How to verify a class extension function is called in Kotlin? - unit-testing

Question

Let's say I want to verify if the ShopCart.itemUpdated() is called:
data class ShopCart(val item: Item)
data class Item(val price: Int)
class ShopCartController(private val cart: ShopCart) {
fun updateCart() {
cart.itemUpdated()
}
fun ShopCart.itemUpdated() = copy(
item = item.copy(price = 20)
)
}
class ShopCartControllerTest {
#Test
fun test() {
val cart = ShopCart(Item(0))
val cartSpy = spy(cart)
val controller = ShopCartController(cartSpy)
controller.updateCart()
with(controller) {
verify(cartSpy).itemUpdated()
}
}
However, this test fails and throws java.lang.NullPointerException.
java.lang.NullPointerException
at com.test.ShopCartController.itemUpdated(Test.kt:28)
at com.test.ShopCartControllerTest.testMe(Test.kt:17)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:498)
at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:50)
at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:47)
at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:17)
at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:325)
at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:78)
at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:57)
at org.junit.runners.ParentRunner$3.run(ParentRunner.java:290)
at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:71)
at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:288)
at org.junit.runners.ParentRunner.access$000(ParentRunner.java:58)
at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:268)
at org.junit.runners.ParentRunner.run(ParentRunner.java:363)
at org.junit.runner.JUnitCore.run(JUnitCore.java:137)
at com.intellij.junit4.JUnit4IdeaTestRunner.startRunnerWithArgs(JUnit4IdeaTestRunner.java:68)
at com.intellij.rt.execution.junit.IdeaTestRunner$Repeater.startRunnerWithArgs(IdeaTestRunner.java:47)
at com.intellij.rt.execution.junit.JUnitStarter.prepareStreamsAndStart(JUnitStarter.java:242)
at com.intellij.rt.execution.junit.JUnitStarter.main(JUnitStarter.java:70)
Is it possible to verify a class extension function is called? I have tried several mocking frameworks (e.g. mockito, mockk), no luck. Do you guys have any ideas?