Swift4_JSON_04.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
    var numOfBestFriends: Int
    
    enum CodingKeys: String, CodingKey {
        case uid = "user_id"
        case userName = "user_name"
        case friends
    }
    
    enum FriendKeys: String, CodingKey {
        case uid = "user_id"
        case isBestFriend = "is_best_friend"
    }
    
    init(from decoder: Decoder) throws {
        let container = try decoder.container(keyedBy: CodingKeys.self)
        self.uid = try container.decode(Int.self, forKey: .uid)
        
        self.numOfBestFriends = 0
        
        var friendsContainer = try container.nestedUnkeyedContainer(forKey: .friends)
        while !friendsContainer.isAtEnd {
            let friendContainer = try friendsContainer.nestedContainer(keyedBy: FriendKeys.self)
            let isBestFriend = try friendContainer.decode(Bool.self, forKey: .isBestFriend)
            if isBestFriend {
                self.numOfBestFriends += 1
            }
        }
    }
    
    func encode(to encoder: Encoder) throws {
        // do something
    }
}

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