Mocking suspend function with Mockito returns null - unit-testing

Question

I have the following classes
interface CarsApi {
suspend fun fetchCar() : Car
}
class FetchCarUseCase(private val carsApi: CarsApi) {
suspend fun execute: Car = withContext(dispatcherProvider.io()) {
carsApi.fetchCar()
}
}
class ViewModel(private val fetchCarUseCase: FetchCarUseCase) {
private var car: Car
suspend fun retrieveCar() {
car = fetchCarUseCase.execute()
}
}
I want to write an ermetic test for the viewModel and the useCase:
#Test
fun testCarFetching() = runBlockingTest {
val aCar = Car()
val mockApi = mock<CarsApi>()
`when`(mockApi.fetchCar()).thenReturn(aCar)
val fetchCarUseCase = FetchCarUseCase(mockApi)
val viewModel = ViewModel(fetchCarUseCase)
viewModel.retrieveCar()
/* assert stuff on viewModel.car*/
}
But the viewModel.car always seems to be null. Inside the test body mockApi.fetchCar() does retrieve the provided value, but inside the FetchCarUseCase it does not. Also if I remove the suspend keyword from the interface, the mocking seems to be working fine.
At the moment, due to some other conditions I cannot use Mockk library, so I'm stuck with Mockito.
Am I missing something?
The used dependencies:
testImplementation 'junit:junit:4.12'
testImplementation 'org.mockito:mockito-core:2.28.2'
testImplementation('com.nhaarman.mockitokotlin2:mockito-kotlin:2.1.0') {
exclude module: 'mockito-core'
}
testImplementation 'org.jetbrains.kotlinx:kotlinx-coroutines-test:1.3.2

Answer 1

In case anyone else has to deal with this problem, here is the infrastructure I have build.
First, in all the classes that launch threads inject through the constructor or property a kotlinx.coroutines.DispatcherProvider. In my case it was just the useCase, but the viewModel might require it, as well.
class FetchCarUseCase(private val dispatcher: CoroutineDispatcher,
private val carsApi: CarsApi) {
suspend fun execute: Car = withContext(dispatcher) {
carsApi.fetchCar()
}
}
In the unit tests project, add a helper rule-class, in order to extract some functionality:
#ExperimentalCoroutinesApi
class CoroutineTestRule(val testDispatcher: TestCoroutineDispatcher = TestCoroutineDispatcher()) : TestWatcher() {
val testDispatcherProvider = object : DispatcherProvider {
override fun default(): CoroutineDispatcher = testDispatcher
override fun io(): CoroutineDispatcher = testDispatcher
override fun main(): CoroutineDispatcher = testDispatcher
override fun unconfined(): CoroutineDispatcher = testDispatcher
}
override fun starting(description: Description?) {
super.starting(description)
Dispatchers.setMain(testDispatcher)
}
override fun finished(description: Description?) {
super.finished(description)
Dispatchers.resetMain()
testDispatcher.cleanupTestCoroutines()
}
}
And finally the unit test looks like this:
#ExperimentalCoroutinesApi
#RunWith(MockitoJUnitRunner::class)
class ViewModelTest {
#get:Rule
var coroutinesTestRule = CoroutineTestRule()
#Test
fun testCarFetching() = coroutinesTestRule.testDispatcher.runBlockingTest {
val aCar = Car()
val mockApi = mock<CarsApi>()
`when`(mockApi.fetchCar()).thenReturn(aCar)
val fetchCarUseCase = FetchCarUseCase(mockApi)
val viewModel = ViewModel(fetchCarUseCase)
viewModel.retrieveCar()
/* assert stuff on viewModel.car*/
}
#Test
fun testCarFetchingError() = coroutinesTestRule.testDispatcher.runBlockingTest {
val aCar = Car()
val mockApi = mock<CarsApi>()
`when`(mockApi.fetchCar()).then {
throw Exception()
}
val fetchCarUseCase = FetchCarUseCase(mockApi)
val viewModel = ViewModel(fetchCarUseCase)
viewModel.retrieveCar()
/* assert stuff on erros*/
}
}
This way all the code in the unit tests runs on the same thread and in the same context.