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)