@Test
public void testOkEmitter() throws Exception {

    Subscription subscription = ObservableExt.periodicGenerator(
            OK_SOURCE, INITIAL_DELAY_MS, INTERVAL_MS, TimeUnit.MILLISECONDS, testScheduler
    ).subscribe(testSubscriber);
    assertThat(testSubscriber.takeNext()).isNull();
    // Check emits
    expectOkEmit(INITIAL_DELAY_MS);
    expectOkEmit(INTERVAL_MS);
    expectOkEmit(INTERVAL_MS);
    // Unsubscribe
    subscription.unsubscribe();
    testSubscriber.onCompleted();
}