How does one get a manually created `CoroutineScope` to use a `TestScope`'s scheduler in `kotlinx-c...

# coroutines

[Ridwan Farouki]

How does one get a manually created CoroutineScope to use a `TestScope`'s scheduler in kotlinx-coroutines-test:1.6.1 for the virtual time behaviour? I would have expected CoroutineScope(Job() + testScheduler) to work but it doesn't. (Note that CoroutineScope(Job() + coroutineContext) does work but then when I try to cancel differentScope it fails the test because it cancels the parent TestScope, making it hard to test code that uses structured concurrency)

fun tickTest() = runTest {
        val differentScope = CoroutineScope(Job() + testScheduler)

        val ticks = mutableListOf<Int>()

        val tickJob = differentScope.launch {
            var time = 0
            while (true) {
                delay(1_000)
                ticks.add(++time)
            }
        }

        advanceTimeBy(5_001)

        tickJob.cancel()

        println(ticks)
        assert(ticks.size == 5)
    }

[julian]

@Ridwan Farouki Try this
val differentScope = CoroutineScope(Job() + coroutineContext[ContinuationInterceptor]!!)

[julian]

I think the reason ^ works is that the test dispatcher has the delay implementation needed for virtual time.

[Ridwan Farouki]

Thank you @julian, it does indeed work!

[julian]

Also, the reason CoroutineScope(Job() + coroutineContext) cancels the TestScope job is that the rightmost context element wins when adding.
Since coroutineContext is rightmost and contains a job, that job overwrites the one on the left of the +.
This, though, would do what you want: CoroutineScope(coroutineContext + Job()) , and then later, differentScope.cancel().

[Ridwan Farouki]

I see, that makes sense, thanks for explaining. The important (and, IMO, rather esoteric) note that the + operator is not commutative caught me off guard.

[Dmitry Khalanskiy [JB]]

Another option is CoroutineScope(StandardTestDispatcher(testScheduler) + Job()) (here, the order doesn't matter), or even just CoroutineScope(StandardTestDispatcher(testScheduler)) because CoroutineScope adds a Job if none is specifie. This creates a new dispatcher that is linked to the test framework.