Cannot test callbacks with mockk: invoke(any())) was not called - kotlin

Question

Given
typealias MyCallback = (s: String) -> Unit
object Hello {
fun main() {
blah { print(it) }
}
fun blah(cb: MyCallback) {
cb.invoke("hi")
}
}
Or
interface MyCallback {
fun invoke (s: String) {}
}
object Hello {
fun main() {
blah(object : MyCallback {
override fun invoke(s: String) {
print(s)
}
})
}
fun blah(cb: MyCallback) {
cb.invoke("hi")
}
}
With both I get the above error (Verification failed: call 1 of 1: MyCallback(#2).invoke(any())) was not called) when doing
#Test
fun testInvoke() {
val mock = mockk<Hello>(relaxed = true)
val cb = mockk<MyCallback>()
mock.blah(cb)
verify { cb.invoke(any()) }
}
how to fix it?

Answer 1

This worked for me. The Hello object doesn't need to be mocked since it is the class being tested. By mocking it, the test was only recording invocations of blah() rather than actually executing them.
Using spyk rather than mockk allows the MyCallback type to be constructed allowing the invoke() function to be defined. So maybe that's more of a workaround than an explanation of why mockk doesn't seem to retain that type info.
typealias MyCallback = (s: String) -> Unit
object Hello {
fun main() {
blah { print(it) }
}
fun blah(cb: MyCallback) {
cb.invoke("hi")
}
}
class MockKTest {
#Test
fun testInvoke() {
val mock = spyk<Hello>()
val cb = mockk<MyCallback>(relaxed = true)
mock.blah(cb) // or just do Hello.blah(cb)
verify { cb.invoke(any()) }
}
}