A correct example of how to unit test the PhotosController

PhotosControllerTest.kt

class PhotosControllerTest : Spek({
    describe("The PhotosController") {

        var mockPhotosApiService: PhotosApiService? = null
        var mockSchedulerForwarder: SchedulerForwarder? = null
        var uut: PhotosController? = null

        beforeEachTest {
            // uut and supporting classes setup and mocked here
        }

        describe("when getPhotos") {

            var ioTestScheduler: TestScheduler? = null
            var mainThreadTestScheduler: TestScheduler? = null

            beforeEachTest {
                // Mock the threads for the call here
            }

            describe("and the call succeeds") {

                beforeEachTest {
                    val photo1 = PhotosModel(
                        id = 42,
                        thumbnailUrl = "https://ddg.co/",
                        title = "DuckDuckGo",
                        url = "https://duckduckgo.com/"
                    )

                    val photo2 = PhotosModel(
                        id = 42,
                        thumbnailUrl = "https://ddg.co/",
                        title = "DuckDuckGo",
                        url = "https://duckduckgo.com/"
                    )

                    val data = listOf(photo1, photo2)
                    whenever(mockPhotosApiService?.getPhotos()).thenReturn(Single.just(data))

                    uut?.photosAdapter = mock()
                    uut?.getPhotos()

                    ioTestScheduler?.triggerActions()
                    mainThreadTestScheduler?.triggerActions()
                }

                it("updates the photos adapter the with fetched data") {
                    verify(uut?.photosAdapter)?.updatePhotos(data!!)
                }

                it("adds a disposable to the composite disposable") {
                    verify(mockCompositeDisposable)?.add(any())
                }
            }

            describe("and the call fails") {
                // Test failure condition here
            }
        }
    }
})