SerializableToUserDefaults.swift

import Foundation

protocol SerializableToUserDefaults {
    static func read(from defaults: UserDefaults, key: String) -> Self
    func write(to defaults: UserDefaults, key: String)
}

extension String: SerializableToUserDefaults {
    static func read(from defaults: UserDefaults, key: String) -> String {
        return defaults.string(forKey: key)!
    }
    func write(to defaults: UserDefaults, key: String) {
        defaults.set(self, forKey: key)
    }
}

struct Pref<T: SerializableToUserDefaults> {
    let key: String
    let defaultValue: T
    init(key: String, default defaultValue: T) {
        self.key = key
        self.defaultValue = defaultValue
    }
    var value: T {
        get {
            let defaults = UserDefaults.standard
            guard defaults.object(forKey: key) != nil else {
                return defaultValue
            }
            return T.read(from: defaults, key: key)
        }
        nonmutating set {
            let defaults = UserDefaults.standard
            newValue.write(to: defaults, key: key)
        }
    }
}

struct OptionalPref<T: SerializableToUserDefaults> {
    let key: String
    init(key: String) {
        self.key = key
    }
    var value: T? {
        get {
            let defaults = UserDefaults.standard
            guard defaults.object(forKey: key) != nil else {
                return nil
            }
            return T.read(from: defaults, key: key)
        }
        nonmutating set {
            let defaults = UserDefaults.standard
            if let newValue = newValue {
                newValue.write(to: defaults, key: key)
            } else {
                defaults.removeObject(forKey: key)
            }
        }
    }
}

struct Prefs {
    static let happy = Pref<String>(key: "happy", default: "possibly")
    static let reasonForHappiness = OptionalPref<String>(key: "reasonForHappiness")
}

for (key, _) in UserDefaults.standard.dictionaryRepresentation() {
    UserDefaults.standard.removeObject(forKey: key)
}

print(Prefs.happy.value)
Prefs.happy.value = "unlikely"
print(Prefs.happy.value)

print(Prefs.reasonForHappiness.value as Any)
Prefs.reasonForHappiness.value = "kittens"
print(Prefs.reasonForHappiness.value as Any)
Prefs.reasonForHappiness.value = nil
print(Prefs.reasonForHappiness.value as Any)