CodableSubclasses.swift

import Foundation

class Vehicle: Codable {
  var seats: Int = 0

  init() { }

  func move() { }
}

extension Vehicle {
  static func decode<Key: CodingKey>(from container: KeyedDecodingContainer<Key>, forKey key: Key) throws -> Vehicle {
    if let car = try? container.decode(Car.self, forKey: key) {
      return car
    }

    if let plane = try? container.decode(Plane.self, forKey: key) {
      return plane
    }

    return try container.decode(Vehicle.self, forKey: key)
  }
}

class Car: Vehicle {
  var wheels: Int = 4

  override func move() {
    // drive car
  }

  override init() {
    super.init()
  }

  enum CodingKeys: String, CodingKey {
    case capacity, wheels
  }

  required init(from decoder: Decoder) throws {
    let container = try decoder.container(keyedBy: CodingKeys.self)
    self.wheels = try container.decode(Int.self, forKey: .wheels)
    try super.init(from: decoder)
  }

  override func encode(to encoder: Encoder) throws {
    var container = encoder.container(keyedBy: CodingKeys.self)
    try container.encode(wheels, forKey: .wheels)
    try super.encode(to: encoder)
  }
}

class Plane: Vehicle {
  var wings: Int = 2

  override func move() {
    // fly plane
  }

  override init() {
    super.init()
  }

  enum CodingKeys: String, CodingKey {
    case capacity, wings
  }

  required init(from decoder: Decoder) throws {
    let container = try decoder.container(keyedBy: CodingKeys.self)
    self.wings = try container.decode(Int.self, forKey: .wings)
    try super.init(from: decoder)
  }

  override func encode(to encoder: Encoder) throws {
    var container = encoder.container(keyedBy: CodingKeys.self)
    try container.encode(wings, forKey: .wings)
    try super.encode(to: encoder)
  }
}

class Person: Codable {
  var name: String
  var vehicle: Vehicle

  init(name: String, vehicle: Vehicle) {
    self.name = name
    self.vehicle = vehicle
  }

  enum CodingKeys: String, CodingKey {
    case name, vehicle
  }

  required init(from decoder: Decoder) throws {
    let container = try decoder.container(keyedBy: CodingKeys.self)
    self.name = try container.decode(String.self, forKey: .name)
    self.vehicle = try Vehicle.decode(from: container, forKey: .vehicle)
  }
}

extension Encodable {
  internal var jsonData: Data? {
      return try? JSONEncoder().encode(self)
  }

  internal var jsonString: String? {
    do {
      let encoder = JSONEncoder()
      encoder.outputFormatting = .prettyPrinted
      return String(data: try encoder.encode(self), encoding: .utf8)
    } catch {
      return nil
    }
  }
}

func test() {
  let pilot = Person(name: "John", vehicle: Plane())
  let driver = Person(name: "David", vehicle: Car())

  if let pilotString = pilot.jsonString,
     let driverString = driver.jsonString {
    print(pilotString)
    print(driverString)
  }

  if let pilotData = pilot.jsonData,
     let driverData = driver.jsonData,
     let decodedPilot = try? JSONDecoder().decode(Person.self, from: pilotData),
     let decodedDriver = try? JSONDecoder().decode(Person.self, from: driverData) {
    print("Pilot - \(decodedPilot.name), Plane - \((decodedPilot.vehicle as! Plane).jsonString!)")
    print("Driver - \(decodedDriver.name), Car - \((decodedDriver.vehicle as! Car).jsonString!)")
  }
}