Cast exception when mocking kotlin lambda callbacks in Mockk

Question

I am having some trouble mocking callback functions with Mockk.

I am trying to mock a task success listener that is called like this:

collection
    .add(Item())
    .addOnSuccessListener {
        update(collection.document(it.id))
    }

Where the java signature for the callback would look like this:

public interface OnSuccessListener<T> {
    void onSuccess(T var1);
}

and the signature for addOnSuccessListener looks like this

public abstract Task<DocumentReference> addOnSuccessListener(@NonNull OnSuccessListener<DocumentReference> var1);

My mock looks like this:

every { documentReferenceTask.addOnSuccessListener(any()) } answers {
    firstArg<(DocumentReference) -> Unit>().onSuccess(documentReference)
    value
}

I have also tried:

every { documentReferenceTask.addOnSuccessListener(any()) } answers {
    firstArg<OnSuccessListener<DocumentReference>>().invoke(documentReference)
    value
}

Either way, I am receiving a cast exception like:

java.lang.ClassCastException: com.package.MyClass$addProduct$1 cannot be cast to kotlin.jvm.functions.Function1

Hi, what is signature for `addOnSuccessListener`? – oleksiyp Dec 17 '18 at 04:45 — oleksiyp, Dec 17 '18 at 04:45
I've updated the original post – CampbellMG Dec 17 '18 at 04:57 — CampbellMG, Dec 17 '18 at 04:57
I not totally understood. The issue is resolved, right? – oleksiyp Dec 17 '18 at 17:48 — oleksiyp, Dec 17 '18 at 17:48

score 1 · Accepted Answer · answered Dec 17 '18 at 05:04

Oops, I was misreading the error.

every { documentReferenceTask.addOnSuccessListener(any()) } answers {
    firstArg<OnSuccessListener<DocumentReference>>().invoke(documentReference)
    value
}

This method works, the cast exception was from the second line. The correction looks like

every { documentReferenceTask.addOnSuccessListener(any()) } answers {
    firstArg<OnSuccessListener<DocumentReference>>().invoke(documentReference)
    **documentReferenceTask**
}

Cast exception when mocking kotlin lambda callbacks in Mockk

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