RxJava 2. Verify TestObserver's invocation order

Question

I have two observables in my API (view model can show the main idea, it's only example):

interface ViewModel {
    val height: Observable<Int>
    val color: Observable<Int>

    fun refresh()
}

How can I verify invocation order of TestObserver in unit test:

@Test
fun `When vm refresh, then color should be applied and then height`() {
    val heightObserver = vm.height.test()
    val colorObserver = vm.color.test()

    vm.refresh()

    // how to verify colorObserver value should be delivered first?
}

In common case we shouldn't rely on order, but question however: are we have a way to verify order and how?

Answer 1

I suggest using the merge operator. For example, these observables implements as ReplaySubject(not necessarily, there may be any other):

class ViewModelImpl : ViewModel {
    private val colorSubject = ReplaySubject.create<Int>()
    private val heightSubject = ReplaySubject.create<Int>()

    override val height: Observable<Int> = heightSubject
    override val color: Observable<Int> = colorSubject

    override fun refresh() {
        colorSubject.onNext(1)
        colorSubject.onNext(2)
        colorSubject.onNext(3)

        heightSubject.onNext(4)
        heightSubject.onNext(5)
        heightSubject.onNext(6)
    }
}

For such an implementation, this test will succeed:

@Test
fun refresh() {
    val vm: ViewModel = ViewModelImpl()
    vm.refresh()
    Observable.merge(vm.color, vm.height)
            .test()
            .assertValues(1, 2, 3, 4, 5, 6)
}

And this a test is performed with an error:

@Test
fun name() {
    val vm: ViewModel = ViewModelImpl()
    vm.refresh()
    Observable.merge(vm.color, vm.height)
            .test()
            .assertValues(4, 5, 6, 1, 2, 3)
}