Android 等到上一个事件完成 Rx

在我的代码中，我有一些事件需要按顺序处理，其他事件需要并行处理.

我成功地完成了并行部分，但无法完成顺序版本.

event.ofType(Message.class)
    .groupBy(Message::getGroup)
    .flatMap(g -> {
        if (g.getKey() == Message.GROUP_PARALLEL)
            return g.flatMap(m -> Observable.just(m)
                        .observeOn(Schedulers.computation())
                        .doOnNext(this::send)
                        .delay(m.getRetryInterval(), TimeUnit.SECONDS)
                        .repeat(m.getRetryCount())
                        .takeUntil(event.ofType(Sent.class)
                            .filter(a -> a.getId() == m.getId())
                        )
                        .map(s -> m)
                    );
        else if (g.getKey() == Message.GROUP_SEQUENTIAL) {
            // ?
        }
    })

我想这样实施:

When an event is received
    If the previous event is still being processed, wait before emission
    Process the event (blocking)

所以:

    event.subscribeOn(Schedulers.computation())
        .waitUntilPreviousComplete()
        .flatMap(this::processEvent) // we assume it take 1 seconds to complete
        .susbcribe(this::processed, this::onError);

    event.onNext(Event.A)
    event.onNext(Event.B)
    event.onNext(Event.C)

结果应该是:

    12h00:00 processed call with event A
    12h00:01 processed call with event B
    12h00:02 processed call with event C

所以我想要看起来像队列的东西.我试过用HashMap和主题做一些奇怪的事情，但我相信有一个干净的方法可以做到这一点.我该如何实现这一点？