How to test async function in Mockk Kotlin

Question

I want to test this function.

suspend fun fetchTwoDocs() =
        coroutineScope {
            val deferredOne = async { fetchDoc(1) }
            val deferredTwo = async { fetchDoc(2) }
            deferredOne.await()
            deferredTwo.await()
        }

How to test this function in mockk

Answer 1

I'm assuming you can't or won't rewrite the code you are writing to use coroutines. In that case, Pablisco offered great ideas here. The one I liked the best was using a queue:

// You create a sync queue
val queue = SynchronousQueue<String>() // Or the type of 'fetch2Docs'
// fetchTwoDocs runs async so it will return eventually
queue.put(fetchTwoDocs())
// queue.take() will wait until there is a value in the queue
assertThat(queue.take(), equalTo("expected value"))

Here an extra example on how to use this in an hypothetical async callback:

val queue = SynchronousQueue<String>()
asyncFunctionThatReturnsWithCallback(someParam) { callbackParam ->
    queue.put(callbackParam)
}
assertTrue(queue.take())

Answer 2

If you just want to test the function, you can simply call it inside runBlocking from your test, or use the kotlinx-coroutines-test library which provides runBlockingTest:

@Test
fun test() = runBlocking {
    val result = fetchTwoDocs()
    // then assert stuff
}