Polling repository with coroutine channels - kotlin

Question

I'm playing with coroutine channels and I wanted to implemented a polling test project. The idea is that a viewmodel will listen for data from a repository that polls an endpoint repeatedly.
When I pass a coroutineScope to the repository, the polling works, however when I create a new coroutineSCope in the repository, I see the data being injected into the channel, but it's not received on the viewmodel.
So this works:
class PollingViewModel : ViewModel() {
val counter = MutableLiveData<String>().apply { value = "uninitialized" }
private val repository = Repository()
init {
viewModelScope.launch {
val channel = repository.poll(this /* scope */)
channel.consumeEach {
Log.d("foo", "Viewmodel received [$it]")
counter.postValue(it.toString())
}
}
}
}
class Repository {
private var startValue = 0
suspend fun poll(coroutineScope: CoroutineScope) =
coroutineScope.produce(capacity = Channel.CONFLATED) {
while (true) {
Log.d("foo", "Sending value [$startValue]")
send(startValue++)
delay(POLLING_PERIOD_MILLIS)
}
}
companion object {
private const val POLLING_PERIOD_MILLIS = 1000L
}
}
But this does not (viewmodel does not receive anything):
class PollingViewModel : ViewModel() {
val counter = MutableLiveData<String>().apply { value = "uninitialized" }
private val repository = Repository()
init {
viewModelScope.launch {
repository.poll().consumeEach {
Log.d("foo", "Viewmodel received [$it]")
counter.postValue(it.toString())
}
}
}
}
class Repository {
private var startValue = 0
suspend fun poll() = coroutineScope {
produce(capacity = Channel.CONFLATED) {
while (true) {
Log.d("foo", "Sending value [$startValue]")
send(startValue++)
delay(POLLING_PERIOD_MILLIS)
}
}
}
companion object {
private const val POLLING_PERIOD_MILLIS = 1000L
}
}
What is the issue with creating a coroutineScope at the repository level?

Answer 1

Looks like the solution is to create a new CoroutineContext in the repository:
class Repository {
private var startValue = 0
private val context: CoroutineContext by lazy(LazyThreadSafetyMode.NONE) {
Job() + Dispatchers.IO
}
suspend fun poll(): ReceiveChannel<Int> = coroutineScope {
produce(
context = context,
capacity = Channel.CONFLATED
) {
while (true) {
send(startValue++)
delay(POLLING_PERIOD_MILLIS)
}
}
}
companion object {
private const val POLLING_PERIOD_MILLIS = 1000L
}
}