CalculatorServiceViewModelTest.kt

@Before
fun setUp() {
    calculatorService = mockk<CalculatorService>()
    viewModel = CalculatorViewModel(calculatorService)
}

@Test
fun test_SumReturnFromCalculator_LiveDataChanged() {
    //Given
    every { calculatorService.sum(a, b) } returns sum
    //When
    viewModel.onSumClick(textA, textB)
    //Then
    verify { calculatorService.sum(a, b) }
    assertEquals(sum, viewModel.sum.getOrAwaitValue())
}