SubclassEncDec.swift

import UIKit

class Animal: Codable {

    var say: String?

    private enum CodingKeys : String, CodingKey {
        case say
    }

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

    init(say: String) {
        self.say = say
    }
}

class Dog: Animal {

    var collarName: String?

    private enum CodingKeys: String, CodingKey {
        case collarName
        case dog // not strictly necessary but nests the subclass specific fields in the resultant JSON.
    }

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

    required init(from decoder: Decoder) throws {
        let container = try decoder.container(keyedBy: CodingKeys.self)
        collarName = try container.decode(String.self, forKey: .collarName)
        let superDecoder = try container.superDecoder(forKey: .dog)
        try super.init(from: superDecoder)
    }

    init(say: String, collarName: String) {
        super.init(say: say)
        self.collarName = collarName
    }
}

let c = Animal(say: "")
c.say = "P in Animal"

let d = Dog(say: "Woof", collarName: "Barney")

print(d.collarName ?? "Nope")

let cData = try! JSONEncoder().encode(c)
let dData = try! JSONEncoder().encode(d)

let cDecoded = try! JSONDecoder().decode(Animal.self, from: cData)
print(cDecoded.say ?? "Nope")

let dDecoded = try! JSONDecoder().decode(Dog.self, from: dData)
print(dDecoded.say ?? "Nope")
print(dDecoded.collarName ?? "Nope")

let subclassDecoded = try! JSONDecoder().decode(Animal.self, from: dData)
print(subclassDecoded.say ?? "Nope")

// Print the JSON:

let encoder = JSONEncoder()
encoder.outputFormatting = .prettyPrinted
let encodedDog = try? encoder.encode(d)
print(String(data: encodedDog!, encoding: .utf8)!)