RxSwift - Debounce/Throttle "inverse"

Question

Let's say I have an instant messaging app that plays a beep sound every time a message arrives. I want to debounce the beeps, but I'd like to play the beep sound for the first message arrived and not for the following ones (in a timespan of, say, 2 seconds).

Another example might be: my app sends typing notifications (so the user I'm chatting with can see that I'm typing a message). I want to send a typing notification when I start typing, but only send new ones in X-seconds intervals, so I don't send a typing notification for every character I type.

Does this make sense? Is there an operator for that? Could it be achieved with the existing operators?

This is my code for the first example. I'm solving it now with debounce, but it's not ideal. If I receive 1000 messages in intervals of 1 second, it won't play the sound until the last message arrives (I'd like to play the sound on the first one).

self.messagesHandler.messages
            .asObservable()
            .skip(1)
            .debounce(2, scheduler: MainScheduler.instance)
            .subscribeNext { [weak self] message in
                    self?.playMessageArrivedSound()
            }.addDisposableTo(self.disposeBag)

Thanks!

Solution 1

Updated for RxSwift 3 and improved throttle operator

With new behavior of throtlle operator, introduced in RxSwift 3.0.0-beta.1, you can use it just like that:

    downloadButton.rx.tap
    .throttle(3, latest: false, scheduler: MainScheduler.instance)
    .subscribe(onNext: { _ in
        NSLog("tap")
    }).addDisposableTo(bag)

---

Old version of answer

Use window operator and then transform Observable<Observable<Type>> to flat Observable using flatMap.

This sample code prints 'tap' only for first tap in every 3 seconds windows (or if tap count exceeds 10000).

    downloadButton.rx_tap
    .window(timeSpan: 3, count: 10000, scheduler: MainScheduler.instance)
    .flatMap({ observable -> Observable<Void> in
        return observable.take(1)
    })
    .subscribeNext { _ in
        NSLog("tap")
    }.addDisposableTo(bag)

Solution 2

Window is a great solution, but I find the sample operator more intuitive and also with correct behavior.

messagesHandler.messages
               .sample(Observable<Int>.timer(0.0, period: 2.0, scheduler: MainScheduler.instance))
               .subscribeNext { [weak self] message in
                    self?.playMessageArrivedSound()
               }.addDisposableTo(self.disposeBag)

The throttle operation does not do what I thought it should.

For people that also find throttle is too confusing:

"throttle will only forward an event once the source observable has stopped sending events for the specified period of time. This does not work well with regular event delivery" for more details.

In this case, the filter you want is

sample(Observable<Int>.timer(0.0, period: 2.0, scheduler: MainScheduler.instance))

Comments

[onmyway133]

This is a good explanation medium.com/@dkhuong291/…

[Bruno Koga]

I'm currently using a similar solution, but I feel there's room for improvement. I'd like to avoid the double subscription. Thanks for answering!

[Evgeny Sureev]

There are definitely was room for improvement. Now it works without double subscription.

[Xav]

"throttle will only forward an event once the source observable has stopped sending events for the specified period of time. This does not work well with regular event delivery" Maybe that was the case in the past but know what is described here is DEBOUNCE not THROTTLE