How can a SharedFlow drop its emission when the do...

# coroutines

[Lukas Lechner]

How can a SharedFlow drop its emission when the downstream is busy?

suspend fun main(): Unit = coroutineScope {

    val sharedFlow = MutableSharedFlow<Int>()

    launch {
        sharedFlow
            .collect {
            println("Process $it")
            delay(1000)
            println("$it processed")
        }
    }

    launch {
        // 1 should be processed
        sharedFlow.emit(1)
        println("sharedFlow emits 1")

        // 2 should not be processed since downstream is busy
        sharedFlow.emit(2)
        println("sharedFlow emits 2")

        // 3 should be processed again
        delay(2000)
        sharedFlow.emit(3)
        println("sharedFlow emits 3")
    }
}

I tried BufferOverflow.DROP_LATEST, but there we need to have a buffer value > 0

[Lukas Lechner]

Okay, I think the solution is to use a channel instead of a SharedFlow …

[Chris Fillmore]

I don’t understand the question. What is the problem you’re trying to solve?

If you want shared flow to have a buffer, pass extraBufferCapacity to its constructor.

[Casey Brooks]

Yeah, this is something that confused me for a while, but DROP_LATEST only works when there is some “buffer” (either replay or extraBufferCapacity), and the behavior is the same with both Channels and SharedFlows. I’m not really sure why this is the case, but I guess it needs that internal buffer to know when its full or busy?

[franztesca]

What would you drop if there is no buffer?

[Joffrey]

Technically there is no guarantee that the first launch has actually reached the collect call by the time you emit the first element to the shared flow