Mocking extension function in Kotlin

Question

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).

In jvm environments you can replace the class name with a function reference:

mockkStatic(Obj::extensionFunc)

Note that this will mock the whole pkg.FileKt class, and not just extensionFunc.

This syntax also applies for extension properties:

val Obj.squareValue get() = value * value

mockkStatic(Obj::squareValue)

More info can be found here: web site and github

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