How to unit test ConflatedBroadcastChannel? - unit-testing

Question

I have a test here, where I am attempting to validate what is being sent to a ConflatedBroadcastChannel (experimental API, I know):
#Test
fun myTest() = runBlockingTest {
val results = ArrayList<String>()
val myChannel = ConflatedBroadcastChannel<String>()
myChannel.openSubscription().consumeEach {
results.add(it)
}
myChannel.send("hello")
assertEquals(1, results.size)
}
But when I run that code, I receive an IllegalStateException with the messge: "This job has not completed yet". Anyone know what's missing? I've tried using TestCoroutineDispatcher as well to no avail.

Answer 1

If you just want to obtain a single value from a channel you can always use receive.
Since ConflatedBroadcastChannel caches the latest value, you can just use it like this:
#Test
fun myTest() = runBlockingTest {
val myChannel = ConflatedBroadcastChannel<String>()
myChannel.send("hello")
val subscription = myChannel.openSubscription()
val result = subscription.receive()
subscription.cancel()
assertEquals("hello", result)
}

Answer 2

So I thought about the message I was getting, and suspected maybe that the "job that's not completed yet" was the .consumeEach operation I was performing. So to be explicit, I wrapped the operation in a launch(myTestCoroutineDispatcher) and called cancel. This doesn't seem like the ideal way to test things, but it makes sense given that the test environment isn't really guessing when your channel should stop receiving things. Anyways, lo and behold, this works:
#Test
fun myTest() = runBlockingTest {
val results = ArrayList<String>()
val myChannel = ConflatedBroadcastChannel<String>()
launch(testMainDispatcher) {
myChannel.openSubscription().consumeEach {
results.add(it)
cancel()
}
}
myChannel.send("hello")
assertEquals(1, results.size)
}
EDIT: another thing you can do to achieve same results is val myJob = launch {...} and then at the end of your test execution, call myJob.cancel() for the same effect.