RxJava: Throttling

- 3 mins

Unlike buffer() and window() operators, throttle() is an operator that omits emissions when they occur rapidly. This is helpful when rapid emissions are considered redundant or unwanted (such as a user clicking on a button repeatedly).

There is multiple throttling operators: throttleLast(), throttleFirst(), throttleWithTimeout(). To understand them, lets start with the following case:

fun main() {
    val source1 = Observable.interval(100, TimeUnit.MILLISECONDS)
        .map { (it + 1) * 100 } // map to elapsed time
        .map { "Source 1: $it" }
        .take(10)

    val source2 = Observable.interval(300, TimeUnit.MILLISECONDS)
        .map { (it + 1) * 300 } // map to elapsed time
        .map { "Source 2: $it" }
        .take(3)

    val source3 = Observable.interval(2000, TimeUnit.MILLISECONDS)
        .map { (it + 1) * 2000 } // map to elapsed time
        .map { "Source 3: $it" }
        .take(2)

    Observable.concat(source1, source2, source3)
        .subscribe { println(it) }

    TimeUnit.SECONDS.sleep(6)
} 

Lets concatenate 3 sources (source1, source2 and source3), with different intervals (100, 300 and 2000), and we take a fixed number of emissions for each one (10, 3 and 2). The output is as follows:

Source 1: 100
Source 1: 200
Source 1: 300
Source 1: 400
Source 1: 500
Source 1: 600
Source 1: 700
Source 1: 800
Source 1: 900
Source 1: 1000
Source 2: 300
Source 2: 600
Source 2: 900
Source 3: 2000
Source 3: 4000

Operator: throttleLast() / sample()

The throttleLast() operator (aliased as sample()) will only emit the last item at a fixed time interval:

Observable.concat(source1, source2, source3)
    .throttleLast(1, TimeUnit.SECONDS)
    .subscribe {println(it) }
Source 1: 900
Source 2: 900
Source 3: 2000

Operator: throttleFirst()

throttleFirst emits the first item that occurs at every fixed time interval:

    Observable.concat(source1, source2, source3)
        .throttleFirst(1, TimeUnit.SECONDS)
        .subscribe { println(it) }
Source 1: 100
Source 2: 300
Source 3: 2000
Source 3: 4000

/Note: throttleFirst() and throttleLast() both emit on the computation Scheduler, however, it’s possible to specify another Scheduler as a third argument./

Operator: throttleWithTimeout()

While emissions are firing rapidly, throttleWithTimeout() (aliased to debounce()) will not emit anything until there is a period of inactivity, and then it will push the last emission forward. This operator takes time interval arguments that specify how long a period of inactivity must be:

    Observable.concat(source1, source2, source3)
        .throttleWithTimeout(1, TimeUnit.SECONDS)
        .subscribe { println(it) }
Source 2: 900
Source 3: 2000
Source 3: 4000

The throttleWithTimeout() is an effective way to handle excessive inputs, noisy, and redundant events that sporadically speed up, slow down, or cease. However, it will delay each winning emission.

Mouaad Aallam

Mouaad Aallam

Computer Software Engineer

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