Working with JSON in Swift: Parsing and Serialization for iOS and macOS

Working with JSON in Swift: Parsing and Serialization

JSON (JavaScript Object Notation) is a lightweight data interchange format widely used in web services and mobile applications. In the Apple ecosystem (iOS, macOS, watchOS, tvOS), working with JSON is an integral part of development: from loading data from a server to saving application settings. In this article, we will take a detailed look at how to perform parsing (decoding) and serialization (encoding) of JSON in Swift using the built-in Codable, JSONDecoder, and JSONEncoder protocols.

1. Codable Basics: Data Model

The modern approach to working with JSON in Swift is based on the Codable protocol, which combines two protocols: Encodable (for serialization) and Decodable (for parsing). For your structure or class to be convertible to and from JSON, simply declare it as Codable.

Consider an example of a user model:

struct User: Codable {    let id: Int    let name: String    let email: String    let isActive: Bool}

If the keys in JSON differ from the property names in Swift (for example, snake_case is used instead of camelCase), you can use the CodingKeys enum:

struct User: Codable {    let id: Int    let fullName: String    let email: String    let isActive: Bool

    enum CodingKeys: String, CodingKey {        case id        case fullName = "full_name"        case email        case isActive = "is_active"    }}

2. Parsing JSON with JSONDecoder

To convert a JSON string or data into Swift objects, JSONDecoder is used. The main method is decode(_:from:), which takes a data type (conforming to Decodable) and a Data object.

Example of parsing a JSON string received from an API:

let jsonString = """{    "id": 1,    "name": "Ivan Petrov",    "email": "ivan@example.com",    "is_active": true}"""

let jsonData = Data(jsonString.utf8)

do {    let decoder = JSONDecoder()    let user = try decoder.decode(User.self, from: jsonData)    print("User: \\(user.name), email: \\(user.email)")} catch {    print("Decoding error: \\(error.localizedDescription)")}

Often JSON comes as an array of objects. In this case, simply specify the type [User].self:

let jsonArrayString = """[    {"id": 1, "name": "Ivan", "email": "ivan@test.com", "is_active": true},    {"id": 2, "name": "Maria", "email": "maria@test.com", "is_active": false}]"""

let jsonArrayData = Data(jsonArrayString.utf8)

do {    let users = try JSONDecoder().decode([User].self, from: jsonArrayData)    users.forEach { print($0.name) }} catch {    print("Error: \\(error)")}

3. Serializing Objects to JSON with JSONEncoder

The reverse process — converting a Swift object to JSON — is performed using JSONEncoder. The encode(_:) method returns a Data object, which can then be converted to a string or sent to a server.

let user = User(id: 3, fullName: "Alexey Smirnov", email: "alex@example.com", isActive: true)

do {    let encoder = JSONEncoder()    encoder.outputFormatting = .prettyPrinted // for readable formatting    let jsonData = try encoder.encode(user)    if let jsonString = String(data: jsonData, encoding: .utf8) {        print(jsonString)    }} catch {    print("Encoding error: \\(error)")}

The output will look like this:

{  "id" : 3,  "full_name" : "Alexey Smirnov",  "email" : "alex@example.com",  "is_active" : true}

Note that JSO