How to unit test Kotlin suspending functions

I see you found out on you own, but I'd like to explain a bit more for the people that might run into the same problem

When you do launch(UI) {}, a new coroutine is created and dispatched to the "UI" Dispatcher, that means that your coroutine now runs on a different thread.

Your runBlocking{} call create a new coroutine, but runBlocking{} will wait for this coroutine to end before continuing, your loadPreviews() function creates a coroutine, start it and then return immediately, so runBlocking() just wait for it and return.

So while runBlocking{} has returned, the coroutine that you created with launch(UI){} is still running in a different thread, that's why the order of your log is messed up

The Unconfined context is a special CoroutineContext that simply create a dispatcher that execute the coroutine right there on the current thread, so now when you execute runBlocking{}, it has to wait for the coroutine created by launch{} to end because it is running on the same thread thus blocking that thread.

I hope my explanation was clear, have a good day

I've found out that it's because of a CoroutineDispatcher. I used to mock UI context with EmptyCoroutineContext. Switching to Unconfined has solved the problem

Update 02.04.20

The name of the question suggests that there'll be an exhaustive explanation how to unit test a suspending function. So let me explain a bit more.

The main problem with testing a suspending function is threading. Let's say we want to test this simple function that updates a property's value in a different thread:

class ItemUpdater(val item: Item) {
  fun updateItemValue() {
    launch(Dispatchers.Default) { item.value = 42 }
  }
}

We need to somehow replace Dispatchers.Default with an another dispatcher only for testing purposes. There're two ways how we can do that. Each has its pros and cons, and which one to choose depends on your project & style of coding:

1. Inject a Dispatcher.

class ItemUpdater(
    val item: Item,
    val dispatcher: CoroutineDispatcher  // can be a wrapper that provides multiple dispatchers but let's keep it simple
) {
  fun updateItemValue() {
    launch(dispatcher) { item.value = 42 }
  }
}

// later in a test class

@Test
fun `item value is updated`() = runBlocking {
  val item = Item()
  val testDispatcher = Dispatchers.Unconfined   // can be a TestCoroutineDispatcher but we still keep it simple
  val updater = ItemUpdater(item, testDispatcher)

  updater.updateItemValue()

  assertEquals(42, item.value)
}

2. Substitute a Dispatcher.

class ItemUpdater(val item: Item) {
  fun updateItemValue() {
    launch(DispatchersProvider.Default) { item.value = 42 }  // DispatchersProvider is our own global wrapper
  }
}

// later in a test class

// -----------------------------------------------------------------------------------
// --- This block can be extracted into a JUnit Rule and replaced by a single line ---
// -----------------------------------------------------------------------------------
@Before
fun setUp() {
  DispatchersProvider.Default = Dispatchers.Unconfined
}

@After
fun cleanUp() {
  DispatchersProvider.Default = Dispatchers.Default
}
// -----------------------------------------------------------------------------------

@Test
fun `item value is updated`() = runBlocking {
  val item = Item()
  val updater = ItemUpdater(item)

  updater.updateItemValue()

  assertEquals(42, item.value)
}

Both of them are doing the same thing - they replace the original Dispatchers.Default in test classes. The only difference is how they do that. It's really really up to you which of them to choose so don't get biased by my own thoughts below.

IMHO: The first approach is a little too much cumbersome. Injecting dispatchers everywhere will result into polluting most of the classes' constructors with an extra DispatchersWrapper only for a testing purpose. However Google recommends this way at least for now. The second style keeps things simple and it doesn't complicate the production classes. It's like an RxJava's way of testing where you have to substitute schedulers via RxJavaPlugins. By the way, kotlinx-coroutines-test will bring the exact same functionality someday in future.