Example about Codable protocol in Swift 4

codable.swift

import Foundation

//
// MARK: - Venue struct representation
//

/**
    Different venue types.
*/
public enum Kind: String, Codable
{
    case library
    case shopping
    case museum
    case cinema
}

/**
    And address representation
*/
public struct Address: Codable
{
	public var street: String
	public var number: Int?
	public var city: String
	
    /**
        Set the JSON key values.

        In `Address` json representation 
        `street` and `city` keys are named 
        in a different way.
    */
	private enum CodingKeys: String, CodingKey
	{
	    case street = "street_name"
	    case city = "city_name"
	    case number
	}
}

/**
    A venue
*/
public struct Venue: Codable
{
	public var name: String
	public var address: Address
	public var kind: Kind
}

//
// MARK: - Encode
//

//
// Let's build Madrid Apple Store
//

let address: Address = Address(street: "Plaza de la Puerta del Sol", number: 1, city: "Madrid")
let appleStore: Venue = Venue(name: "Apple Store Madrid Flagship", address: address, kind: .shopping)

let encoder: JSONEncoder = JSONEncoder()
encoder.outputFormatting = .prettyPrinted // output looks pretty

if let data = try? encoder.encode(appleStore), let json = String(data: data, encoding: .utf8)
{
    print(json)
}

//
// MARK: - Decode
//

// 
// Now simulate a HTTP request...
//

var jsonLibrary: String = """
{
  "kind": "library",
  "address": {
    "city_name": "Madrid",
    "street_name": "Calle de San Justo",
    "number": 5
  },
  "name": "Biblioteca Pública Municipal Iván de Vargas"
}
"""

let decoder: JSONDecoder = JSONDecoder()

if let data = jsonLibrary.data(using: .utf8), let venue = try? decoder.decode(Venue.self, from: data)
{
    dump(venue)
}