nonmutating-full.swift

import Foundation

protocol Settings {
    subscript(key: String) -> AnyObject? { get nonmutating set }
}

struct Defaults: Settings {
    typealias Set = (String, AnyObject?) -> Void
    typealias Get = (String) -> AnyObject?
    
    let set: Set
    let get: Get
    
    subscript(key: String) -> AnyObject? {
        get { return get(key) }
        nonmutating set { set(key, newValue) }
    }
}

let standardUserDefaults = Defaults(
    set: { key, value in NSUserDefaults.standardUserDefaults().setObject(value, forKey: key) },
    get: { NSUserDefaults.standardUserDefaults().objectForKey($0) })

standardUserDefaults["x"] = "y"

extension NSUserDefaults: Settings {
    subscript(key: String) -> AnyObject? {
        get { return objectForKey(key) }
        set { setObject(newValue, forKey: key) }
    }
}

let settingsRef: Settings = NSUserDefaults.standardUserDefaults()
print(settingsRef["myProp"])
settingsRef["myProp"] = "someValue"