Swift4.0 で追加される Codable ref: http://qiita.com/hanawat/items/f3fffac2a4ea2e8a1767

file0.swift

struct Person {

  let name: String
  let age: Int
}

extension Person {

  class Helper: NSObject, NSCoding {

    let person: Person?

    init(person: Person) {
      self.person = person
      super.init()
    }

    required init?(coder aDecoder: NSCoder) {
      guard
        let name = aDecoder.decodeObject(forKey: "name") as? String,
        let age = aDecoder.decodeObject(forKey: "age") as? Int else {
          return nil
      }

      person = Person(name: name, age: age)
      super.init()
    }

    func encode(with aCoder: NSCoder) {
      guard let person = person else { return }

      aCoder.encode(person.name, forKey: "name")
      aCoder.encode(person.age, forKey: "age")
    }
  }
}

let person = Person(name: "Foo", age: 10)
let helper = Person.Helper(person: person)
let data = NSKeyedArchiver.archivedData(withRootObject: helper)

file1.swift

struct Person: Codable {

  enum Child: String, Codable {
    case son, daughter
  }

  let name: String
  let age: Int
  let children: [Child]
}

let person = Person(name: "Foo", age: 10, children: [.son, .daughter])
let data = try? JSONEncoder().encode(person)

file2.swift

/// A type that can convert itself into and out of an external representation.
public typealias Codable = Decodable & Encodable

file3.swift

let person = try? JSONDecoder().decode(Person.self, from: data)

file4.swift

let data = NSKeyedArchiver.archivedData(withRootObject: helper) // 299 bytes
let data = try? JSONEncoder().encode(person)                    // 53 bytes