How to mock a suspend function on an existing mock object?

I have mock A built and given to me. Now, I need to mock a suspend API to return a value foo.

Example:

@Inject lateinit var mockA: A // using dagger to provide mocks configured in a separate module.

@Test
fun test1() = runBlocking {
    // how do i mock a suspend method on mockA?
}

_Originally posted by @andrewborba10 in https://github.com/nhaarman/mockito-kotlin/issues/269#issuecomment-429467218_

[Hurricaaane]

With mockito-kotlin:2.1.0 :

@Test
fun getFoo_validObject_returnsBarData() = runBlocking {
    aRepoMock.stub {
        onBlocking { getFoo() }.doReturn(BarData(42))
    }
   
    val result = classUnderTest.methodThatCallsGetFoo()
    assertTrue(result is BarData)
}

I’m not entirely sure though.

[stawil15]

I know this comment will probably be considered unhelpful, but I recently spent a lot of time trying to do the same thing and I eventually came to the conclusion that, assuming no actual business logic is is taking place in your async block/function, I don’t really get anything out of including it in my tests. Testing that a mocked class returns the mocked data that I mocked for it isn’t actually useful.

That is to say, my suspend functions are only there to retrieve data (network calls) and I should focus on testing the function that contains the business logic for once those calls are returned.

Granted, if there is a way to do what you’re asking for it can still be very useful and I want to know.