RxJava fromCallable 和取消订阅

Question

我有以下代码（RxJava 1.3.8）：Scheduler scheduler = Schedulers.from(Executors.newSingleThreadExecutor());Subscription subscription1 = Completable.fromCallable(() -> {  Thread.sleep(1000);  System.out.println("first Callable executed");  return 0;})    .subscribeOn(scheduler)    .subscribe();Subscription subscription2 = Completable.fromCallable(() -> {  Thread.sleep(1000);  System.out.println("second Callable executed");  return 0;})    .subscribeOn(scheduler)    .subscribe();CompositeSubscription subscriptions = new CompositeSubscription();subscriptions.addAll(subscription1, subscription2);subscriptions.clear();输出：第一个 Callable 执行第二个 Callable 执行问题是 - 为什么第二个 Callable 被执行？在它运行之前，如果取消订阅，我希望订阅检查并取消执行。