Swift Codable

Encode and decode Swift types using Codable (Encodable & Decodable) with JSONEncoder, JSONDecoder, and related APIs. Targets Swift 6.3 / iOS 26+.

Contents

• Basic Conformance
• Custom CodingKeys
• Custom Decoding and Encoding
• Nested and Flattened Containers
• Heterogeneous Arrays
• Date Decoding Strategies
• Data and Key Strategies
• Lossy Array Decoding
• Single Value Containers
• Default Values for Missing Keys
• Encoder and Decoder Configuration
• Codable with URLSession
• Codable with SwiftData
• Codable with UserDefaults
• Common Mistakes
• Review Checklist
• References

Basic Conformance

When all stored properties are themselves Codable, the compiler synthesizes conformance automatically:

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

let user = try JSONDecoder().decode(User.self, from: jsonData)
let encoded = try JSONEncoder().encode(user)

Prefer Decodable for read-only API responses and Encodable for write-only. Use Codable only when both directions are required.

Custom CodingKeys

Rename JSON keys without writing a custom decoder by declaring a CodingKeys enum:

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"
    }
}

Every stored property must appear in the enum. Omitting a property from CodingKeys excludes it from encoding/decoding -- provide a default value or compute it separately.

Custom Decoding and Encoding

Override init(from:) and encode(to:) for transformations the synthesized conformance cannot handle:

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)
    }
}

Nested and Flattened Containers

Use nestedContainer(keyedBy:forKey:) to navigate and flatten nested JSON:

// 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)
    }
}

Chain multiple nestedContainer calls to flatten deeply nested structures. Also use nestedUnkeyedContainer(forKey:) for nested arrays.

Heterogeneous Arrays

Decode arrays of mixed types using a discriminator field:

// JSON: [{"type":"text","content":"Hello"},{"type":"image","url":"pic.jpg"}]
enum ContentBlock: Decodable {
    case text(String)
    case image(URL)

    enum CodingKeys: String, CodingKey { case type, content, url }

    init(from decoder: Decoder) throws {
        let container = try decoder.container(keyedBy: CodingKeys.self)
        let type = try container.decode(String.self, forKey: .type)
        switch type {
        case "text":
            let content = try container.decode(String