Swift nonmutating Example

AppDefault.swift

import Foundation

enum AppDefault {
    case APIServer, runMode, homeDirectory, reactHost
    
    var stringValue: String? {
        get {
            return NSUserDefaults.standardUserDefaults().stringForKey(String(self))
        }
        
        nonmutating set {
            if let newValue = newValue {
                NSUserDefaults.standardUserDefaults().setObject(newValue, forKey: String(self))
            } else {
                NSUserDefaults.standardUserDefaults().removeObjectForKey(String(self))
            }
        }
    }
}


let server = AppDefault.APIServer

// `AppDefault`'s `nonmutating` setter on `stringValue` allows for this work on immutable instances
server.stringValue = "127.0.0.1:4000/"