Equivalent of doReturn(x).when(y)... in mockk? - kotlin

Question

I am looking for a mockk equivalent of doReturn(...).when(...).*
I am working on writing some unit tests (testing contracts) that involves a lot of system classes and so need to intercept the methods that I don't control and return some call backs (which the method in code would have eventually returned). In mockito, I could do something like doReturn(...).when(...).*
I Wasn't able to find a similar thing in mockK. Seems like every{} always runs the block before answers or returns.
class Vehicle: Listener {
fun displayCar(listener:Listener){
OtherClass().fetchCar(listener)
}
override fun showCarSuccessful() {
//do something
}
}
class OtherClass {
//assume its an outside function that returns nothing but invokes a method of listener call back
fun fetchCar(listener: Listener) {
//... Some system level operations that I don't have control to generate mock objects but in the test I want to use the listener to call some method so that I can
// test some contracts
listener.showCarSuccessful()
}
}
class Tests {
#Test
fun testCarSuccess() {
val listener: Listener = mockk(relaxed = true)
val vehicle = Vehicle()
//also tried with mockkClass and others
val other: OtherClass = mockk(relaxed = true)
every { other.fetchCar(listener) } returns {listener.showCarSuccessful()}
vehicle.displayCar(listener)
//do some verification checks here
}
}
interface Listener {
fun showCarSuccessful()
}

Answer 1

The every{} block is your when clause. You can set up multiple conditions for returning different results. See the example of setting up fixed returns and performing progrommatic answers
import io.mockk.MockKException
import io.mockk.every
import io.mockk.mockk
import org.junit.jupiter.api.Assertions
import org.junit.jupiter.api.Test
class MyClass {
fun add(operand1: Int, operand2: Int): Int {
TODO()
}
}
class MockkTest {
#Test
fun testMocking() {
val myClassMock = mockk<MyClass> {
every { add(1, 2) } returns 3 // Set behaviour
every { add(2, 2) } returns 4 // Set behaviour
every { add(3, 4)} answers {args[0] as Int * args[1] as Int} // Programmatic behaviour
}
Assertions.assertEquals(3, myClassMock.add(1, 2))
Assertions.assertEquals(4, myClassMock.add(2, 2))
Assertions.assertEquals(12, myClassMock.add(3, 4))
Assertions.assertThrows(MockKException::class.java) {
myClassMock.add(5, 6) // This behaviour has not been set up.
}
}
}
But, in your example in particular, I find this line:
every { other.fetchCar(listener) } returns listener.showCarSuccessful()
very strange. First it is not doing what you think it is doing - it is going to make that call as you set this behaviour up you are telling your mock to return the result of that call, not to do that cal. To do what you want to do, you should rather do this:
every { other.fetchCar(listener) } answers {listener.showCarSuccessful()}
But even then, this line is setting up the mock behaviour after you have called your class under test - set up your mock behaviour first.
Also, it is strange that you are setting up side effects in a top level mock in a nested mock. Surely for testing your Vehicle class all you want to do is verify that its inner class was called with the correct arguments. Also, how does Vehicle get a reference to your OtherClass mock, it is instantiating a new one and calling that function.
Here is an attempt to make your example work:
import io.mockk.mockk
import io.mockk.verify
import org.junit.jupiter.api.Test
interface Listener {
fun showCarSuccessful()
}
class Vehicle(val other: OtherClass) : Listener {
fun displayCar(listener: Listener) {
other.fetchCar(listener)
}
override fun showCarSuccessful() {
//do something
}
}
class OtherClass {
//assume its an outside function that returns nothing but invokes a method of listener call back
fun fetchCar(listener: Listener) {
}
}
class VehicleTest{
#Test
fun testDisplayCar(){
val listener: Listener = mockk(relaxed = true)
val other: OtherClass = mockk(relaxed = true) //also tried with mockkClass and others
val vehicle = Vehicle(other)
vehicle.displayCar(listener)
verify{ other.fetchCar(listener) }
}
}
Even this I think is maybe still a bit off - I suspect that the listener you want Vehicle to pass to OtherClass is itself, not an argument...
You should also then write a separate test for OtherClass to make sure it does what you expect it to when you call fetchCar