Hello, i'm trying to mock Instant but I can't seem...

# mockk

[tim]

Hello, i'm trying to mock Instant but I can't seem to get it to work. My approach:

object MockkInstant {
    var value: Instant = Instant.now()

    fun mock() {
        mockkStatic(Instant::class)

        every {
            Instant.now()
        } answers {
            value.minusMillis(0) // line 22 from stack trace below
        }
    }

    fun clear() {
        clearStaticMockk(Instant::class)
    }

    fun step(duration: Duration = 1.seconds) {
        value = value.add(duration) // extension function defined elsewhere
    }
}

Now in my tests i'm trying to do this:

"fixes calls to Instant.now()" {
    val a = Instant.now()
    delay(1)
    val b = Instant.now()
    a.shouldBe(b)
}

But I'm getting an error from the answers block:

Empty list doesn't contain element at index 0.
java.lang.IndexOutOfBoundsException: Empty list doesn't contain element at index 0.
	at kotlin.collections.EmptyList.get(Collections.kt:35)
	at kotlin.collections.EmptyList.get(Collections.kt:23)
	at io.mockk.MockKAnswerScope.getValueAny(API.kt:3834)
	at io.mockk.MockKAnswerScope.getValue(API.kt:2188)
	at io.glimpseprotocol.testing.mockk.MockkInstant$mock$2.invoke(MockkInstant.kt:22)
	at io.glimpseprotocol.testing.mockk.MockkInstant$mock$2.invoke(MockkInstant.kt:13)
	at io.mockk.MockKStubScope$answers$1.invoke(API.kt:2092)
	at io.mockk.MockKStubScope$answers$1.invoke(API.kt:2069)
	at io.mockk.FunctionAnswer.answer(Answers.kt:19)
...

I've seen other people using returns, but afaik that returns the same Instant object whereas I want to create a new one each time Instant.now() is called. Any suggestions?

[LeoColman]

Are you using Kotest?

[LeoColman]

If you are, I'd recommend https://kotest.io/extensions/#current-instant-listeners

[tim]

Yes using KoTest ... this looks great i will check that out 🙏

[tim]

Hmm doesn't quite appear to have the features I'm trying to accomplish above with mockk. For example being able to 'step' forward/backword in time is a requirement as I'm walking through a dataset and want to tightly control the value of Instant.now().

From what I can tell with kotest constant intant functions, they appear to be more about fixing Instant.now() with in a block whereas I want to fix it across my running test and then step forward, and then step forward again. I guess i could wrap each step in withConstantInstant and then manually create the instant for each step... just seems a bit verbose especially if say i wanted to test it over 50 steps

[LeoColman]

Kotest uses mockk inside itself

[LeoColman]

You can take a look at the implementation and copy how the listeners do

[LeoColman]

(for the instant part specifically)

[tim]

Ahh k

[tim]

If they're providing constant time my hunch is they'll be using returns rather than answers

[tim]

will check out thier source

[tim]

ty

[tim]

Managed to get 'answers' to work correctly:

@OptIn(ExperimentalTime::class)
object MockkInstant {
    var step: Duration = 0.seconds
    private lateinit var base: Instant

    fun mock(from: Instant = Instant.now()) {
        base = from
        mockkStatic(Instant::class)

        every {
            Instant.now()
        } answers {
            base.add(step)
        }
    }

    fun clear() {
        clearStaticMockk(Instant::class)
    }

    fun step(duration: Duration = 1.seconds) {
        step += duration
    }
}