Codable：JSONのエンコードとデコード

Codableプロトコル

struct User: Codable {
  var name: String
  var age: Int
}

自動的な実装合成

struct Product: Codable {
  var id: Int
  var title: String
  var price: Double
  // init(from:) and encode(to:) are auto-generated
}

JSONのデコード

let json = #"{"name":"Alice","age":30}"#.data(using: .utf8)!
let user = try JSONDecoder().decode(User.self, from: json)
print(user.name) // Alice

JSONへのエンコード

let user = User(name: "Bob", age: 25)
let data = try JSONEncoder().encode(user)
print(String(data: data, encoding: .utf8)!)

カスタム CodingKeys

struct Article: Codable {
  var createdAt: Date
  enum CodingKeys: String, CodingKey {
    case createdAt = "created_at"
  }
}

日付のデコード戦略

let decoder = JSONDecoder()
decoder.dateDecodingStrategy = .iso8601
let article = try decoder.decode(Article.self, from: data)

ネストされたコンテナ

struct Response: Codable {
  struct Data: Codable {
    var users: [User]
  }
  var data: Data
}
let resp = try JSONDecoder().decode(Response.self, from: json)

カスタムデコードロジック

struct Rating: Codable {
  var stars: Int
  init(from decoder: Decoder) throws {
    let container = try decoder.singleValueContainer()
    let raw = try container.decode(Double.self)
    stars = Int(raw.rounded())
  }
  func encode(to encoder: Encoder) throws {
    var container = encoder.singleValueContainer()
    try container.encode(Double(stars))
  }
}

keyDecodingStrategy

let decoder = JSONDecoder()
decoder.keyDecodingStrategy = .convertFromSnakeCase
let user = try decoder.decode(User.self, from: data)
// "first_name" → firstName

エンコードのオプション

let encoder = JSONEncoder()
encoder.outputFormatting = [.prettyPrinted, .sortedKeys]
let data = try encoder.encode(product)

Optional フィールドの処理

struct User: Codable {
  var name: String
  var bio: String?  // nil if "bio" key is absent from JSON
}

確認問題

レッスンのまとめ