system.reactive - Is there an Rx operator for combining the latest from streams 1 and 2 only when stream 2 emits things? -

here's attempt @ drawing marble diagram --

stream 1 = a----b----c---------d------> (magical operator) stream 2 = 1----------2-----3-----4---> stream 3 = 1a---------2c----3c----4d-->

i looking generates stream 3 streams 1 , 2. basically, whenever emitted stream 2, combines latest stream 1. combinelatest similar want want things emitted stream 3 when emitted stream 2, not stream 1. operator exist?

there operator need: 1 overload of sample takes observable instead of duration parameter. documentation here: https://github.com/reactivex/rxjava/wiki/filtering-observables#sample-or-throttlelast

the usage (i'll give examples in scala):

import rx.lang.scala.observable import scala.concurrent.duration import duration._  def o = observable.interval(100.milli) def sampler = observable.interval(180.milli)  // often, need sampled observable o.sample(sampler).take(10).subscribe(x ⇒ println(x +  ", ")) thread.sleep(2000) // or, use case o.combinelatest(sampler).sample(sampler).take(10).subscribe(x ⇒ println(x +  ", ")) thread.sleep(2000) 

the output:

0,  2,  4,  6,  7,  9,  11,  13,  15,  16,   (2,0),  (4,1),  (6,2),  (7,3),  (9,4),  (11,5),  (13,6),  (15,7),  (16,8),  (18,9), 

there slight catch in duplicate entries sampled observable swallowed (see discussion @ https://github.com/reactivex/rxjava/issues/912). other that, think looking for.