Rxswift - how to show a progress bar

You may to use ActivityIndicator from RxSwift repo. I using MBProgressHUD in my project. At first you need to create extension for this library:

extension MBProgressHUD {

    /**
     Bindable sink for MBProgressHUD show/hide methods.
     */
    public var rx_mbprogresshud_animating: AnyObserver<Bool> {
        return AnyObserver { event in
            MainScheduler.ensureExecutingOnScheduler()

            switch (event) {
            case .Next(let value):
                if value {
                    let loadingNotification = MBProgressHUD.showHUDAddedTo(UIApplication.sharedApplication().keyWindow?.subviews.last, animated: true)
                    loadingNotification.mode = self.mode
                    loadingNotification.labelText = self.labelText
                    loadingNotification.dimBackground = self.dimBackground
                } else {
                    MBProgressHUD.hideHUDForView(UIApplication.sharedApplication().keyWindow?.subviews.last, animated: true)
                }
            case .Error(let error):
                let error = "Binding error to UI: \(error)"
                #if DEBUG
                    rxFatalError(error)
                #else
                    print(error)
                #endif
            case .Completed:
                break
            }
        }
    }
}

import RxSwift
import RxCocoa

extension Reactive where Base: MBProgressHUD {
    public var animation: Binder<Bool> {
        return Binder(self.base) { hud, show in
            guard let view = UIApplication.shared.keyWindow?.subviews.last()! else { return }
            if show {
                if hud.superview == nil {
                    view.addSubview(hud)
                }
                hud.show(animated: true)
            } else {
                hud.hide(animated: true)
            }
        }
    }
}

Next you need to create ActivityIndicator object in your ViewController class:

let progress = MBProgressHUD()
progress.mode = MBProgressHUDMode.Indeterminate
progress.labelText = "Loading..."
progress.dimBackground = true

let indicator = ActivityIndicator()
indicator.asObservable()
    .bindTo(progress.rx_mbprogresshud_animating)
    .addDisposableTo(bag)

Next just use trackActivity() function into your sequences:

apiMethod
.trackActivity(indicator)
.subscribeNext { stringArray in 
    items.value = stringArray
}
.addDisposableTo(bag)