LoadingBarButton.swift

import UIKit
import RxSwift
import RxCocoa

final class LoadingBarButton {

    fileprivate weak var navigationItem: UINavigationItem?
    fileprivate var initialBarButton: UIBarButtonItem?

    init(navigationItem: UINavigationItem) {
        self.navigationItem = navigationItem
        self.initialBarButton = navigationItem.rightBarButtonItem
    }

    fileprivate func setState(waiting: Bool = false) {
        if waiting {
            let activityIndicatorItem =  UIBarButtonItem.activityIndicator
            navigationItem?.rightBarButtonItem = activityIndicatorItem.barButton
            activityIndicatorItem.activityIndicator.startAnimating()
        } else {
            navigationItem?.rightBarButtonItem = initialBarButton
        }
    }

}

extension Observable {

    func changeLoadingUI(using barButton: LoadingBarButton) -> Observable<Observable.E> {
        return observeOn(MainScheduler.instance)
            .do(onSubscribe: {
                barButton.setState(waiting: true)
            }, onDispose: {
                barButton.setState(waiting: false)
            })
    }

}