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:

#2024

#2019

#2016

#2015