swift-codable

# 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](#decode-and-verify-workflow)
- [Basic Conformance](#basic-conformance)
- [Custom CodingKeys](#custom-codingkeys)
- [Custom Decoding and Encoding](#custom-decoding-and-encoding)
- [Nested and Flattened Containers](#nested-and-flattened-containers)
- [Heterogeneous Arrays](#heterogeneous-arrays)
- [Date Decoding Strategies](#date-decoding-strategies)
- [Data and Key Strategies](#data-and-key-strategies)
- [Lossy Array Decoding](#lossy-array-decoding)
- [Single Value Containers](#single-value-containers)
- [Default Values for Missing Keys](#default-values-for-missing-keys)
- [Encoder and Decoder Configuration](#encoder-and-decoder-configuration)
- [Codable with URLSession](#codable-with-urlsession)
- [Codable with SwiftData](#codable-with-swiftdata)
- [Codable with UserDefaults](#codable-with-userdefaults)
- [Common Mistakes](#common-mistakes)
- [Review Checklist](#review-checklist)
- [References](#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:

```swift
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:

```swift
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:

```swift
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:

```swift
// 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](references/codable-advanced-patterns.md#heterogeneous-arrays)
for discriminator-based mixed arrays.

## Date Decoding Strategies

Configure `JSONDecoder.dateDecodingStrategy` to match your API:

```swift
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 S