Hey guys, I am trying to create a polling mechanism which fetches data when at least one Subscriber ...

# coroutines

[Omkar Amberkar]

Hey guys, I am trying to create a polling mechanism which fetches data when at least one Subscriber is active and would stop otherwise, I started with sharedflow but no luck there so I tried to work it with channelflow but the problem is that the isClosedForSend flag is still false in case I cancel the job, and there is no other way to close the channelflow that I am aware of. need some help 🙂

fun poll(gameId: String, dispatcher: CoroutineDispatcher, ) = channelFlow {
            while (!isClosedForSend) {
                try {
                    send(repository.getDetails(id))
                    delay(MIN_REFRESH_TIME_MS)
                } catch (throwable: Throwable) {
                    Timber.e("Debug: error -> ${throwable.message}")
                }
                invokeOnClose { Timber.e("Debug: channel flow closed.") }
        }
    }

[Nick Allen]

Sounds like you want to use the shareIn operator with WhileSubscribed()

[Omkar Amberkar]

yes, did try that too using callbackFlow shareIn but still, the problem of how to stop polling once there are no terminal operators, or lets say when all terminator operators are canceled.

my problem is idk how to stop polling when there are no terminal operators and start it when there is at least one.

with the above flow, it still keeps polling since the channel is still open to send

[Nick Allen]

shareIn will cancel the callbackFlow and then resubscribe if a new poller starts. Your lambda will be canceled and then completely restarted.

[Nick Allen]

Just realized you are catching the CancellationException. Rethrow if the exception is CancellationException.

[Nick Allen]

Actually you need to ditch the whole try/catch.

[Nick Allen]

It's called exception transparency, if the collector throws, you are supposed to make sure that your lambda throws that same exception. You can log it, but you need to rethrow it.

[Nick Allen]

Cant get the link to the section on my phone 😅 but it's in these docs https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines.flow/-flow/index.html