UserDefaultsObservable.swift

extension UserDefaults {
    @objc dynamic var yourKey: Bool {
        return bool(forKey: "yourKey")
    }
}

protocol UserDefaultsObservable: AnyObject {
    var observation: Any? { get set }
    typealias UserDefaultsCompletionHandler = (_ defaults: UserDefaults) -> Void
    func addObservation(completion: UserDefaultsCompletionHandler?)
}

extension UserDefaultsObservable {
    func addObservation(completion: UserDefaultsCompletionHandler?) {
        observation = UserDefaults.standard.observe(\.yourKey, options: [.new], changeHandler: { defaults, _ in
            completion?(defaults)
        })
    }
}