How to obtain a UIAlertController observable (Reac

I implemented a "reactive" UIAlertController so I can get an Observable<Int> of the button press. (See code below).

My question, or questions, are:

Is this implementation correct? I don't like storing the observers; I wonder if there's a better solution.
Or... is there already an implementation of this in ReactiveCocoa or RxSwift?

Here is the implementation. I removed the parts not relevant to te question.

class AlertBuilder {

    typealias AlertAction = (Int) -> ()

    private let alert: UIAlertController

    /** If observable() is called, we keep here the observers to notify them */
    private var observers: [AnyObserver<Int>] = []

    init(alert: UIAlertController) {
        self.alert = alert
    }

    /** When using observable(), the action is not needed. */
    func button(_ title: String, style: UIAlertActionStyle = .default, action: AlertAction? = nil) -> AlertBuilder {

        let buttonIndex = alert.actions.count
        alert.addAction( UIAlertAction(title: title, style: style, handler: { [weak self] _ in

            // Callback via action
            action?(buttonIndex)

            // Callback via observers
            if let this = self {
                for observer in this.observers {
                    observer.onNext(buttonIndex)
                    observer.onCompleted()
                }
                this.observers = []
            }
        }) )
        return self
    }

    /**
     * Returns an Observable that will emit the pressed button index and complete.
     * It's important to keep a reference to the AlertBuilder, otherwise the events won't be received.
     */
    func observable() -> Observable<Int> {

        return Observable<Int>.create { observer in
            self.observers.append(observer)
            return Disposables.create()
        }
    }
}

You would use it from a controller, like this:

let alert = UIAlertController(title: "title", message: "msg", preferredStyle: .actionSheet)

let builder = AlertBuilder(alert: alert)
    .button("no", style: .destructive)
    .button("yes")

self.present(alert, animated: true, completion: nil)

self.builder.observable()
    .subscribe(onNext: { buttonIndex in /* ... */ })
    .disposed(by: bag)

// keep reference to builder so observable() works
self.builder = builder

Solution, that allows to keep all code in one place, is extension to UIAlertViewController:

extension UIAlertController {

    struct AlertAction {
        var title: String?
        var style: UIAlertActionStyle

        static func action(title: String?, style: UIAlertActionStyle = .default) -> AlertAction {
            return AlertAction(title: title, style: style)
        }
    }

    static func present(
        in viewController: UIViewController,
        title: String?, 
        message: String?,
        style: UIAlertControllerStyle,
        actions: [AlertAction])
        -> Observable<Int>
    {
        return Observable.create { observer in
            let alertController = UIAlertController(title: title, message: message, preferredStyle: style)

            actions.enumerated().forEach { index, action in
                let action = UIAlertAction(title: action.title, style: action.style) { _ in
                    observer.onNext(index)
                    observer.onCompleted()
                }
                alertController.addAction(action)
            }

            viewController.present(alertController, animated: true, completion: nil)
            return Disposables.create { alertController.dismiss(animated: true, completion: nil) }
        }

    }

}

And usage:

let actions: [UIAlertController.AlertAction] = [
    .action(title: "no", style: .destructive),
    .action(title: "yes")
]

UIAlertController
    .present(in: self, title: "Alert", message: "message", style: .alert, actions: actions)
    .subscribe(onNext: { buttonIndex in
        print(buttonIndex)
    })
    .disposed(by: bag)

Code and logic is pretty straightforward, so i provide you no explanation here. Ask if you have any questions.