Ah ok. So far I have been testing extension functi...

# android

[Pascal How]

Ah ok. So far I have been testing extension functions using assert to verify expected and actual results. However, since on the surface, extensions look like methods of the class being extended, there may be scenarios where you would want a way of mocking that function if that class is being injected as a dependency

[gildor]

Do you have example of such scenario? Usually when you want to mock extension function you actually don’t need extension function, but a class or member function

[Pascal How]

So let’s say we want to extend a Car class with an extension function hadAccident() because the original class did not have this method and it is from a third party library so we cannot add member functions

Then if another class ServiceCompany takes a car and a mechanic as dependency and one of its methods does

fun organiseRepair() {
    if(car.hadAccident) {
        mechanic.doFullRepair()
    } else {
        mechanic.doMinimalCheck()
    }
}

When testing with Mockito you would then want something like

fun givenCarHadAccident_whenOrganiseRepair_thenDoFullRepair() {
    `when`(car.hadAccident).thenReturn(true)

    serviceCompany.organiseRepair()

    verify(mechanic).doFullRepair()
}

[gildor]

Where did you get mechanic?

[gildor]

actually it doesn’t matter in this case

[Pascal How]

So both mechanic and car are dependencies to the class ServiceCompany. Then we want to test the ServiceCompany class

[gildor]

Just create a Car object for case when hadAccident returns true or returns false

[gildor]

I don’t see why you should mock extension function in this case if you can construct required Car object

[Pascal How]

class ServiceCompany(val car:Car, val mechanic: Mechanic) {
fun organiseRepair() {
    if(car.hadAccident) {
        mechanic.doFullRepair()
    } else {
        mechanic.doMinimalCheck()
    }
}
}

[Pascal How]

Yes I can construct car but what if Car is from a third party library and it does not have the hadAccident method?

[gildor]

I don’t see any problem

[gildor]

extension function uses only public methods

[gildor]

so you can mock Car or create car with required data (second even more clear approach)

[gildor]

for example if hadAccident is something like:

fun Car.hadAccident() = this.accidents.size > 0

You can just do:

fun givenCarHadAccident_whenOrganiseRepair_thenDoFullRepair() {
    val carWithAccident = Car(accidents = listOf(Accident()))
    val serviceCompany = ServiceCompany(carWithAccident, mechanic)

    serviceCompany.organiseRepair()

    verify(mechanic).doFullRepair()
}

[gildor]

you don’t need any mocking there

[gildor]

this is why I asked about case

[gildor]

and even if it’s impossible to create Car with required data (no interfaces, no required constructors), you can just mock Car instance but do not mock extension function

[gildor]

And just in case
Great Mockk library http://mockk.io/ from @oleksiyp can mock extension functions, but my opinion that in 99% cases you should avoid it, because you can and should use another test approaches.
In general mocking static members consider by many as bad practice. One of the reasons, because developers starts to use it for cases when it shouldn’t be used, because you can easily test without mocking at all, like in your example

[Pascal How]

Yea I was wondering more about cases where you cannot create Car with accidents as parameter and all you can do is val car = Car() (might be a bad example)
Then one way around that is to provide an interface and the class that implements that can provide a method wrapping the call to the extension function.
During test, you would then test the mock instance instead

Thanks! I saw these libraries but I did not think it was a good idea to use them

[gildor]

If you cannot create an instance of class, just use mocks, you probably need mock of car even if you mock hadAccident.
So I really don’t see any reason to use mocked extension function in this case

[gildor]

also, you actually test implementation details of serviceCompany instead of testing serviceCompany results, so it’s also bad practice imho

[gildor]

even if hadAccident would be member function I still suggest to use real instance of Car with proper data for cases when it’s possible

[Pascal How]

Cool! Makes more sense now. Thanks for your time 👍