Unit testing Kotlin's ConflatedBroadcastChannel behavior - kotlin

Question

In the new project that I'm currently working on I have no RxJava dependency at all, because until now I didn't need that - coroutines solve threading problem pretty gracefully.
At this point I stumbled upon on a requirement to have a BehaviorSubject-alike behavior, where one can subscribe to a stream of data and receive the latest value upon subscription. As I've learned, Channels provide very similar behavior in Kotlin, so I decided to give them a try.
From this article I've learned, that ConflatedBroadcastChannel is the type of channel that mimics BehaviorSubject, so I declared following:
class ChannelSender {
val channel = ConflatedBroadcastChannel<String>()
fun sendToChannel(someString: String) {
GlobalScope.launch(Dispatchers.Main) { channel.send(someString) }
}
}
For listening to the channel I do this:
class ChannelListener(val channelSender: ChannelSender) {
fun listenToChannel() {
channelSender.channel.consumeEach { someString ->
if (someString == "A") foo.perform()
else bar.perform()
}
}
}
This works as expected, but at this point I'm having difficulties understanding how to unit test ChannelListener.
I've tried to find something related here, but none of example-channel-**.kt classes were helpful.
Any help, suggestion or correction related to my incorrect assumptions is appreciated. Thanks.

Answer 1

With the help of Alexey I could manage to end up having following code, which answers the question:
class ChannelListenerTest {
private val val channelSender: ChannelSender = mock()
private val sut = ChannelListener(channelSender)
private val broadcastChannel = ConflatedBroadcastChannel<String>()
private val timeLimit = 1_000L
private val endMarker = "end"
#Test
fun `some description here`() = runBlocking {
whenever(channelSender.channel).thenReturn(broadcastChannel)
val sender = launch(Dispatchers.Default) {
broadcastChannel.offer("A")
yield()
}
val receiver = launch(Dispatchers.Default) {
while (isActive) {
val i = waitForEvent()
if (i == endMarker) break
yield()
}
}
try {
withTimeout(timeLimit) {
sut.listenToChannel()
sender.join()
broadcastChannel.offer(endMarker) // last event to signal receivers termination
receiver.join()
}
verify(foo).perform()
} catch (e: CancellationException) {
println("Test timed out $e")
}
}
private suspend fun waitForEvent(): String =
with(broadcastChannel.openSubscription()) {
val value = receive()
cancel()
value
}
}