swift-codable

struct Product: Codable {
    let id: Int
    let displayName: String
    let imageURL: URL
    let priceInCents: Int
 
    enum CodingKeys: String, CodingKey {
        case id
        case displayName = "display_name"
        case imageURL = "image_url"
        case priceInCents = "price_in_cents"
    }
}

struct Event: Codable {
    let name: String
    let timestamp: Date
    let tags: [String]
 
    enum CodingKeys: String, CodingKey {
        case name, timestamp, tags
    }
 
    init(from decoder: Decoder) throws {
        let container = try decoder.container(keyedBy: CodingKeys.self)
        name = try container.decode(String.self, forKey: .name)
        // Decode Unix timestamp as Double, convert to Date
        let epoch = try container.decode(Double.self, forKey: .timestamp)
        timestamp = Date(timeIntervalSince1970: epoch)
        // Default to empty array when key is missing
        tags = try container.decodeIfPresent([String].self, forKey: .tags) ?? []
    }
 
    func encode(to encoder: Encoder) throws {
        var container = encoder.container(keyedBy: CodingKeys.self)
        try container.encode(name, forKey: .name)
        try container.encode(timestamp.timeIntervalSince1970, forKey: .timestamp)
        try container.encode(tags, forKey: .tags)
    }
}

// JSON: { "id": 1, "location": { "lat": 37.7749, "lng": -122.4194 } }
struct Place: Decodable {
    let id: Int
    let latitude: Double
    let longitude: Double
 
    enum CodingKeys: String, CodingKey { case id, location }
    enum LocationKeys: String, CodingKey { case lat, lng }
 
    init(from decoder: Decoder) throws {
        let container = try decoder.container(keyedBy: CodingKeys.self)
        id = try container.decode(Int.self, forKey: .id)
        let location = try container.nestedContainer(
            keyedBy: LocationKeys.self, forKey: .location)
        latitude = try location.decode(Double.self, forKey: .lat)
        longitude = try location.decode(Double.self, forKey: .lng)
    }
}

let decoder = JSONDecoder()
 
// ISO 8601 (e.g., "2024-03-15T10:30:00Z")
decoder.dateDecodingStrategy = .iso8601
 
// Unix timestamp in seconds (e.g., 1710499800)
decoder.dateDecodingStrategy = .secondsSince1970
 
// Custom DateFormatter
let formatter = DateFormatter()
formatter.dateFormat = "yyyy-MM-dd"
formatter.locale = Locale(identifier: "en_US_POSIX")
formatter.timeZone = TimeZone(secondsFromGMT: 0)
decoder.dateDecodingStrategy = .formatted(formatter)
 
// Custom closure for multiple formats
decoder.dateDecodingStrategy = .custom { decoder in
    let container = try decoder.singleValueContainer()
    let string = try container.decode(String.self)
    if let date = ISO8601DateFormatter().date(from: string) { return date }
    throw DecodingError.dataCorruptedError(
        in: container, debugDescription: "Cannot decode date: \(string)")
}

let decoder = JSONDecoder()
decoder.dataDecodingStrategy = .base64           // Base64-encoded Data fields
decoder.keyDecodingStrategy = .convertFromSnakeCase  // simple keys only; not URL/ID spelling
// {"user_name": "Alice"} maps to `var userName: String` -- no CodingKeys needed
 
let encoder = JSONEncoder()
encoder.dataEncodingStrategy = .base64
encoder.keyEncodingStrategy = .convertToSnakeCase

func fetchUser(id: Int) async throws -> User {
    let url = URL(string: "https://api.example.com/users/\(id)")!
    let (data, response) = try await URLSession.shared.data(from: url)
    guard let http = response as? HTTPURLResponse,
          (200...299).contains(http.statusCode) else {
        throw APIError.invalidResponse
    }
    let decoder = JSONDecoder()
    decoder.keyDecodingStrategy = .convertFromSnakeCase
    decoder.dateDecodingStrategy = .iso8601
    return try decoder.decode(User.self, from: data)
}
 
// Generic API envelope. Configure a decoder inside this helper because
// fetchUser's decoder is out of scope.
struct APIResponse<T: Decodable>: Decodable {
    let data: T
    let meta: Meta?
    struct Meta: Decodable { let page: Int; let totalPages: Int }
}
 
func decodeUsersEnvelope(from data: Data) throws -> [User] {
    let decoder = JSONDecoder()
    decoder.keyDecodingStrategy = .convertFromSnakeCase
    decoder.dateDecodingStrategy = .iso8601
    return try decoder.decode(APIResponse<[User]>.self, from: data).data
}

// DON'T -- crashes if key is absent
let value = try container.decode(String.self, forKey: .bio)
// DO -- falls back when the key is absent or null
let value = try container.decodeIfPresent(String.self, forKey: .bio) ?? ""

// DON'T -- one bad element kills the whole decode
let items = try container.decode([Item].self, forKey: .items)
// DO -- decode elements individually only when partial success is allowed

// DON'T -- default strategy expects Double, but API sends ISO string
let decoder = JSONDecoder()  // dateDecodingStrategy defaults to .deferredToDate
// DO -- set strategy to match your API format
decoder.dateDecodingStrategy = .iso8601

// DON'T
let user = try? decoder.decode(User.self, from: data)
print(user!.name)
// DO
guard let user = try? decoder.decode(User.self, from: data) else { return }

// DON'T -- unnecessarily constrains the type to also be Encodable
struct APIResponse: Codable { let id: Int; let message: String }
// DO -- use Decodable for read-only API responses
struct APIResponse: Decodable { let id: Int; let message: String }

// DON'T -- verbose boilerplate for every model
enum CodingKeys: String, CodingKey {
    case userName = "user_name"
    case avatarUrl = "avatar_url"
}
// DO -- configure once on the decoder for simple cases
decoder.keyDecodingStrategy = .convertFromSnakeCase
// Keep CodingKeys for `imageURL`, `baseURI`, `userID`, and similar names.