I want `scanAudios()` and `scanVideos()` to execut...

# coroutines

[The Monster]

I want scanAudios() and scanVideos() to execute first before other stuff in this function.

private suspend fun getFolderDataList(): List<FolderData> {
        thisScope.async() {
            scanAudios()
            scanVideos()
        }.await()

        val audioFolderData = viewModel.getAudioUris().map { uri ->
            FolderData("Audio", uri)
        }

        val videoFolderData = viewModel.getVideoUris().map { uri ->
            FolderData("Video", uri)
        }

        return audioFolderData.plus(videoFolderData)
    }

Am I doing it correctly?

[Dominaezzz]

Short answer is yes.

[Dominaezzz]

I'm a little sceptical about the immediate await after the async, but it should do what you want.

[Dominaezzz]

Why have you used async there?

[The Monster]

I needed the function await(). launch() does not have it. launch() only has invokeOnCompletion() which I cannot use because the other code are also suspend functions.

[Dominaezzz]

launch has join .

[Dominaezzz]

Why not just call scanAudios and scanVideos without wrapping it in a coroutine?

[The Monster]

oh I didn't know of join. Let me check it out.

[The Monster]

scanAudios and scanVideos need to complete first for the code after to be accurate.

[Dominaezzz]

Sure but you can do that without wrapping them.

[Dominaezzz]

The wrapping you've added there doesn't change anything with regards to ordering.

[The Monster]

oh how do I wait for them without the join or await then?

[Dominaezzz]

private suspend fun getFolderDataList(): List<FolderData> {
        scanAudios()
        scanVideos()

        val audioFolderData = viewModel.getAudioUris().map { uri ->
            FolderData("Audio", uri)
        }

        val videoFolderData = viewModel.getVideoUris().map { uri ->
            FolderData("Video", uri)
        }

        return audioFolderData.plus(videoFolderData)
    }

[Dominaezzz]

What do you mean by wait? What does scanAudios do? It's a suspend function right?

[The Monster]

are you sure getAudioUris() and getVideoUris() won't be executed when `scanAudios`/`scanVideos` are running?

[The Monster]

yes scanAudios is a suspend function. getAudioUris and getVideoUris are also suspend.

[Dominaezzz]

Yes, suspend functions suspend the coroutine when they are called. You only get concurrency when you introduce it with launch or async.

[The Monster]

wait here means I want scanAudios and scanVideos to finish first before calls to getAudioUris and getVideoUris

[The Monster]

ohh

[Dominaezzz]

Do you also want scanAudios and scanVideos to run at the same time?

[The Monster]

Thanks for the help. I haven't used coroutines for a while; I must have confused between concurrency and suspension.

[The Monster]

yes I want them to run at the same time

[Dominaezzz]

Ah, in that case.....

[Dominaezzz]

private suspend fun getFolderDataList(): List<FolderData> {
        coroutineScope {
            launch { scanAudios() }
            launch { scanVideos() }
        }

        val audioFolderData = viewModel.getAudioUris().map { uri ->
            FolderData("Audio", uri)
        }

        val videoFolderData = viewModel.getVideoUris().map { uri ->
            FolderData("Video", uri)
        }

        return audioFolderData.plus(videoFolderData)
    }

[The Monster]

perfect. Thanks. Your code helps this part of the doc makes more sense now. https://kotlinlang.org/docs/composing-suspending-functions.html#concurrent-using-async

[Joffrey]

Calling suspend functions is like regular functions, they don't return until they're done. Using async{...}.await() is pointless and actually equivalent to just calling the functions. The only difference here is you're using a particular scope, so the coroutine context could be different, but in terms of what waits and what doesn't it's equivalent

[Joffrey]

If you want to call scanAudios and scanVideos concurrently, it's another story. But it doesn't seem to be your goal here, so you can keep things very simple