What is the better way to mock date using mockk?

# mockk

[Recca Chao]

What is the better way to mock date using mockk?

[Emil Kantis]

why would you need to do that?

[Recca Chao]

to separate my test and the current time

[Quincy]

Use java.time.Clock, if you are on the JVM.

[Quincy]

See https://kotlinlang.slack.com/archives/C9EJFT6DB/p1631801269005500?thread_ts=1631800907.005200&cid=C9EJFT6DB for more info.

[Emil Kantis]

Or object Time { fun now() = Instant.now() } , for instance, and you can easily do mockkObject(Time) every { Time.now() } returns myTimestamp

[Emil Kantis]

as @Quincy mentions in the linked thread, depending your use case

[Recca Chao]

thanks! @Emil Kantis @Quincy

[Quincy]

You're welcome.

[Recca Chao]

My current test code

val fixedClock = Clock.fixed(
		Instant.parse("2022-01-01T23:59:59Z"),
		ZoneId.of("UTC")
	)
	mockkStatic(Clock::class)
	every { Clock.systemUTC() } returns fixedClock
	Assert.assertThat(isValidDate(), `is`(false))

[Quincy]

You don't need to mock the Clock itself. Using the fixed clock gives you control over how timestamps are created. Now what you need to do is ensure the SUT takes a Clock as a parameter and uses that Clock to create timestamps.

[Quincy]

class Sut(private val clock: Clock) {
    fun doSomething(): OffsetDateTime {
       return OffsetDateTime.now(clock)
    }
}

[Quincy]

For example, with the above class we can now write a test that can take control of the timestamp creation.

[Quincy]

fun `my_test`() {
    val now = Instant.of(1L)
    val clock = Clock.fixed(now)

    assertThat(
        Sut(clock).doSomething(),
        equalTo(OffsetDateTime.of(...))
    )
}

[Quincy]

Something like that.

[Quincy]

This give you the ability to let the production code use the real system clock when running in production, but within tests you can specify the behavior of the clock. And you don't need to mock anything.

[Recca Chao]

understood, I think this is a better way to invert the dependency, thanks again

[Quincy]

You're welcome again. 🙂

[thanksforallthefish]

yesterday there was a similar question in #kotest, if you are using that testing framework you can also wrap your test with withConstantNow(Instant.now()) . behind the scene it uses mockkStatic so it only works when in your code you do val date = Instant.now() and it won’t with val date = LocalDate.now().

you can also do withConstantNow(LocalDate.now()) or every other time-setting instruction, though personally I still prefer to use a clock. not a big fan of static mocking in general (as much as I would actually remove it from the library)