Swift UserDefault property wrapper with @propertyWrapper, thanks to https://medium.com/@nimjea

Settings.swift

class Settings {
    static let shared: Settings = {
        Settings()
    }()
    
    private init() {
        // Private, due to singleton
    }
    
    @UserDefaultProperty("on_off", defaultValue: false)
    var isEnabled: Bool
    
    @UserDefaultProperty("number", defaultValue: 3)
    var number: Int
    
    @UserDefaultProperty("name", defaultValue: nil)
    var name: String?
}

UserDefaultProperty.swift

@propertyWrapper
struct UserDefaultProperty<T> {
    let key: String
    let defaultValue: T

    init(_ key: String, defaultValue: T) {
        self.key = key
        self.defaultValue = defaultValue
    }

    var wrappedValue: T {
        get {
            return UserDefaults.standard.object(forKey: key) as? T ?? defaultValue
        }
        set {
            UserDefaults.standard.set(newValue, forKey: key)
        }
    }
}