I thought it would be easy because I will always h...

# coroutines

[leandro]

I thought it would be easy because I will always have an initial value (ZonedDateTime.now()), but if that's too cumbersome I can make its consumers suspend

[wbertan]

Could you use the SharedFlow? This week I’ve received help here and a suggestion was to use it, and for me it works as expected, here is the thread: https://kotlinlang.slack.com/archives/C1CFAFJSK/p1606233013349500?thread_ts=1606232057.348900&cid=C1CFAFJSK

[leandro]

although SharedFlow does more than the StateFlow, it doesn't seem like I can sneak in some transformation to have it emit regularly 😕

[wbertan]

You could use onSubscription 🤔

MutableSharedFlow<Long>(
    0,
    extraBufferCapacity = 1,
    onBufferOverflow = BufferOverflow.DROP_OLDEST
).onSubscription {
    while(true) {
        emit(System.currentTimeMillis())
        delay(5_000)
    }
}

[elizarov]

It would not work. On subscription will suspend the whole flow. You cannot work around having to provide scope. Still, why it has to be SharedFlow? Why a regular flow does not work for you?

[wbertan]

It is only recent I’ve start to look more in Coroutines, but from my test it executes either way, for example:

class AsasTest {

    val a = MutableSharedFlow<Long>(
        replay = 0, extraBufferCapacity = 1, onBufferOverflow = BufferOverflow.DROP_OLDEST
    ).onSubscription {
        val id = UUID.randomUUID().toString()
        println("creating flow $id")
        while (true) {
            println("emit flow $id")
            emit(System.currentTimeMillis())
            delay(5_000)
        }
    }

    @Test
    fun testAsasA() {
        runBlocking {
            a.onEach { println("a1 $it") }.launchIn(this)
            a.onEach { println("a2 $it") }.launchIn(this)
        }
    }
}

It will result in this output:

creating flow 7e83c9c0-af30-4621-99a7-87dfc3c4fbc9
emit flow 7e83c9c0-af30-4621-99a7-87dfc3c4fbc9
a1 1606339289726
creating flow 83a59d4a-8394-49e8-b11b-ca7309917fde
emit flow 83a59d4a-8394-49e8-b11b-ca7309917fde
a2 1606339289739
emit flow 7e83c9c0-af30-4621-99a7-87dfc3c4fbc9
a1 1606339294743
emit flow 83a59d4a-8394-49e8-b11b-ca7309917fde
a2 1606339294743
emit flow 7e83c9c0-af30-4621-99a7-87dfc3c4fbc9
a1 1606339299747
emit flow 83a59d4a-8394-49e8-b11b-ca7309917fde
a2 1606339299748

I understand it would be the same as doing:

val b = flow {
        val id = UUID.randomUUID().toString()
        println("creating flow $id")
        while(true) {
            println("emit flow $id")
            emit(System.currentTimeMillis())
            delay(5_000)
        }
    }

And my understand of the question would be how to share it to not have to “create” a new Flow or SharedFlow for each subscription, but I’ve tried in the past and still don’t know how to do it, I believe it would be something like:

val c = flow {
        val id = UUID.randomUUID().toString()
        println("creating flow $id")
        while(true) {
            println("emit flow $id")
            emit(System.currentTimeMillis())
            delay(5_000)
        }
    }.shareIn(<What I should put here to share?>)

But I’m struggling with the shareIn as it is a val and I don’t have a CoroutineScope to provide it. To understand my original scenario it was to replace a Rx Observable.share() .

[ursus]

well, you want to share state, it needs a scope

however your state is basically in now() so ju might just prepend that value on every subscription? onStart(emit(now()) something like that