Android limit specific API call to one in 3 seconds

Question

I want to limit /test API call to be called once in 3 seconds, so for instance:

2021-09-21 14:09:19.920 V/OkHttp: --> GET https://xxx/test
2021-09-21 14:09:20.031 V/OkHttp: <-- 200 https://xxx/test (109ms)
2021-09-21 14:09:20.038 V/OkHttp: --> GET https://xxx/test
2021-09-21 14:09:20.136 V/OkHttp: <-- 200 https://xxx/test (96ms)
2021-09-21 14:09:20.146 V/OkHttp: --> GET https://xxx/test
2021-09-21 14:09:20.315 V/OkHttp: <-- 200 https://xxx/test (168ms)
2021-09-21 14:09:20.325 V/OkHttp: --> GET https://xxx/test
2021-09-21 14:09:20.499 V/OkHttp: <-- 200 https://xxx/test (172ms)
2021-09-21 14:09:20.514 V/OkHttp: --> GET https://xxx/test
2021-09-21 14:09:20.615 V/OkHttp: <-- 200 https://xxx/test (100ms)
2021-09-21 14:09:20.628 V/OkHttp: --> GET https://xxx/test
2021-09-21 14:09:20.721 V/OkHttp: <-- 200 https://xxx/test (91ms)
2021-09-21 14:09:20.734 V/OkHttp: --> GET https://xxx/test
2021-09-21 14:09:20.827 V/OkHttp: <-- 200 https://xxx/test (87ms)

Would be called once:

2021-09-21 14:09:19.920 V/OkHttp: --> GET https://xxx/test
2021-09-21 14:09:20.031 V/OkHttp: <-- 200 https://xxx/test (109ms)

For test purpose, I'm doing:

repeat(10) { index ->
    apiRepo.test() //Returns Single
        .toObservable()
        .debounce(3, TimeUnit.SECONDS)
        .subscribe({}, { Timber.e(it) })
    }

I come from Coroutine world, so I think I've misunderstood how debounce works, because I still get 10 calls (within 2 seconds), while I expect 1 in 3 seconds.

Answer 1

You create a new debounce ten times. That is why it is not working as expected. To achieve what you want you can use PublishSubject.

val apiCallSubject = PublishSubject.create<Unit>()

apiCallSubject
    .debounce(3, TimeUnit.SECONDS)
    .flatMapSingle { apiRepo.test() }
    .subscribe({}, { Timber.e(it) })

repeat(10) { apiCallSubject.onNext(Unit) }

The point is that Observable which you debounce has to emit multiple items. In your case it is emitting only one but ten times.

Answer 2

You create new observable each iteration. Your apiRepo is returning new Observable each time. You need to do something like this:

val publisher = PublishSubject.create<Unit>()
Observable.fromPublisher(publisher).debounce(3, TimeUnit.SECONDS).flatMap { api.testApi() }.subscribe({}, { Timber.e(it) })

repeat(1000000){
   publisher.onNext(Unit)
}

From the loop you are emiting new items and as soon onNext() is called, the chain will react upon it.