CounterViewModelTest.kt

class CounterViewModelTest {

    private lateinit var viewModel: CounterViewModel

    @Rule
    @JvmField
    val instantTaskExecutorRule: TestRule = InstantTaskExecutorRule()

    @Before
    fun setUp() {
        viewModel = CounterViewModel()
    }

    @Test
    fun clickingButtonIncreasesCounter() {
        // given

        // when
        viewModel.onButtonClick()

        // then
        assertEquals(1, viewModel.counter.value)
    }
}