Property Wrapper for User Defaults.

UserDefaulter.swift

@propertyWrapper
struct UserDefaulter<A: RawRepresentable> {
    let key: String
    init(_ key: String) {
        self.key = key
    }
    
    private var fetchedValue: A?
    var wrappedValue: A? {
        mutating get {
            if let fetchedValue = fetchedValue {
                return fetchedValue
            }
            if let rawValue = UserDefaults.standard.value(forKey: key) as? A.RawValue {
                let result = A(rawValue: rawValue)
                self.fetchedValue = result
                return result
            }
            return nil
        }
        set {
            UserDefaults.standard.set(newValue?.rawValue, forKey: key)
            self.fetchedValue = newValue
        }
    }
}