Swift NSCoding Archiving and Unarchiving

example.swift

import Foundation

class Foo: NSObject, NSCoding {
  let identifier:String
  init(identifier:String) {
    self.identifier = identifier
  }
  
  override var description:String {
    return "Foo: \(identifier)"
  }
  
  func encodeWithCoder(aCoder: NSCoder) {
    aCoder.encodeObject(identifier, forKey: "identifier")
  }
  
  required convenience init?(coder aDecoder: NSCoder) {
    guard let identifier = aDecoder.decodeObjectForKey("identifier") as? String
      else {
        return nil
    }
    self.init(identifier:identifier)
  }
}

class Bar:Foo {
  let tag:String
  
  init(identifier:String, tag:String) {
    self.tag = tag
    super.init(identifier: identifier)
  }
  
  override func encodeWithCoder(aCoder: NSCoder) {
    super.encodeWithCoder(aCoder)
    aCoder.encodeObject(tag, forKey: "tag")
  }

  required convenience init?(coder aDecoder: NSCoder) {
    guard let
      identifier = aDecoder.decodeObjectForKey("identifier") as? String,
      tag        = aDecoder.decodeObjectForKey("tag") as? String else {
        return nil
    }
    
    self.init(identifier:identifier, tag:tag)
    
  }
  
  override var description:String {
    return "Bar: \(identifier) is \(tag)"
  }
}

let b = Bar(identifier: "name", tag: "red")
NSKeyedArchiver.archiveRootObject(b, toFile: "archive")
let _b = NSKeyedUnarchiver.unarchiveObjectWithFile("archive")!