Unit testing class with own coroutine scope

Question

I have a class that looks something like

class Foo {
  private val scope = Job() + Dispatchers.IO
  val emissions = PublishSubject.create<Bar>()

  fun doSomething() {
    scope.launch {
      // Do a whole bunch of work...
      withContext(Dispatchers.Main) emissions.onNext(bar)
    }
  }
}

And I'm trying to come up with a way to unit test it. I've tried making the scope injectable and writing something like

@Test fun testFoo() = runBlockingTest {
  Dispatchers.setMain(TestCoroutineDispatcher())
  val foo = Foo(this)
  foo.doSomething()
  foo.emissions.assertStuff()
}

but that doesn't seem to work. The assertion happens before the coroutine inside doSomething() has finished.

I've also tried making this dispatchers injectable, providing Dispatchers.Unconfined, but that didn't help either. Is there something wrong with this approach?

Answer 1

If you are able to expose the job for public API, you can try

class Foo {
  private val scope: CoroutineScope = CoroutineScope(Job() + Dispatchers.IO)
  val emissions = PublishSubject.create<Bar>()

  fun doSomething() = scope.launch {
    // Do a whole bunch of work...
    withContext(Dispatchers.Main) { emissions.onNext(bar) }
  }
}

class Test {
  private val testFoo = Foo()
  private val testObserver: TestObserver<Bar> = TestObserver.create()

  @BeforeEach
  fun setUp() {
    testFoo.emissions.subscribe(testObserver)
  }

  @Test fun testFoo() {
    runBlockingTest {
      Dispatchers.setMain(TestCoroutineDispatcher())
      testFoo.doSomething().join()
      testObserver.assertValue(bar)
    }
  }
}

Answer 2

Testing asynchronous code can be done well with this library: Awaitility (or its kotlin extension)

You would write something like:

@Test fun testFoo() {

  val foo = Foo()

  foo.doSomething()

  await().atMost(5, MILLIS).until(/* your check like - foo.emissions.onNext(bar) */);
}