observeOn.swift

//Main thread at point of observable creation
_ = Observable.just("hello")
      .map {
          return $0.characters.count  //On main thread
      }
      .map {
          return $0 + 5 //On main thread
      }
      .observeOn(ConcurrentDispatchQueueScheduler(globalConcurrentQueueQOS: .UserInteractive)) // User Interactive dispatch queue passed to observe on operator
      .map {
          return $0 + 10 //On user interactive dispatch queue
      }
      .subscribeNext { //Observable subscribed to on main thread
          print($0) //Printed on user interactive dispatch queue
      }