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Unsure how to unit test interface in Kotlin with chained block

I'm working with Kotlin (for Android), along with jUnit and Koin.
I have an interface to consume that looks like this:
interface MyInterface {
fun vehicle(id: String, block: VehicleInterface.() -> Unit)
}
interface VehicleInterface {
fun requestVersion()
}
The code that calls this method looks like this:
import com.domain.protocol.thelibrary.sdk.MyInterface
// The below is injected via Koin
class SomeClass(private val myInterface: MyInterface) {
fun functionIWantToUnitTest(id: String) {
myInterface.vehicle(id) {
requestVersion()
}
}
}
I would like to unit test that when I call functionIWantToUnitTest and pass in the id "1" then myInterface.vehicle is called with id "1" and I'd also like to verify that requestVersion is then called.
I wrote a unit test as such:
#Test
fun `Ensure functionIWantToUnitTest calls requestVersion with the correct id`() {
every { myInterface.vehicle("1", any()) } returns Unit
someClass.functionIWantToUnitTest("1")
verifySequence {
myInterface.vehicle("1") {
requestVersion()
}
}
}
This fails as such:
java.lang.AssertionError: Verification failed: call 1 of 1:
MyInterface(myInterface#1).vehicle(eq(1), eq(lambda {}))). Only one matching call to
MyInterface(myInterface#1)/vehicle(String, Function1) happened, but arguments are not
matching:
[0]: argument: 1, matcher: eq(1), result: +
[1]: argument: lambda {}, matcher: eq(lambda {}), result: -
Can anyone please help me to understand what I'm doing wrong...
Any help gratefully accepted.

Problems with Kotlin Result<T> on unit tests

I am working on an android Application and I opted to use Kotlin Result class so as to handle success/failure on my operations. I made the changes to the code, but the tests stop working and I cannot understand why. Here I show you some snippets:
FireStoreClient.kt
suspend fun items(): Result<ItemsResponse>
NetworkDataSource.kt
suspend fun getItems(): List<Item> =
fireStoreClient.items().fold({ it.items.map { item -> item.toDomain() } }, { emptyList() })
NetworkDataSourceTest.kt
#ExperimentalCoroutinesApi
#Test
fun `Check getItems works properly`() = runBlockingTest {
whenever(fireStoreClient.items()).doReturn(success(MOCK_ITEMS_DOCUMENT))
val expectedResult = listOf(
Item(
id = 1,
desc = "Description 1"
),
Item(
id = 2,
desc = "Description 2"
)
)
assertEquals(expectedResult, dataSource.getItems())
}
And this is the exception I am getting right now. Any clue? It appears that the fold() method is not being executed when unit testing.
java.lang.ClassCastException: kotlin.Result cannot be cast to ItemsResponse
at NetworkDataSource.getItems(NetworkDataSource.kt:31)

I've found a different workaround for this result-wrapping issue, for those who don't want to make their own Result type.
This issue appears to happens specifically when using Mockito's .thenReturn on suspend functions. I've found that using .thenAnswer doesn't exhibit the problem.
So instead of writing this in your unit test (changed doReturn to thenReturn here):
whenever(fireStoreClient.items()).thenReturn(success(MOCK_ITEMS_DOCUMENT))
Use:
whenever(fireStoreClient.items()).thenAnswer { success(MOCK_ITEMS_DOCUMENT) }
Edit: I should note that I was still experiencing this issue when running Kotlin 1.5.0.
Edit: On Kotlin 1.5.20 I can use .thenReturn again.

After a deep dive into the problem, finally, I've found a temporary workaround that works in the testing environment. The problem is, somehow the value of the Result object is wrapped by another Result, and we can pull the desired value or exception using reflection.
So, I've created an extension function called mockSafeFold, which implements the fold behavior in normal calls, and acts fine when you are executing unit-tests.
inline fun <R, reified T> Result<T>.mockSafeFold(
onSuccess: (value: T) -> R,
onFailure: (exception: Throwable) -> R
): R = when {
isSuccess -> {
val value = getOrNull()
try {
onSuccess(value as T)
} catch (e: ClassCastException) {
// This block of code is only executed in testing environment, when we are mocking a
// function that returns a `Result` object.
val valueNotNull = value!!
if ((value as Result<*>).isSuccess) {
valueNotNull::class.java.getDeclaredField("value").let {
it.isAccessible = true
it.get(value) as T
}.let(onSuccess)
} else {
valueNotNull::class.java.getDeclaredField("value").let {
it.isAccessible = true
it.get(value)
}.let { failure ->
failure!!::class.java.getDeclaredField("exception").let {
it.isAccessible = true
it.get(failure) as Exception
}
}.let(onFailure)
}
}
}
else -> onFailure(exceptionOrNull() ?: Exception())
}
Then, simply call it instead of fold:
val result: Result = myUseCase(param)
result.mockSafeFold(
onSuccess = { /* do whatever */ },
onFailure = { /* do whatever */ }
)

I had the same issue.
I noticed that my method of injected class which should return Result<List<Any>> returns actually Result<Result<List<Any>>> which causes the ClassCastException. I used the Evaluate Expression option for the result from the method and I got
Success(Success([]))
The app works well but unit tests didn't pass due this problem.
As a temporary solution I built a new simple implementation of Result sealed class with fold() extension function. It should be easy to replace in future to kotlin.Result
Result sealed class:
sealed class Result<T> {
data class Success<T>(val value: T) : Result<T>()
data class Failure<T>(val error: Throwable) : Result<T>()
}
fold() extension function:
inline fun <R, T> Result<T>.fold(
onSuccess: (value: T) -> R,
onFailure: (exception: Throwable) -> R
): R = when (this) {
is Result.Success -> onSuccess(value)
is Result.Failure -> onFailure(error)
}

Using Mockk to mock private function that takes a lambda

I am trying to write a unit test for a implementation of an abstract class I wrote. The method I'm trying to mock takes a lambda as it's only parameter. I'm trying to capture this lambda, so I can invoke it and get the result.
This is the method I'm trying to mock:
protected fun update(block: suspend S.() -> S?): Unit
I am using an extension function in my tests like this:
suspend inline fun <reified T : Model<S>, S : State> T.blah(
state: S,
block: (T) -> Unit
): S? {
val spy = spyk(this, recordPrivateCalls = true)
val slot = slot<suspend S.() -> S?>()
every { spy["update"](capture(slot)) } answers { Unit }
block(spy)
return slot.captured.invoke(state)
}
So I am creating a spy, then a slot, then when the update function is called, capture it so that it blocks the actual class from performing the call. Then I invoke the lambda myself and return the value.
However I keep getting this error:
io.mockk.MockKException: can't find function update(kotlin.jvm.functions.Function2$Subclass1#6bfa228c) for dynamic call
at io.mockk.InternalPlatformDsl.dynamicCall(InternalPlatformDsl.kt:122)
at io.mockk.MockKMatcherScope$DynamicCall.invoke(API.kt:1969)
I followed the stacktrace and set a breakpoint in the InternalPlatformDsl.kt class, and traced it to this block of code:
for ((idx, param) in it.parameters.withIndex()) {
val classifier = param.type.classifier
val matches = when (classifier) {
is KClass<*> -> classifier.isInstance(params[idx])
is KTypeParameter -> classifier.upperBounds.anyIsInstance(params[idx])
else -> false
}
if (!matches) {
return#firstOrNull false
}
}
It successfully matches the first parameter which is the class under test Model in this case, but it fails matching the second parameter because it is wrapped in the capture function.
Any ideas on how I can intercept this update call?
I'm using the latest version of mockk, and JUnit 4

Unable to use lambda arrow expression for Livedata observe method in Kotlin

I am new to kotlin and I am a little confused while using lambda expression in LiveData observe method.
The signature for observe method is as follows
observe(LifecycleOwner owner, Observer<? super T> observer)
where Observer is an interface with a single method
void onChanged (T t)
However,calling the above observe method in kotlin as follows gives type mismatch error :
val myViewModel = ViewModelProviders.of(this).get(AnimeListViewModel::class.java)
myViewModel.animes.observe(this, { anime -> println(anime) })
Isn't this the same as calling the setOnClickListener on a view. The following piece of code works perfectly without any compilation error:
val myView = View(this)
myView.setOnClickListener { view -> println(view) }
I have already read this answer which shows how to call the method using lambda expression (using SAM conversion). However, I am still not sure why a simple arrow expression would fail.

LiveData doesn't have a lambda expression, you should pass the observer interface as an object
myViewModel.animes.observe(this, Observer { anime -> println(anime) })
Or by creating an extension function like this
fun <T : Any> LiveData<T>.observe(lifecycleOwner: LifecycleOwner, block: (T) -> Unit) = observe(lifecycleOwner, Observer(block))
And calling it like this
myViewModel.animes.observe(this) { anime -> println(anime) }
Or like this
fun main() {
myViewModel.animes.observe(this, ::handleLiveData)
}
fun handleLiveData(anime: Anime) {
println(anime)
}

There are some problems on kotlin to resolve generics, so that's the reason. Kotlin has been working on this, and you will find the whole explanation
here.

Mockito Wanted but not Invoked

I'm new to writing tests and using Mockito.
I've read the similar topics here on Stackoverflow and made the suggested changes, making sure that regarded classes / interfaces / methods are open.
I tried to follow this
Mocking the constructor injected dependencies
This is the test I came up with so far
class RegistrationPresenterTest {
#Test
fun testRegisterSuccess() {
val mockService = mock<IHerokuInteractor>()
val mockLocal = mock<ILocalStorageInteractor>()
val mockView = mock<RegisterView>()
val mockRegistrationResponse = HerokuRegisterResponse("hash")
val mockPair = ImeiPair("imei","hash")
val presenter = RegisterPresenterImpl(mockLocal,mockService)
whenever(mockService.register(any())).thenReturn(Observable.just(mockRegistrationResponse))
whenever(mockLocal.clearPreferences()).thenReturn(Observable.just(true))
whenever(mockLocal.putImeiPair(any())).thenReturn(Observable.just(true))
//whenever(presenter.writeImeiPairLocally(any())) How do I specify parameters since it uses a parameter from the register method?
presenter.bindView(mockView)
presenter.register("imei","male")
verify(mockService, times(1)).register(any())
verify(mockLocal,times(1)).clearPreferences()
verify(mockLocal,times(1)).putImeiPair(any())
verify(mockView,times(1)).moveToMain()
}
but the response I keep getting is
Wanted but not invoked:
registerPresenterImpl.writeImeiPairLocally(
<any com.company.appname.model.ImeiPair>
);
Actually, there were zero interactions with this mock.
I got this response even when I don't mention that method in the test.
This is my presenter register method. I've changed the classes / interfaces & methods involved to open (kotlin). I believe override methods are open by nature in kotlin.
open class RegisterPresenterImpl #Inject constructor(val localStorage : ILocalStorageInteractor, var herokuService : IHerokuInteractor)
override fun register(imei : String, gender : String){
subscription = herokuService.register(RegisterObject(imei,gender)).subscribeOn(Schedulers.io())
.observeOn(AndroidSchedulers.mainThread()).subscribe(
{
registrationResult ->
Log.d(TAG,"${registrationResult}")
if(registrationResult.imei_hash != null){
writeImeiPairLocally(ImeiPair(imei,registrationResult.imei_hash))
}
else{
Log.e(TAG,"User already exists")
}
},
{
errorResponse -> Log.e(TAG,"Could not register user ${errorResponse.message}")
}
)
addSubscription(subscription)
}
and similarly the
open fun writeImeiPairLocally(pair : ImeiPair){
subscription = localStorage.clearPreferences().flatMap {
cleared -> localStorage.putImeiPair(pair)}.subscribeOn(Schedulers.io())
.observeOn(AndroidSchedulers.mainThread()).subscribe(
{
booleanResult -> view?.moveToMain()
},
{
errorResponse -> Log.e(TAG,"Could not write ImeiPair to SharedPreferences ${errorResponse.message}")
}
)
addSubscription(subscription)
}
Here is interfaces
open interface ILocalStorageInteractor : ILocalStorage{
fun getImeiPair() : Observable<ImeiPair>
fun putImeiPair(pair: ImeiPair) : Observable<Boolean>
}
open interface ILocalStorage {
fun clearPreferences() : Observable<Boolean>
}
All help is appreciated.

If you are using plain jUnit, then your AndroidSchedulers.mainThread() is null. That's why onNext is not called.
You need to override Schedulers in a setUp() method with:
RxAndroidPlugins.getInstance().registerSchedulersHook(new RxAndroidSchedulersHook() {
#Override
public Scheduler getMainThreadScheduler() {
return Schedulers.immediate(); // or .test()
}
});
To avoid concurrency in tests, I would recommend to override Schedulers.io() like this:
RxJavaHooks.setOnIOScheduler(scheduler1 -> Schedulers.immediate());
If you are going to use TestScheduler, don't forget to call TestScheduler.triggerActions() method.
Also don't forget to unregister Schedulers in tearDown() like this:
RxJavaHooks.reset();
RxAndroidPlugins.getInstance().reset();
AndroidSchedulers.reset();
Schedulers.reset();