Reactor StepVerifier.withVirtualTime blocks indefinitely

Question

I'm trying to use Reactor's virtual time feature, but the test blocks indefinitely (without timeout) or throws an AssertionError (with timeout):

@Test
public void test() {
    StepVerifier.withVirtualTime(() -> 
            Flux.just(1, 2, 3, 4).delayElements(Duration.ofSeconds(1)))
            .expectSubscription()
            .expectNextCount(4)
            .expectComplete()
            .verify(Duration.ofSeconds(10));
}

The exception is:

java.lang.AssertionError: VerifySubscribertimed out on reactor.core.publisher.FluxConcatMap$ConcatMapImmediate@66d1af89

The same example with real time works as expected:

@Test
public void test2() {
    StepVerifier.create(Flux.just(1, 2, 3, 4).delayElements(Duration.ofSeconds(1)))
            .expectSubscription()
            .expectNextCount(4)
            .expectComplete()
            .verify(Duration.ofSeconds(10));
}

I can not see an error in my first example following Manipulating Time from the reference.

What is wrong?

Answer 1

You need to use .thenAwait(Duration), otherwise the (virtual) clock won't move, and the delay never happens. You can also use .expectNoEvent(Duration) after the expectSubscription().

For example:

@Test
public void test() {
  StepVerifier.withVirtualTime(() -> 
        Flux.just(1, 2, 3, 4).delayElements(Duration.ofSeconds(1)))
        .expectSubscription() //t == 0
//move the clock forward by 1s, and check nothing is emitted in the meantime
        .expectNoEvent(Duration.ofSeconds(1))
//so this effectively verifies the first value is delayed by 1s:
        .expectNext(1)
//and so on...
        .expectNoEvent(Duration.ofSeconds(1))
        .expectNext(2)
//or move the clock forward by 2s, allowing events to take place,
//and check last 2 values where delayed
        .thenAwait(Duration.ofSeconds(2))
        .expectNext(3, 4)
        .expectComplete()
//trigger the verification and check that in realtime it ran in under 200ms
        .verify(Duration.ofMillis(200));
}