Kotlin: Unit testing a function that takes a function as parameter

Question

I have a function that retrieves a list of items from a repository. Instead of using a regular callback I pass in a function and invoke this with the result. But how can you unittest this kind of function. Is there some way to verify that the passed in function is being invoked or should I refactor and use a regular callback and test it using a mocked callback-interface?

My code:

class WorklistInteractor @Inject
constructor(private val worklistRepository: WorklistRepository,
            private val preferenceManager: PreferenceManager,
            private val executor: Executor)
    : WorklistDialogContract.Interactor, Executor by executor {

    @Volatile private var job: Job? = null

    override fun getWorklist(callback: (Result<List<WorklistItem>>) -> Unit) {
        job = onWorkerThread {
            val result = worklistRepository.getWorklist().awaitResult()
            onMainThread { callback(result) }
        }
    }

    override fun cancel() {
        job?.cancel()
    }
}

Answer 1

To check it is called, something like that would work:

var hasBeenCalled = false
interactor.getWorklist({ result -> hasBeenCalled = true })

assertTrue(hasBeenCalled)

Of course you could also check that the expected result is passed, etc.