[Swift] Codable class and inheritance

CodableInheritanceExample.swift

//
//  CodableInheritanceExample.swift
//
//  Created by 홍경표 on 2021/05/28.
//

class BaseResponse: Codable {
    var retCode: Int = 0
    
    private enum CodingKeys: String, CodingKey {
        case retCode
    }
    
    required init(from decoder: Decoder) throws {
        let container = try decoder.container(keyedBy: CodingKeys.self)
        retCode = try container.decode(Int.self, forKey: .retCode)
    }
    
    func encode(to encoder: Encoder) throws {
        var container = encoder.container(keyedBy: CodingKeys.self)
        try container.encode(retCode, forKey: .retCode)
    }
}

final class SpecialResponse: BaseResponse {
    var temp: Int?
    var message: String?
    
    private enum CodingKeys: String, CodingKey {
        case temp
        case message
    }
    
    required init(from decoder: Decoder) throws {
        let container = try decoder.container(keyedBy: CodingKeys.self)
        
        try super.init(from: decoder)
        
        temp = try? container.decode(Int?.self, forKey: .temp)
        message = try? container.decode(String?.self, forKey: .message)
    }
    
    override func encode(to encoder: Encoder) throws {
        var container = encoder.container(keyedBy: CodingKeys.self)
        
        try super.encode(to: encoder)
        
        try? container.encode(temp, forKey: .temp)
        try? container.encode(message, forKey: .message)
    }
}

// !! Need to use `try?`, not `try` at 39, 40 lines. !!
//
// Because `temp` and `message` are optional, so you don't know if they exist or not.
// If there is no `message`(or `temp) and if you use `try`(not `try?`)
//     ,then the decoder will recognize `message` as `Int with nil`.
// and you will get this error message.
/*
   DecodingError
     ▿ keyNotFound : 2 elements
       - .0 : CodingKeys(stringValue: "message", intValue: nil)
       ▿ .1 : Context
         - codingPath : 0 elements
         - debugDescription : "No value associated with key CodingKeys(stringValue: \"message\", intValue: nil) (\"message\")."
         - underlyingError : nil
*/

/*
let encoder = JSONEncoder()
let decoder = JSONDecoder()

let dict: [String: Int] = ["retCode": 1, "temp": 1000]
let data = try encoder.encode(dict)
debugPrint(String(data: data, encoding: .utf8) as Any)
let decoded = try decoder.decode(SpecialResponse.self, from: data)
dump(decoded)
*/