Is it possible to test arguments in mocked constructor with mockk?

Question

Purpose

I want to test the result of a function of a class. And it returns an instance of an Android class, which should be mocked in a unit test since it seems to be stubbed in any unit test.

Example code

Here's a minimized example code of the simulated android class, AndroidData , and the class to be tested, MyHelper .

/**
 * Should be mocked in a unit test.
 */
class AndroidData(val value: Float)

/**
 * Target class to be tested.
 */
class MyHelper() {
    fun createData(flag: Boolean): AndroidData {
        // simplified logic
        return if (flag) AndroidData(20f) else AndroidData(10f)
    }
}

What I want to achieve is something like this( note that this is incorrect code ):

class MyHelperTest : TestCase() {
    @Test
    fun testCreateData() {
        mockkConstructor(AndroidData::class)
        val valueSlot = slot<Float>()
        every { constructedWith<AndroidData>(capture(valueSlot)) }  // Type mismatch in IDE
        val helper = MyHelper()
        
        val returnedData = helper.createData(true)
        assertTrue(returnedData.value >= 15f)
    }
}

I want to test that returned AndroidData instance has a correct value , or matches some conditions. I cannot find any tutorial about this case or any correct way to implement it.

What I tried

These 2 test cases would both fail.

    @Test
    fun testCreateData1() {
        mockkConstructor(AndroidData::class)

        val helper = mockk<MyHelper>(relaxed = true)
        val createdData = helper.createData(true)

        println("createdData=$createdData")  // AndroidData(child of #1#2)
        println("createdData.value=${createdData.value}")  // 0.0

        // test if the argument in AndroidData's constructor was larger than 15
        assertTrue(createdData.value >= 15f)  // assertion failed
    }

    @Test
    fun testCreateData2() {
        mockkConstructor(AndroidData::class)
        // raise error: Missing mocked calls inside every { ... } block: make sure the object inside the block is a mock
        every {
            constructedWith<AndroidData>(LargerThan15Matcher())
        }

        val helper = mockk<MyHelper>(relaxed = true)
        val createdData = helper.createData(true)

        println("createdData=$createdData")  // AndroidData(child of #1#2)
        println("createdData.value=${createdData.value}")  // 0.0

        // test if the argument in AndroidData's constructor was larger than 15
        assertTrue(createdData.value >= 15f)
    }

The testing idea was pretty easy and intuitive for Python unittest in my experience. Yet it seems impossible in Java or Android? I haven't tried the mockito and roboletric library because I was told that mockk provides support on Android projects. Or I just haven't found the correct way to do this, or the whole testing idea is completely wrong? Please end my days of searching and struggle.

Answer 1

It is not really clear to me what you are trying to achieve, but I can try to help you with what I think I understand.

Let's start with your test-case testCreateData1 :

• A relaxed mock in MockK tries to return a "simple value" (mostly "default" values or null if possible) for each function call of the mock.
• Therefore, your relaxed mock of MyHelper returns AndroidData(0.0) which does not satisfy your assertion.

Regarding test-case testCreateData2 :

• every {... } alone is not a proper expression, but it is intended to specify what should happen on every ... . For instance, if you have a mock m with a function f you could write something like the following:

every { m.f() } returns 42
every { m.f() } answers { callOriginal() }

• In pretty much the same way, constructedWith<AndroidData>(LargerThan15Matcher()) is not a proper expression, because it is intended to specify what should happen with an AndroidData constructed with an argument matched by the LargerThan15Matcher . For instance, if you want to say that the value of such an object should always return 1 you could write

 every { constructedWith<AndroidData>(LargerThan15Matcher()).value } returns 1f