How to mock a Kotlin singleton object?

Question

Given a Kotlin singleton object and a fun that call it's method

object SomeObject {    fun someFun() {} }  fun callerFun() {    SomeObject.someFun() } 

Is there a way to mock call to SomeObject.someFun()?

Answer 1

There's a very nice mocking library for Kotlin - Mockk, which allows you to mock objects, the exact same way you're desiring.

As of its documentation:

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Objects can be transformed to mocks following way:

object MockObj {   fun add(a: Int, b: Int) = a + b }  mockkObject(MockObj) // aplies mocking to an Object  assertEquals(3, MockObj.add(1, 2))  every { MockObj.add(1, 2) } returns 55  assertEquals(55, MockObj.add(1, 2)) 

To revert back use unmockkAll or unmockkObject:

@Before fun beforeTests() {     mockkObject(MockObj)     every { MockObj.add(1,2) } returns 55 }  @Test fun willUseMockBehaviour() {     assertEquals(55, MockObj.add(1,2)) }  @After fun afterTests() {     unmockkAll()     // or unmockkObject(MockObj) } 

Despite Kotlin language limits you can create new instances of objects if testing logic needs that:

val newObjectMock = mockk<MockObj>() 

Answer 2

Just make you object implement an interface, than you can mock you object with any mocking library. Here example of Junit + Mockito + Mockito-Kotlin:

import com.nhaarman.mockito_kotlin.mock import com.nhaarman.mockito_kotlin.whenever import org.junit.Assert.assertEquals import org.junit.Test  object SomeObject : SomeInterface {     override fun someFun():String {         return ""     } }  interface SomeInterface {     fun someFun():String }  class SampleTest {      @Test     fun test_with_mock() {         val mock = mock<SomeInterface>()          whenever(mock.someFun()).thenReturn("42")          val answer = mock.someFun()          assertEquals("42", answer)     } } 

Or in case if you want mock SomeObject inside callerFun:

import com.nhaarman.mockito_kotlin.mock import com.nhaarman.mockito_kotlin.whenever import org.junit.Assert.assertEquals import org.junit.Test  object SomeObject : SomeInterface {     override fun someFun():String {         return ""     } }  class Caller(val someInterface: SomeInterface) {     fun callerFun():String {         return "Test ${someInterface.someFun()}"     } }  // Example of use val test = Caller(SomeObject).callerFun()  interface SomeInterface {     fun someFun():String }  class SampleTest {      @Test     fun test_with_mock() {         val mock = mock<SomeInterface>()         val caller = Caller(mock)          whenever(mock.someFun()).thenReturn("42")          val answer = caller.callerFun()          assertEquals("Test 42", answer)     } }