How to produce one Observable from another

Question

Let's say we have source Observable of Ints:

val source:Observable[Int]

I would like to create another Observable, producing values whose difference to first appeared value at source is greater than 10:

def detect() = Observable[Int](
  subscriber =>
    if (!subscriber.isUnsubscribed) {
      var start:Option[Int] = None
      source.subscribe(
        item => {
          if (start.isEmpty) {
            start = Option(item)
          }
          else {
            start.filter(v => Math.abs(item - v) > 10).foreach {
              item => subscriber.onNext(item)
            }
          }
        }
      )
      subscriber.onCompleted()
    }
)

Here I've used var start to hold first value of source Observable.

Is there a way to simplify this code? I don't like this approach with assigning value to a var

Answer 1

Here is what I came up with:

import rx.lang.scala.Observable

val source = Observable.from(List(5, 2, 3, 16, -40, 2, -70, 50))

source.scan(Option.empty[(Int, Int)]) { (acc, next) =>
  acc.map(_.copy(_2 = next)) orElse Some((next, next))
}.collect {
  case Some((start, current)) if math.abs(start - current) > 10 => current
}.subscribe(x => println(x))

prints

16
-40
-70
50

basically scan keeps an accumulator that can be uninitialized (None), or can hold a pair: the first value and the last element emitted from source. Then we collect only those elements that meet the your predicate.

Answer 2

You just need to apply the filter operator, which produces a new Observable that mirrors emissions of the source observable, but skips those for which the predicate tests false:

val filtered = source.filter(v => Math.abs(item - v) > 10)