bufferByKey.scala

val o: Observable[Int] = Observable(1 to 20)

def bufferByKey[T, K](o: Observable[T], keyFunc: T => K): Observable[Seq[T]] = {
  type S = (Option[K], List[T], Option[List[T]])
  o.scan[S](None, List(), None) { (lastState: S, newVal: T) =>
    val (lastKey, acc, emit) = lastState
    val nk = keyFunc(newVal)
    lastKey match {
      case None => (Some(nk), newVal :: acc, None)
      case Some(k) =>
        if (k == nk)
          (Some(k), newVal :: acc, None)
        else
          (Some(nk), List(newVal), Some(acc))
    }
  } flatMap {
    case (_, _, None) => Observable()
    case (_, _, Some(acc)) => Observable(acc.reverse)
  }
}

val x = bufferByKey(o, { x: Int => x % 3 == 0 })
x subscribe { x => println("x= " + x) }