Swift4_JSON_03.swift

import Foundation

let json = """
{
    "user_id": 1,
    "user_name": "magicien",
    "friends": [
        {
            "user_id": 2,
            "is_best_friend": true
        }
    ]
}
""".data(using: .utf8)!

struct UserInfo: Codable {
    let uid: Int
    let userName: String
    let url: URL?
    var counter: Int = 0
    
    enum CodingKeys: String, CodingKey {
        case uid = "user_id"
        case userName = "user_name"
        case url
    }
}

let decoder = JSONDecoder()
do {
    let userInfo = try decoder.decode(UserInfo.self, from: json)
    print(userInfo)
} catch DecodingError.keyNotFound(let key, let context) {
    print("keyNotFound: \(key): \(context)")
} catch {
    print("\(error.localizedDescription)")
}