Rx_subsribeOn.swift

let backgroundScheduler = SerialDispatchQueueScheduler(qos: .background)

Observable.just(1)
    .map { _ in
        print(Thread.isMainThread)     // false
    }
    .subscribeOn(backgroundScheduler)
    .map { _ in
        print(Thread.isMainThread)     // false
    }
    .subscribe(onNext: {
        print(Thread.isMainThread)     // false
    })