UserDefaults+NotificationCenter+RxSwift.swift

extension Reactive where Base: UserDefaults {

    public func `default`<E: Equatable>(_ type: E.Type, _ keyPath: String) -> Observable<E?> {
        let center = NotificationCenter.default
        let initial = base.object(forKey: keyPath) as? E
        let changes = center.rx.notification(UserDefaults.didChangeNotification)
            .map { _ in
                self.base.object(forKey: keyPath) as? E
        }

        return Observable<E?>.just(initial)
            .concat(changes)
            .distinctUntilChanged { previous, next in
                guard let previous = previous, let next = next else {
                    return false
                }
                return previous == next
        }
    }
}