How to mock lambda with mockito in kotlin

Question

I have a kotlin Android app. There is a function that loads compositions from the backend and returns them to a callback:

getCompositons(callback: (Array<Composition>) -> Unit)

How can I mock the callback using mockito. So that I then can do something like this:

var callback = //mockito mock
getCompositons(callback) 
verify(callback, timeout(10000)).apply()

I read that lambda are matched to the java type function and therefore I assume apply could be the method invoked. Maybe I could mock a function and use that? But the Kotlin function interface only seems to have one return type, no parameters. java.util.Function says unresolved reference function.

Any help appreciated.

Answer 1

This is really no different to mocking any other type:

val callback = mock<(Array<Composition>) -> Unit>()

getCompositons(callback)

verify(callback)(any())  // Or verify(callback).invoke(any()) to be explicit

(In case you weren't aware of them, I'm using the mockito-kotlin bindings here.)

Answer 2

You can do that like this:

val function: Array<Composition>) -> Unit = {}
val callback = mock(function::class.java)

getCompositons(callback)

verify(callback)(any()) // or for example verifyNoInteractions(callback)

No extra libraries besides the standard mockito are needed