swift-codable

name	swift-codable
description	Implement Swift Codable models for JSON and property-list encoding and decoding with JSONDecoder, JSONEncoder, CodingKeys, and custom init(from:) or encode(to:). Use when parsing API responses, remapping keys, flattening nested JSON, handling date or data decoding strategies, decoding heterogeneous arrays, or integrating Codable with URLSession, SwiftData, or UserDefaults.

Swift Codable

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

Contents

• Decode and Verify Workflow
• 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

Decode and Verify Workflow

1. Decode representative success, missing, null, malformed, acronym-key, and date fixtures.
2. On failure, inspect DecodingError, its codingPath, and the raw payload.
3. Correct only the mismatched model, key, container, or strategy; do not hide contract failures with lossy decoding.
4. Rerun fixtures and encode/decode round trips where both directions are part of the contract.

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: .)
         container.encode(name, forKey: .name)
         container.encode(timestamp.timeIntervalSince1970, forKey: .timestamp)
         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

Load Advanced Codable Patterns for discriminator-based mixed arrays.

Date Decoding Strategies

Configure JSONDecoder.dateDecodingStrategy to match your API:

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)")
}

Set the matching strategy on JSONEncoder: encoder.dateEncodingStrategy = .iso8601

Data and Key Strategies

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

Use key strategies only for mechanical snake_case-to-camelCase mappings. convertFromSnakeCase maps by spelling, not Swift acronym/initialism policy: image_url, base_uri, and user_id match imageUrl, baseUri, and userId only. If the Swift model uses imageURL, baseURI, or userID, declare explicit CodingKeys; the strategy will not synthesize those names.

Lossy Array Decoding

Use lossy arrays only when partial success is part of the product contract; load Lossy Arrays.

Single Value Containers

Use singleValueContainer() for type-safe primitive wrappers; see Single-Value Wrappers.

Default Values for Missing Keys

Stored defaults do not make synthesized decoding tolerate missing nonoptional keys. Load Missing-Key Defaults when the contract assigns explicit fallback behavior to missing or null values.

Encoder and Decoder Configuration

Keep matching strategies at the transport/file-format boundary. Load Encoder Configuration for nonconforming floats and property-list guidance.

Codable with URLSession

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:  {  page: ;  totalPages:  }
}

 ( : )  -> [] {
     decoder  ()
    decoder.keyDecodingStrategy  .convertFromSnakeCase
    decoder.dateDecodingStrategy  .iso8601
      decoder.decode(<[]>., from: data).data
}

Codable with SwiftData

Keep schema values typed and route persistence design to swiftdata; see Persistence Boundaries.

Codable with UserDefaults

Use primitives for small preferences. Load Persistence Boundaries for a small Codable RawRepresentable/@AppStorage handoff; use a real persistence layer for larger or durable data.

Common Mistakes

1. Not handling missing defaulted fields:

// 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) ?? ""

2. Failing entire array when one element is invalid:

// 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

3. Date strategy mismatch:

// 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

4. Force-unwrapping decoded optionals:

// 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 }

5. Using Codable when only Decodable is needed:

// 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 }

6. Manual CodingKeys for simple snake_case APIs:

// 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.

Review Checklist

• Types conform to Decodable only when encoding is not needed
• decodeIfPresent used with defaults for optional or missing keys
• keyDecodingStrategy = .convertFromSnakeCase used for simple snake_case APIs, with CodingKeys retained for acronym spellings
• dateDecodingStrategy matches the API date format
• Arrays of unreliable data use lossy decoding to skip invalid elements
• Custom init(from:) validates and transforms data instead of post-decode fixups
• JSONEncoder.outputFormatting includes .sortedKeys for deterministic test output
• Wrapper types (UserID, etc.) use singleValueContainer for clean JSON
• Generic APIResponse<T> wrapper used for consistent API envelope handling
• No force-unwrapping of decoded values
• Persistence boundary is explicit: SwiftData only for compatible noncomputed model properties, @AppStorage/UserDefaults only for small primitive or RawRepresentable preferences

References

• Advanced Codable patterns -- mixed arrays, lossy decoding, wrappers, defaults, configuration, and persistence boundaries
• Codable -- protocol combining Encodable and Decodable
• JSONDecoder -- decodes JSON data into Codable types
• JSONEncoder -- encodes Codable types as JSON data
• CodingKey -- protocol for encoding/decoding keys
• JSONDecoder.KeyDecodingStrategy.convertFromSnakeCase -- snake-case conversion behavior and limitations
• Encoding and Decoding Custom Types -- Apple guide on custom Codable conformance
• Using JSON with Custom Types -- Apple sample code for JSON patterns
• Preserving your app's model data across launches -- SwiftData model property compatibility