「最大でn回のイベントを発行する + Completeしない」Observableの作り方

extension Reactive where Base: UIViewController {
    var viewWillAppear: Observable<Void> {
        return sentMessage(#selector(UIViewController.viewWillAppear(_:))).map { _ in () }.shareReplay(1)
    }
}

viewController.rx.viewWillAppear
    .subscribe(...)
    .addDisposableTo(disposeBag)

extension Reactive where Base: UIViewController {
    var viewWillAppear: Observable<Void> {
        return sentMessage(#selector(UIViewController.viewWillAppear(_:))).map { _ in () }.shareReplay(1)
    }

    var viewWillAppearOnce: Observable<Void> {
        return viewWillAppear.take(1)
    }
}

next()
complete

extension Reactive where Base: UIViewController {
    var viewWillAppear: Observable<Void> {
        return sentMessage(#selector(UIViewController.viewWillAppear(_:))).map { _ in () }.shareReplay(1)
    }

    var viewWillAppearOnce: Observable<Void> {
        return Observable.concat ([viewWillAppear.take(1), Observable.never()])
    }
}