Using Swift Codable for JSON Encoding and Decoding: Features, Tips, and Advanced Scenarios

Swift 4 introduced the Codable protocol to replace NSCoding, allowing structs, enums, and classes to be easily serialized to and from JSON and other formats.

By conforming to Codable, a type automatically gets init(from:) and encode(to:) implementations, reducing boilerplate code.

Example of a simple Student struct and how to make it Codable:

struct Student: Codable {
    var id: String
    var name: String
    var grade: Int
}

Decoding JSON to a Swift type uses JSONDecoder :

let student = try JSONDecoder().decode(Student.self, from: jsonData)
print(student)

Encoding a Swift value back to JSON uses JSONEncoder :

let jsonData = try JSONEncoder().encode(student)
let jsonString = String(data: jsonData, encoding: .utf8)

For nested objects, arrays, and dictionaries, Codable works as long as the contained types also conform to Codable. Example of a Class containing an array of Student :

struct Class: Codable {
    var classNumber: String
    var students: [Student]
}

Optional properties handle missing or null values; making the property optional lets the decoder produce nil for empty objects or null.

class SourceFeed: Codable {
    var feedId: String?
    var title: String?
}

Key mismatches between JSON snake_case and Swift camelCase can be solved with JSONDecoder.keyDecodingStrategy or by defining a CodingKeys enum.

var decoder = JSONDecoder()
decoder.keyDecodingStrategy = .convertFromSnakeCase

private enum CodingKeys: String, CodingKey {
    case chatMessage = "chat_message"
}

Date values can be decoded by setting JSONDecoder.dateDecodingStrategy to .iso8601 , .secondsSince1970 , or a custom DateFormatter .

decoder.dateDecodingStrategy = .iso8601

Enums can conform to Codable by providing raw values, allowing JSON strings or integers to map directly to Swift enum cases.

enum FeedTemplate: String, Codable {
    case video = "video"
    case pic = "pic"
    case link = "link"
}

Dynamic payloads where only one of several possible sub‑objects is present require custom init(from:) and encode(to:) implementations, using decodeIfPresent for optional sub‑objects.

struct Feed: Codable {
    var template: FeedTemplate
    var videoFeed: VideoFeed?
    var picFeed: PicFeed?
    var linkFeed: LinkFeed?
    // custom init omitted for brevity
}

When a property’s type does not itself conform to Codable (e.g., CLLocationCoordinate2D ), you can encode/decode its constituent values manually or wrap it in a Codable struct.

struct Destination: Codable {
    var location: Coordinate
    var name: String
}
struct Coordinate: Codable {
    let latitude, longitude: Double
}

Overall, Codable dramatically simplifies JSON handling in Swift, reducing boilerplate and improving safety across a wide range of use‑cases.