Mocking extension function in Kotlin

Question

How to mock Kotlin extension function using Mockito or PowerMock in tests? Since they are resolved statically should they be tested as static method calls or as non static?

Answer 1

I think MockK can help you.

It supports mocking extension functions too.

You can use it to mock object-wide extensions:

data class Obj(val value: Int)

class Ext {
    fun Obj.extensionFunc() = value + 5
}

with(mockk<Ext>()) {
    every {
        Obj(5).extensionFunc()
    } returns 11

    assertEquals(11, Obj(5).extensionFunc())

    verify {
        Obj(5).extensionFunc()
    }
}

If you extension is a module-wide, meaning that it is declared in a file (not inside class), you should mock it in this way:

data class Obj(val value: Int)

// declared in File.kt ("pkg" package)
fun Obj.extensionFunc() = value + 5

mockkStatic("pkg.FileKt")

every {
    Obj(5).extensionFunc()
} returns 11

assertEquals(11, Obj(5).extensionFunc())

verify {
    Obj(5).extensionFunc()
}

By adding mockkStatic("pkg.FileKt") line with the name of a package and file where extension is declared (pkg.File.kt in the example).

More info can be found here: web site and github