How can I collect values from a mutableSharedFlow in a unit test? - unit-testing

Question

The following test doesn't pass. The string "before adding XXX" is never printed.
#Test
fun testFoo() = runBlocking {
val testCoroutineScope = TestCoroutineScope().apply {
pauseDispatcher() // This needs to be here because the actual test handles time.
}
val sharedFlow = MutableSharedFlow<Int>()
val values = mutableListOf<Int>()
println("before launch")
val job = testCoroutineScope.launch {
println("before collect")
sharedFlow.collect {
println("before adding $it")
values.add(it)
}
}
println("before emits")
sharedFlow.emit(1)
sharedFlow.emit(2)
testCoroutineScope.runCurrent()
assertEquals(mutableListOf(1, 2), values)
job.cancel()
}
I would like to have a way to emit values once the collect part is handled. I can't set a replay value, nor can I use an onSubscription because that would change the business logic.
Adding delay or yield don't seem to make any difference so it might not be a racing condition.
The test passes if pauseDispatcher() is commented.
My understanding was that emit would suspend until the collect lambda were called.

Answer 1

The way I solved it is via extraBufferCapacity and one more testCoroutineScope.runCurrent()
#Test
fun testFoo() = runBlocking {
val testCoroutineScope = TestCoroutineScope().apply {
pauseDispatcher()
}
val sharedFlow = MutableSharedFlow<Int>(
extraBufferCapacity = 2 // Without it, sharedFlow.emit won't have a space to save data. It will be collected
// next time there's a testCoroutineScope.runCurrent()
)
val values = mutableListOf<Int>()
println("before launch")
val job = testCoroutineScope.launch {
println("before collect")
sharedFlow.collect {
println("before adding $it")
values.add(it)
}
}
testCoroutineScope.runCurrent() // Allows the previous launch to start collecting
println("before emits")
sharedFlow.emit(1)
sharedFlow.emit(2)
testCoroutineScope.runCurrent()
assertEquals(mutableListOf(1, 2), values)
job.cancel()
}

Answer 2

UPDATE: Turns out my initial attempt wasn't bulletproof either, a race condition still existed, here's my revised solution, so far so good:
val msf = MutableSharedFlow<Int>()
runBlocking {
val job1 = GlobalScope.launch {
msf
.onStart {
println("${Thread.currentThread().name} - on start ")
}
.onEach { println("got $it") }
.collect()
}
val job2 = launch {
msf.subscriptionCount
.filter { it > 0 }
.onEach {
msf.emit(42)
cancel()
}
.collect()
}
delay(100)
job1.cancel()
println(job2)
}
println("${Thread.currentThread().name} - escaped!")
Old solution (incorrect):
runBlocking {
val msf = MutableSharedFlow<Int>()
val d = CompletableDeferred<Boolean>()
launch {
msf
.onStart {
d.complete(true)
}
.onEach { println("got $it") }
.collect()
}
d.join()
msf.emit(42)
}
using the invokeOnCompletion() handler cannot use suspended functions, using it with tryEmit() did not work for me (returns true but does nothing).