assertLastValue for RxJava TestObservers

The TestObserver is an RxJava staple for testing.

It allows you to assert values in a stream, in the specific order they were emitted. Here’s a quick code snippet from the movies-usf repository ¹:

@Test
fun onSearchingForMovieBladeRunner_shouldSeeSearchResult() {
    viewModel = MSMainVm(mockApp, mockMovieRepo)
    val viewStateTester = viewModel.viewState.test()

    viewModel.processInput(SearchMovieEvent("blade runner 2049"))

    viewStateTester.assertValueAt(1) {
        assertThat(it.searchedMovieTitle).isEqualTo("Searching Movie...")
        true
    }

    viewStateTester.assertValueAt(2) {
        assertThat(it.searchedMovieTitle).isEqualTo("Blade Runner 2049")
        // ...
        true
    }
}

If you look at the source for the base TestObserver, there are a bunch of these useful methods:

RxJava Tip for the Day - Share, Publish, Refcount and All That Jazz

Ok, so in my previous post I innocuously introduced the .share() operator.

Observable<Object> tapEventEmitter = _rxBus.toObserverable().share();

The .share() operator is basically just a wrapper to the chained call .publish().refcount().

You’ll find the chained combo .publish().refcount() used in quite a few Rx examples on the web. It allows you to “share” the emission of the stream. Considering how powerpacked and frequently used this combo is, RxJava basically introduced the friendlier more useful operator share(). This mechanism is sometimes referred to as “multicasting”.

#2024

#2019

#2015

#2014