Swift 4 Codable Protocol example in Playground

Swift4Codable.swift

//
// Created by Dejan Skledar
// Equaleyes Solutions Ltd.
//

import UIKit

struct Sword: Codable {

    private enum CodingKeys: String, CodingKey {
        case name = "sword_name"
        case size = "length"
        case isValyrianSteel = "is_valyrian_steel"
    }

    let name: String
    let size: Int
    let isValyrianSteel: Bool

    init(name: String, size: Int, isValyrianSteel: Bool) {
        self.name = name
        self.size = size
        self.isValyrianSteel = isValyrianSteel
    }

    init(from decoder: Decoder) throws {
        let values = try decoder.container(keyedBy: CodingKeys.self)

        name = try values.decode(String.self, forKey: .name)
        size = try values.decode(Int.self, forKey: .size)
        isValyrianSteel = try values.decode(Bool.self, forKey: .isValyrianSteel)
    }

    public func encode(to encoder: Encoder) throws {
        var values = encoder.container(keyedBy: CodingKeys.self)

        try values.encode(name, forKey: .name)
        try values.encode(size, forKey: .size)
        try values.encode(isValyrianSteel, forKey: .isValyrianSteel)
    }
}

let oathKeeper = Sword(name: "Oathkeeper", size: 120, isValyrianSteel: true)

if let jsonData = try? JSONEncoder().encode(oathKeeper) {
    let jsonString = String(data: jsonData, encoding: .utf8)
}

let json = """
{
    "length": 120,
    "is_valyrian_steel": true,
    "sword_name": "Oathkeeper"
}
"""

let data = json.data(using: .utf8)
let sword = try? JSONDecoder().decode(Sword.self, from: data!)