How to mock and verify Lambda expression in Kotlin? - testing

Question

In Kotlin (and Java 8) we can use Lambda expression to remove boilerplate callback interface. For example,
data class Profile(val name: String)
interface ProfileCallback {
fun onSuccess(profile: Profile)
}
class ProfileRepository(val callback: ProfileCallback) {
fun getProfile() {
// do calculation
callback.onSuccess(Profile("name"))
}
}
We can change remove ProfileCallback and change it into Kotlin's Lambda:
class ProfileRepository(val callback: (Profile) -> Unit) {
fun getProfile() {
// do calculation
callback(Profile("name"))
}
}
This works fine, but I'm not sure how to mock and then verify that function. I have
tried using Mockito like this
#Mock
lateinit var profileCallback: (Profile) -> Unit
#Test
fun test() {
// this wouldn't work
Mockito.verify(profileCallback).invoke(any())
}
but it throw an Exception:
org.mockito.exceptions.base.MockitoException: ClassCastException
occurred while creating the mockito mock : class to mock :
'kotlin.jvm.functions.Function1', loaded by classloader :
'sun.misc.Launcher$AppClassLoader#7852e922'
How to mock and verify Lambda expression in Kotlin? Is it even possible?

Answer 1

Here is example how you can achieve that using mockito-kotlin:
Given repository class
class ProfileRepository(val callback: (Int) -> Unit) {
fun getProfile() {
// do calculation
callback(1)
}
}
Using mockito-kotlin lib - you can write test mocking lambdas like this:
#Test
fun test() {
val callbackMock: (Int) -> Unit = mock()
val profileRepository = ProfileRepository(callbackMock)
profileRepository.getProfile()
argumentCaptor<Int>().apply {
verify(callbackMock, times(1)).invoke(capture())
assertEquals(1, firstValue)
}
}