@Test public void demonstrateAnAsynchronousSideChannel() throws Exception { // tests: CompletableFuture<Done> switchF = new CompletableFuture<>(); Graph<FlowShape<Integer, Integer>, NotUsed> killSwitch = Flow.fromGraph(new KillSwitch<>(switchF)); ExecutionContext ec = system.dispatcher(); CompletionStage<Integer> valueAfterKill = switchF.thenApply(in -> 4); CompletionStage<Integer> result = Source.from(Arrays.asList(1, 2, 3)) .concat(Source.fromCompletionStage(valueAfterKill)) .via(killSwitch) .runFold(0, (n, sum) -> n + sum, mat); switchF.complete(Done.getInstance()); assertEquals(new Integer(6), result.toCompletableFuture().get(3, TimeUnit.SECONDS)); }