subscribeOn.swift

_ = Observable.just("hello")
      .map {
          return $0.characters.count  //On main thread
      }
      .map {
          return $0 + 5 // On main thread
      }
      .map {
          return $0 + 10 // On main thread
      }
      .subscribeNext { //Observable subscribed to on main thread
              print($0)
      }

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