In kotlin, how do I mock a suspend function that wraps a callback?

Let's say there's an interface with a callback:

interface SomeInterface {
fun doSomething(arg: String, callback: (Exception?, Long) -> Unit)
}

which I extend into a suspend function like this:

suspend fun SomeInterface.doSomething(arg: String): Long = suspendCoroutine { cont ->
this.doSomething(arg) { err, result ->
if (err == null) {
cont.resume(result)
} else {
cont.resumeWithException(err)
}
}
}

I'd like to mock this in tests, but am failing.
Ideally I'd like to use something like this:

@Test
fun checkService() {
runBlocking {
val myService = mock<SomeInterface>()
whenever(myService.doSomething(anyString())).thenReturn(1234L)
val result = myService.doSomething("")
assertEquals(result, 1234L)
}
}

The above syntax fails with a mockito exception because it's expecting a matcher for the callback.

org.mockito.exceptions.misusing.InvalidUseOfMatchersException:
Invalid use of argument matchers!
2 matchers expected, 1 recorded:

How can I mock a suspend function like that?
If a similar syntax is not possible how can I have the mock call back with the desired arguments such that the suspend variant that is used throughout my code returns the desired result during tests?


**Update**:
It seems it's not possible when it's an extension function. Based on [Marko Topolnik][1]'s comment, I gather it's because an extension is simply a static function which is out of mockito's capability.

When the suspend function is a member function, then it works as expected, with my original syntax.

Here is a gist with some demo code:



[1]:

[To see links please register here]

I suggest using [MockK](

[To see links please register here]

) for your tests, which is more coroutine-friendly.

To mock a coroutine, you can use `coEvery` and `returns` like below:

val interf = mockk<SomeInterface>()
coEvery { a.doSomething(any()) } returns Outcome.OK

When you need to

> on .... do return ..

and method is suspended, with **mockito** you can use this:

- use this lib [mockito-kotlin][1]
- Mock your object, lets name it myObject (myObject has suspend method named isFoo)

then:

myObject.stub {
onBlocking { isFoo() }.doReturn(true)
}


[1]:

[To see links please register here]