchStory(story)
            }
        }
    }
}

`coroutineScope`will wait for all children.

[Joffrey]

@uli pretty close 😄

[uli]

🤣

[uli]

you were first though. and i guess we are right. or at least “the same wrong”

[Joffrey]

I'm not sure this qualifies as being first, given how close in time we posted our answers - but I find it pretty funny how similar they are 😆

[Joffrey]

@ursus if you needed to gather results from prefetchStory calls, you would indeed write something similar to what you have, although you can simplify it a bit using `map`:

private suspend fun prefetchStories(stories: List<Story>): List<PREFETCH_RETURN_TYPE> {
    return coroutineScope {
        stories.map { story ->
            async {
                prefetchStory(story)
            }
        }.awaitAll()
    }
}

[ursus]

the scope reference was what I was after in a plain suspend fun, thanks!