RxJava单元测试-可观察的在测试中不发出事件

Question

I'm testing a view model which has the following definition:

class PostViewModel(private val postApi: PostApi): ViewModel() {

    private val _post: PublishSubject<Post> = PublishSubject.create()
    val postAuthor: Observable<String> = _post.map { it.author }

    fun refresh(): Completable {
        return postApi.getPost() // returns Single<Post>
            .doOnSuccess {
                _post.onNext(it)
            }
            .ignoreElement()
        }
    }
}

My fragment then displays the post author by subscribing to viewModel.postAuthor in its onActivityCreated and calling and subscribing to refresh() whenever the user wants an updated post and everything is fine and dandy.

The issue I'm running into is trying to verify this behaviour in a unit test: specifically, I am unable to get postAuthor to emit an event in my testing environment.

My test is defined as follows:

    @Test
        fun `When view model is successfully refreshed, display postAuthor`() {


        val post = Post(...)


        whenever(mockPostApi.getPost().thenReturn(Single.just(post))
            viewModel.refresh()
                .andThen(viewModel.postAuthor)
                .test()
                .assertValue { it == "George Orwell" }
        }

The test fails due to no values or errors being emitted, even though I can verify through the debugger that the mock does in-fact return the Post as expected. Is there something obvious that I'm missing, or am I completely wrong in my testing approach?

Answer 1

viewModel.postAuthor is a hot-observable. It emits value when you call _post.onNext(it) .

Unlike a cold-observable, the late subscribers cannot receive the values that got emitted before they subscribe. So in your case I think the viewModel.postAuthor is subscribed after you call viewModel.refresh() , so it cannot receive the value.

Answer 2

The observable could be emitting on a different thread so that's why it's empty when the test is checking the values/errors.

You could try forcing your observable to emit on the same thread. Depending on which scheduler you're using, it'd be something like:

RxJavaPlugins.setIoSchedulerHandler { Schedulers.trampoline() }