Hi, im trying to write some tests around some code that delays before taking an action. however, del...

# flow

[abbic]

Hi, im trying to write some tests around some code that delays before taking an action.
however, delays are skipped when using runTest and its making testing the timeout impossible.
am i missing something? im trying to use advanceTimeBy to maneuver around this delay for tests but it seems useless as the delays are just skipped

[CLOVIS]

What is the timeout for? How does the system behave differently depending on whether the timeout finished or not?

[abbic]

a lamda is run if the timeout completes

[abbic]

and the timeout completes if a flow doesn't emit in that time window

[abbic]

fun <T: Any> Flow<T>.ifDelayed(timeMillis: Long, block: () -> Unit): Flow<T> = channelFlow {
     val delayJob = launch {
         delay(timeMillis)
         block()
     }
 
     collect {
         delayJob.cancelAndJoin()
         send(it)
     }
 }

[abbic]

may not be correct, found an answer on stackoverflow that I'm still working to adapt and understand, and testing it properly is a part of that process

[CLOVIS]

How I would do it:

@Test
fun lambdaIsNotCalledIfTimeoutDidntEnd() = runTest {
    var called = false
    flow {
        delay(100)
        emit(Unit)
    }.ifDelayed(1000) {
        called = true
    }.collect()
    assertFalse(called)
}

@Test
fun lambdaIsCalledIfTimeoutEnds() = runTest {
    var called = false
    flow {
        delay(5000)
        emit(Unit)
    }.ifDelayed(1000) {
        called = true
    }.collect()
    assertTrue(called)
}

[abbic]

ooh brilliant

[abbic]

the thing for me here is understanding advanceTimeBy is useless for testing delays

[CLOVIS]

It's not useless, it's just that what you want to test is whether the lambda is called or not. You don't really care about the control of time at all

[CLOVIS]

In general, I recommend testing the effect of things rather than the things themselves. It's called black box testing, if you want to read more about it

[Kirill Zhukov]

You don’t really care about the control of time at all

I think in many cases you actually do! So, I would agree that advanceTimeBy doesn’t appear to be very helpful in situations like this. I’ve noticed a common sentiment among developers, including myself, who are confused about what advanceTimeBy is supposed to do and how to use it.

[Kirill Zhukov]

This thread alone… https://kotlinlang.slack.com/archives/C1CFAFJSK/p1677339643532769