If you don't want any delay, why don't you simply resume the continuation in the schedule call?:

class TestUiContext : CoroutineDispatcher(), Delay {    override fun scheduleResumeAfterDelay(time: Long, unit: TimeUnit, continuation: CancellableContinuation<Unit>) {        continuation.resume(Unit)    }    override fun dispatch(context: CoroutineContext, block: Runnable) {        //CommonPool.dispatch(context, block)  // dispatch on CommonPool        block.run()  // dispatch on calling thread    }}

That way delay() will resume with no delay. Note that this still suspends at delay, so other coroutines can still run (like yield())

@Testfun `test with delay`() {    runBlocking(TestUiContext()) {        launch { println("launched") }        println("start")        delay(5000)        println("stop")    }}

Runs without delay and prints:

startlaunchedstop

EDIT:

You can control where the continuation is run by customizing the dispatch function.

In kotlinx.coroutines v1.2.1 they added the kotlinx-coroutines-test module. It includes the runBlockingTest coroutine builder, as well as a TestCoroutineScope and TestCoroutineDispatcher. They allow auto-advancing time, as well as explicitly controlling time for testing coroutines with delay.

Use TestCoroutineDispatcher, TestCoroutineScope, or Delay

TestCoroutineDispatcher, TestCoroutineScope, or Delay can be used to handle a delay in a Kotlin coroutine made in the production code tested.

Implement

In this case SomeViewModel's view state is being tested. In the ERROR state a view state is emitted with the error value being true. After the defined Snackbar time length has passed using a delay a new view state is emitted with the error value set to false.

SomeViewModel.kt

private fun loadNetwork() {    repository.getData(...).onEach {        when (it.status) {            LOADING -> ...            SUCCESS ...            ERROR -> {                _viewState.value = FeedViewState.SomeFeedViewState(                    isLoading = false,                    feed = it.data,                    isError = true                )                delay(SNACKBAR_LENGTH)                _viewState.value = FeedViewState.SomeFeedViewState(                    isLoading = false,                    feed = it.data,                    isError = false                )            }        }    }.launchIn(coroutineScope)}

There are numerous ways to handle the delay. advanceUntilIdle is good because it doesn't require specifying a hardcoded length. Also, if injecting the TestCoroutineDispatcher, as outlined by Craig Russell, this will be handled by the same dispatcher used inside of the ViewModel.

SomeTest.kt

private val testDispatcher = TestCoroutineDispatcher()private val testScope = TestCoroutineScope(testDispatcher)// Code that initiates the ViewModel emission of the view state(s) here.testDispatcher.advanceUntilIdle()

These will also work:

• testScope.advanceUntilIdle()
• testDispatcher.delay(SNACKBAR_LENGTH)
• delay(SNACKBAR_LENGTH)
• testDispatcher.resumeDispatcher()
• testScope.resumeDispatcher()
• testDispatcher.advanceTimeBy(SNACKBAR_LENGTH)
• testScope.advanceTimeBy(SNACKBAR_LENGTH)

Error without handling the delay

kotlinx.coroutines.test.UncompletedCoroutinesError: Unfinished coroutines during teardown. Ensure all coroutines are completed or cancelled by your test.

at kotlinx.coroutines.test.TestCoroutineDispatcher.cleanupTestCoroutines(TestCoroutineDispatcher.kt:178) at app.topcafes.FeedTest.cleanUpTest(FeedTest.kt:127) at app.topcafes.FeedTest.access$cleanUpTest(FeedTest.kt:28) at app.topcafes.FeedTest$topCafesTest$1.invokeSuspend(FeedTest.kt:106) at app.topcafes.FeedTest$topCafesTest$1.invoke(FeedTest.kt) at kotlinx.coroutines.test.TestBuildersKt$runBlockingTest$deferred$1.invokeSuspend(TestBuilders.kt:50) at kotlin.coroutines.jvm.internal.BaseContinuationImpl.resumeWith(ContinuationImpl.kt:33) at kotlinx.coroutines.DispatchedTask.run(DispatchedTask.kt:56) at kotlinx.coroutines.test.TestCoroutineDispatcher.dispatch(TestCoroutineDispatcher.kt:50) at kotlinx.coroutines.DispatchedContinuationKt.resumeCancellableWith(DispatchedContinuation.kt:288) at kotlinx.coroutines.intrinsics.CancellableKt.startCoroutineCancellable(Cancellable.kt:26) at kotlinx.coroutines.CoroutineStart.invoke(CoroutineStart.kt:109) at kotlinx.coroutines.AbstractCoroutine.start(AbstractCoroutine.kt:158) at kotlinx.coroutines.BuildersKt__Builders_commonKt.async(Builders.common.kt:91) at kotlinx.coroutines.BuildersKt.async(Unknown Source) at kotlinx.coroutines.BuildersKt__Builders_commonKt.async$default(Builders.common.kt:84) at kotlinx.coroutines.BuildersKt.async$default(Unknown Source) at kotlinx.coroutines.test.TestBuildersKt.runBlockingTest(TestBuilders.kt:49) at kotlinx.coroutines.test.TestBuildersKt.runBlockingTest(TestBuilders.kt:80) at app.topcafes.FeedTest.topCafesTest(FeedTest.kt:41) at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) at java.lang.reflect.Method.invoke(Method.java:498) at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:50) at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12) at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:47) at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:17) at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:325) at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:78) at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:57) at org.junit.runners.ParentRunner$3.run(ParentRunner.java:290) at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:71) at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:288) at org.junit.runners.ParentRunner.access$000(ParentRunner.java:58) at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:268) at org.junit.runners.ParentRunner.run(ParentRunner.java:363) at org.junit.runner.JUnitCore.run(JUnitCore.java:137) at com.intellij.junit4.JUnit4IdeaTestRunner.startRunnerWithArgs(JUnit4IdeaTestRunner.java:68) at com.intellij.rt.junit.IdeaTestRunner$Repeater.startRunnerWithArgs(IdeaTestRunner.java:33) at com.intellij.rt.junit.JUnitStarter.prepareStreamsAndStart(JUnitStarter.java:230) at com.intellij.rt.junit.JUnitStarter.main(JUnitStarter.java:58)