Codable / Encodable / Decodable

import Foundation

// A struct that conforms to Codable
struct User: Codable {
    let id: Int
    let name: String
    let email: String
    var isActive: Bool
}

// Encode: Swift → JSON
let user = User(id: 1, name: "Alice", email: "alice@example.com", isActive: true)

let encoder = JSONEncoder()
encoder.outputFormatting = .prettyPrinted  // Human-readable format

if let jsonData = try? encoder.encode(user),
   let jsonString = String(data: jsonData, encoding: .utf8) {
    print("JSON:")
    print(jsonString)
}

// Decode: JSON → Swift
let jsonString = """
{
    "id": 2,
    "name": "Bob",
    "email": "bob@example.com",
    "isActive": false
}
"""

let decoder = JSONDecoder()
if let data = jsonString.data(using: .utf8),
   let decodedUser = try? decoder.decode(User.self, from: data) {
    print("\nDecoded: \(decodedUser.name) (\(decodedUser.email))")
}

// CodingKeys: mapping between JSON key names and property names
struct Article: Codable {
    let articleId: Int
    let articleTitle: String

    enum CodingKeys: String, CodingKey {
        case articleId    = "article_id"    // snake_case → camelCase
        case articleTitle = "article_title"
    }
}

let articleJson = """
{"article_id": 10, "article_title": "Swift入門"}
"""

if let data = articleJson.data(using: .utf8),
   let article = try? decoder.decode(Article.self, from: data) {
    print("Article: \(article.articleTitle) (ID: \(article.articleId))")
}

If the property names in your type match the keys in the JSON, adopting Codable is all you need — encoding and decoding happen automatically. When customization is required, define a CodingKeys enum.

To map snake_case JSON keys to camelCase properties, you can either handle each key individually in CodingKeys, or use decoder.keyDecodingStrategy = .convertFromSnakeCase. If decoding fails, an exception is thrown. Use try? to get nil on failure, or use do-catch to retrieve detailed error information.

For the basics of protocols, see Protocol Basics / Conformance.