I'm looking for a way to subscribe to a conflated `BroadcastChannel` but ignore any items that may a...

# coroutines

[Dan Lowe]

I'm looking for a way to subscribe to a conflated BroadcastChannel but ignore any items that may already be in that channel at the time of subscription. Is this possible? (example in thread)

[Dan Lowe]

val demoScope = CoroutineScope(<http://Dispatchers.IO|Dispatchers.IO>)

val myBroadcastChannel = BroadcastChannel<String>(Channel.CONFLATED)

myBroadcastChannel.send("1")

demoScope.launch {
    myBroadcastChannel.openSubscription().consumeEach {
        println("subscriber1: $it")
    }
}

delay(1000)
myBroadcastChannel.send("2")
delay(1000)
myBroadcastChannel.send("3")
delay(1000)
myBroadcastChannel.send("4")


demoScope.launch {
    myBroadcastChannel.openSubscription().consumeEach {
        println("subscriber2: $it")
    }
}

delay(1000)
myBroadcastChannel.send("5")

/* current output 
    subscriber1: 1
    subscriber1: 2
    subscriber1: 3
    subscriber1: 4
    subscriber2: 4
    subscriber2: 5
    subscriber1: 5
 */

/* deesired output 
    subscriber1: 2
    subscriber1: 3
    subscriber1: 4
    subscriber2: 5
    subscriber1: 5
 */

[octylFractal]

I think your best bet is going to be checking valueOrNull, and if it is non-null, drop the first element from the subscription

[Dan Lowe]

Good suggestion. I'm still trying to see if there are any edge cases, but it looks like I can get away with providing the channel like:

myBroadcastChannel.openSubscription().apply {
        receiveOrNull()
}

in order to get a receive channel

[Dan Lowe]

Thanks!

[Dan Lowe]

I'll have to check it out later, but I have some tests that appear to be getting hung on receiveOrNull() that's preventing further execution.

[octylFractal]

are you expecting receiveOrNull() to return if the channel is empty?

[Dan Lowe]

I was, but see that isn't what it's supposed to do. I can see why it's getting hung up now

[octylFractal]

https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines.channels/-receive-channel/poll.html

[Dan Lowe]

There it is, poll looks like it works. Still waiting on some tests, but the problematic ones have passed

[Dan Lowe]

Thanks again