Hi, as far as i know using suspend in function tha...

# flow

[Václav Škorpil]

Hi, as far as i know using suspend in function that returns flow is bad practice and you should use CoroutineContext.() insted. But what to do when it is a member function and you need to call suspend fun inside?

This is how i would call it with suspend

class KeywordService(val keywordRepo: KeywordRepo) {

    fun getKeywordData(
        keywordId: UUID,
    ): Flow<String> {
        val name = keywordRepo.getKeywordName() // suspend function 
        return keywordRepo.getKeywordData(name) // function that returns flow
    }
}

Currently i am calling it with a flow builder, but i am not sure if this is correct.

class KeywordService(val keywordRepo: KeywordRepo) {

    fun getKeywordData(
        keywordId: UUID,
    ): Flow<String> = flow {
        val name = keywordRepo.getKeywordName() // suspend function 
        val dataFlow = keywordRepo.getKeywordData(name) // function that returns flow
        emitAll(dataFlow)
    }
}

This way it feels little bit clunky, is there any better way to do it?

[Francesc]

You could do it like this

fun getKeywordData(
            keywordId: UUID,
        ): Flow<String> = flow {
            emit(keywordRepo.getKeywordName())
        }.flatmapLatest { name ->
            keywordRepo.getKeywordData(name)
        }

[Václav Škorpil]

But that would get pretty messy when more suspend fun calls are needed, wouldn't it?

[Francesc]

do you have a more concrete example of your use case?

[Francesc]

you can always wrap your suspend functions in an extension function that converts them to a flow and then you could collect those and emit a single item that you flaptmap on

[Václav Škorpil]

fun getKeywordData(
        keywordId: UUID,
    ): Flow<String> = flow {
        val name = keywordRepo.getKeywordName() // suspend function 
        val secondArgument = differentRepo.getArgument() // suspend function 
        val thirdArgument = differentRepo.getThirdArgument() // suspend function 
        val dataFlow = keywordRepo.getKeywordData(name, secondArgument, thirdArgument) // function that returns flow
        emitAll(dataFlow)
    }

If you have something like this, then you would need to create a class to wrap them and return them in a flow right?

[Francesc]

yes, but you could either create a suspend function that calls each suspending functiona and returns when it has collected all the data, and then you flatmap on that, or you could convert each to a flow and then use combine to collect them all

[Francesc]

combine(
    flow { emit(suspendOne()) },
    flow { emit(suspendTwo()) },
) { one, two ->
    MyData(one, two)
}.flatmapLatest { data ->
    myFlow(data)
}

[Francesc]

the other option is

suspend fun aggregator(): MyData {
    val one = getOne()
    val two = getTwo()
    return MyData(one, two)
}

and you could also use async to make these 2 run in parallel

suspend fun aggregator(): MyData = coroutineScope {
    val one = async { getOne() }
    val two = async { getTwo() }
    return MyData(one.await(), two.await())
}

[Václav Škorpil]

Yes, that solution with combine looks nice, Thank you 🙂 Also could you please, explain me why that first solution that i currently use is wrong?

[Francesc]

it'snot wrong, it's perfectly valid

[Francesc]

I think it's a bit less idiomatic, but it's otherwise good

[Václav Škorpil]

I feel like it is the most straight forward, but not sure about performance as i am emiting all items to a new flow. Anyways thank you very much for your time, i am very happy that i know new ways how to work with flows.