SomeViewModelTestWithBuilder.kt

class SomeViewModelTest {

    private val someInteractor: SomeInteractor = mockk(relaxed = true)
    private val dataToLoad: String = "Data to load"

    @Before
    fun setUp() {
        clearAllMocks()
    }

    @Test
    fun `Loader state should be shown while data is being loaded`() = runTest {
        // Replaced with the builder
        val viewModel = ViewModelBuilder()
            .withLoadingData()
            .build(testScheduler)

        assertThat(viewModel)
            .loaderIsShown()
            .errorIsHidden()
            .contentIsHidden()
    }

    @Test
    fun `Should show content if data is loaded`() = runTest {
        // Replaced with the builder
        val viewModel = ViewModelBuilder()
            .withLoadedData(dataToLoad)
            .build(testScheduler)

        assertThat(viewModel)
            .contentIsShow(dataToLoad)
            .errorIsHidden()
            .loaderIsHidden()
    }

    @Test
    fun `Should show error state if error appeared`() = runTest {
        val throwable = Throwable()

        // Replaced with the builder
        val viewModel = ViewModelBuilder()
            .withDataLoadedWithError(throwable)
            .build(testScheduler)

        assertThat(viewModel)
            .errorIsShown()
            .contentIsHidden()
            .loaderIsHidden()
    }
    
    @Test
    fun `Should show content state if user clicks on retry button after loading data with error`() = runTest {
        // Replaced with the builder
        val viewModel = ViewModelBuilder()
            .withDataLoadedWithError(throwable)
            .build(testScheduler)
        
        coEvery { someInteractor.loadSomething() } returns dataToLoad
        someViewModel.onRetryButtonClicked()
        
        assertThat(viewModel)
            .contentIsShow(dataToLoad)
            .errorIsHidden()
            .loaderIsHidden()
    }
    
    private inner class ViewModelBuilder {

        fun withLoadingData(): ViewModelBuilder {
            coEvery { someInteractor.loadSomething() } coAnswers {
                delay(1_000)
                "result is not important"
            }
            return this
        }

        fun withDataLoadedWithError(throwable: Throwable): ViewModelBuilder {
            coEvery { someInteractor.loadSomething() } coAnswers { throw throwable }
            return this
        }

        fun withLoadedData(dataToLoad: String): ViewModelBuilder {
            coEvery { someInteractor.loadSomething() } returns dataToLoad
            return this
        }

        fun build(testScheduler: TestCoroutineScheduler): SomeViewModel {
            val dispatechers = TestDispatchersProvider(testScheduler)
            val viewModel = SomeViewModel(someInteractor, dispatechers)
            return viewModel
        }
    }
}