Agent Skills: Swift Codable Patterns

Swift Codable Patterns

Comprehensive guide to Codable protocol conformance for JSON and PropertyList encoding/decoding in Swift 6.x.

Quick Reference

Decision Tree: When to Use Each Approach

Has your type...
├─ All properties Codable? → Automatic synthesis (just add `: Codable`)
├─ Property names differ from JSON keys? → CodingKeys customization
├─ Needs to exclude properties? → CodingKeys customization
├─ Enum with associated values? → Check enum synthesis patterns
├─ Needs structural transformation? → Manual implementation + bridge types
├─ Needs data not in JSON? → DecodableWithConfiguration (iOS 15+)
└─ Complex nested JSON? → Manual implementation + nested containers

Common Triggers

| Error | Solution | |-------|----------| | "Type 'X' does not conform to protocol 'Decodable'" | Ensure all stored properties are Codable | | "No value associated with key X" | Check CodingKeys match JSON keys | | "Expected to decode X but found Y instead" | Type mismatch; check JSON structure or use bridge type | | "keyNotFound" | JSON missing expected key; make property optional or provide default | | "Date parsing failed" | Configure dateDecodingStrategy on decoder |

Part 1: Automatic Synthesis

Swift automatically synthesizes Codable conformance when all stored properties are Codable.

Struct Synthesis

// ✅ Automatic synthesis
struct User: Codable {
    let id: UUID              // Codable
    var name: String          // Codable
    var membershipPoints: Int // Codable
}

// JSON: {"id":"...", "name":"Alice", "membershipPoints":100}

Requirements:

• All stored properties must conform to Codable
• Properties use standard Swift types or other Codable types
• No custom initialization logic needed

Enum Synthesis Patterns

Pattern 1: Raw Value Enums

enum Direction: String, Codable {
    case north, south, east, west
}

// Encodes as: "north"

The raw value itself becomes the JSON representation.

Pattern 2: Enums Without Associated Values

enum Status: Codable {
    case success
    case failure
    case pending
}

// Encodes as: {"success":{}}

Each case becomes an object with the case name as the key and empty dictionary as value.

Pattern 3: Enums With Associated Values

enum APIResult: Codable {
    case success(data: String, count: Int)
    case error(code: Int, message: String)
}

// success case encodes as:
// {"success":{"data":"example","count":5}}

Gotcha: Unlabeled associated values generate _0, _1 keys:

enum Command: Codable {
    case store(String, Int)  // ❌ Unlabeled
}

// Encodes as: {"store":{"_0":"value","_1":42}}

Fix: Always label associated values for predictable JSON:

enum Command: Codable {
    case store(key: String, value: Int)  // ✅ Labeled
}

// Encodes as: {"store":{"key":"value","value":42}}

When Synthesis Breaks

Automatic synthesis fails when:

1. Computed properties - Only stored properties are encoded
2. Non-Codable properties - Custom types without Codable conformance
3. Property wrappers - @Published, @State (except @AppStorage with Codable types)
4. Class inheritance - Subclasses must implement init(from:) manually

Part 2: CodingKeys Customization

Use CodingKeys enum to customize encoding/decoding without full manual implementation.

Renaming Keys

struct Article: Codable {
    let url: URL
    let title: String
    let body: String

    enum CodingKeys: String, CodingKey {
        case url = "source_link"      // JSON uses "source_link"
        case title = "content_name"   // JSON uses "content_name"
        case body                     // Matches JSON key
    }
}

// JSON: {"source_link":"...", "content_name":"...", "body":"..."}

Excluding Properties

Omit properties from CodingKeys to exclude them from encoding/decoding:

struct NoteCollection: Codable {
    let name: String
    let notes: [Note]
    var localDrafts: [Note] = []  // ✅ Must have default value

    enum CodingKeys: CodingKey {
        case name
        case notes
        // localDrafts omitted - not encoded/decoded
    }
}

Rule: Excluded properties require default values or you must implement init(from:) manually.

Snake Case Conversion

For consistent snake_case → camelCase conversion:

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

// JSON: {"first_name":"Alice", "last_name":"Smith"}
// Decodes to: User(firstName: "Alice", lastName: "Smith")

Enum Associated Value Keys

Customize keys for enum associated values using {CaseName}CodingKeys:

enum Command: Codable {
    case store(key: String, value: Int)
    case delete(key: String)

    enum StoreCodingKeys: String, CodingKey {
        case key = "identifier"  // Renames "key" to "identifier"
        case value = "data"      // Renames "value" to "data"
    }

    enum DeleteCodingKeys: String, CodingKey {
        case key = "identifier"
    }
}

// store case encodes as: {"store":{"identifier":"x","data":42}}

Pattern: {CaseName}CodingKeys with capitalized case name.

Part 3: Manual Implementation

For structural differences between JSON and Swift models, implement init(from:) and encode(to:).

Container Types

| Container | When to Use | |-----------|-------------| | Keyed | Dictionary-like data with string keys | | Unkeyed | Array-like sequential data | | Single-value | Wrapper types that encode as a single value | | Nested | Hierarchical JSON structures |

Nested Containers Example

Flatten hierarchical JSON:

// JSON:
// {
//   "latitude": 37.7749,
//   "longitude": -122.4194,
//   "additionalInfo": {
//     "elevation": 52
//   }
// }

struct Coordinate {
    var latitude: Double
    var longitude: Double
    var elevation: Double  // Nested in JSON, flat in Swift

    enum CodingKeys: String, CodingKey {
        case latitude, longitude, additionalInfo
    }

    enum AdditionalInfoKeys: String, CodingKey {
        case elevation
    }
}

extension Coordinate: Decodable {
    init(from decoder: Decoder) throws {
        let values = try decoder.container(keyedBy: CodingKeys.self)
        latitude = try values.decode(Double.self, forKey: .latitude)
        longitude = try values.decode(Double.self, forKey: .longitude)

        let additionalInfo = try values.nestedContainer(
            keyedBy: AdditionalInfoKeys.self,
            forKey: .additionalInfo
        )
        elevation = try additionalInfo.decode(Double.self, forKey: .elevation)
    }
}

extension Coordinate: Encodable {
    func encode(to encoder: Encoder) throws {
        var container = encoder.container(keyedBy: CodingKeys.self)
        try container.encode(latitude, forKey: .latitude)
        try container.encode(longitude, forKey: .longitude)

        var additionalInfo = container.nestedContainer(
            keyedBy: AdditionalInfoKeys.self,
            forKey: .additionalInfo
        )
        try additionalInfo.encode(elevation, forKey: .elevation)
    }
}

Bridge Types for Structural Mismatches

When JSON structure fundamentally differs from Swift model:

// JSON: {"USD": 1.0, "EUR": 0.85, "GBP": 0.73}
// Want: [ExchangeRate]

struct ExchangeRate {
    let currency: String
    let rate: Double
}

// Bridge type for decoding
private extension ExchangeRate {
    struct List: Decodable {
        let values: [ExchangeRate]

        init(from decoder: Decoder) throws {
            let container = try decoder.singleValueContainer()
            let dictionary = try container.decode([String: Double].self)
            values = dictionary.map { ExchangeRate(currency: $0, rate: $1) }
        }
    }
}

// Public interface
extension ExchangeRate {
    static func decode(from data: Data) throws -> [ExchangeRate] {
        let list = try JSONDecoder().decode(List.self, from: data)
        return list.values
    }
}

Part 4: Date Handling

Built-in Strategies

let decoder = JSONDecoder()

// 1. ISO 8601 (recommended)
decoder.dateDecodingStrategy = .iso8601
// Expects: "2024-02-15T17:00:00+01:00"

// 2. Unix timestamp (seconds)
decoder.dateDecodingStrategy = .secondsSince1970
// Expects: 1708012800

// 3. Unix timestamp (milliseconds)
decoder.dateDecodingStrategy = .millisecondsSince1970
// Expects: 1708012800000

// 4. Custom formatter
let formatter = DateFormatter()
formatter.dateFormat = "yyyy-MM-dd"
formatter.locale = Locale(identifier: "en_US_POSIX")  // ✅ Always set
formatter.timeZone = TimeZone(secondsFromGMT: 0)      // ✅ Always set
decoder.dateDecodingStrategy = .formatted(formatter)

// 5. Custom closure
decoder.dateDecodingStrategy = .custom { decoder in
    let container = try decoder.singleValueContainer()
    let dateString = try container.decode(String.self)

    if let date = ISO8601DateFormatter().date(from: dateString) {
        return date
    }

    throw DecodingError.dataCorruptedError(
        in: container,
        debugDescription: "Cannot decode date string \(dateString)"
    )
}

ISO 8601 Nuances

Default: 2024-02-15T17:00:00+01:00 Timezone required: Without timezone offset, decoding may fail across regions

// ❌ No timezone - parsing depends on device locale
"2024-02-15T17:00:00"

// ✅ With timezone - unambiguous
"2024-02-15T17:00:00+01:00"

Performance Consideration

Custom closures run for every date - optimize expensive operations:

// ❌ Creates new formatter for every date
decoder.dateDecodingStrategy = .custom { decoder in
    let formatter = DateFormatter()  // Expensive!
    // ...
}

// ✅ Reuse formatter
let sharedFormatter = DateFormatter()
sharedFormatter.dateFormat = "yyyy-MM-dd"

decoder.dateDecodingStrategy = .custom { decoder in
    // Use sharedFormatter
}

Part 5: Type Transformation

StringBacked Wrapper

Handle APIs that encode numbers as strings:

protocol StringRepresentable: CustomStringConvertible {
    init?(_ string: String)
}

extension Int: StringRepresentable {}
extension Double: StringRepresentable {}

struct StringBacked<Value: StringRepresentable>: Codable {
    var value: Value

    init(from decoder: Decoder) throws {
        let container = try decoder.singleValueContainer()
        let string = try container.decode(String.self)

        guard let value = Value(string) else {
            throw DecodingError.dataCorruptedError(
                in: container,
                debugDescription: "Cannot convert '\(string)' to \(Value.self)"
            )
        }

        self.value = value
    }

    func encode(to encoder: Encoder) throws {
        var container = encoder.singleValueContainer()
        try container.encode(value.description)
    }
}

// Usage
struct Product: Codable {
    let name: String
    private let _price: StringBacked<Double>

    var price: Double {
        get { _price.value }
        set { _price = StringBacked(value: newValue) }
    }

    enum CodingKeys: String, CodingKey {
        case name
        case _price = "price"
    }
}

// JSON: {"name":"Widget","price":"19.99"}
// Decodes to: Product(name: "Widget", price: 19.99)

Type Coercion

For loosely typed APIs that may return different types:

struct FlexibleValue: Codable {
    let stringValue: String

    init(from decoder: Decoder) throws {
        let container = try decoder.singleValueContainer()

        if let string = try? container.decode(String.self) {
            stringValue = string
        } else if let int = try? container.decode(Int.self) {
            stringValue = String(int)
        } else if let double = try? container.decode(Double.self) {
            stringValue = String(double)
        } else {
            throw DecodingError.dataCorruptedError(
                in: container,
                debugDescription: "Cannot decode value to String, Int, or Double"
            )
        }
    }
}

Warning: Avoid this pattern unless the API is truly unpredictable. Prefer strict types.

Part 6: Advanced Patterns

DecodableWithConfiguration (iOS 15+)

For types that need data unavailable in JSON:

struct User: Encodable, DecodableWithConfiguration {
    let id: UUID
    var name: String
    var favorites: Favorites  // Not in JSON, injected via configuration

    enum CodingKeys: CodingKey {
        case id, name
    }

    init(from decoder: Decoder, configuration: Favorites) throws {
        let container = try decoder.container(keyedBy: CodingKeys.self)
        id = try container.decode(UUID.self, forKey: .id)
        name = try container.decode(String.self, forKey: .name)
        favorites = configuration  // Injected
    }
}

// Usage (iOS 17+)
let favorites = try await fetchFavorites()
let user = try JSONDecoder().decode(
    User.self,
    from: data,
    configuration: favorites
)

userInfo Workaround (iOS 15-16)

extension JSONDecoder {
    private struct ConfigurationDecodingWrapper<T: DecodableWithConfiguration>: Decodable {
        var wrapped: T

        init(from decoder: Decoder) throws {
            let config = decoder.userInfo[configurationUserInfoKey] as! T.DecodingConfiguration
            wrapped = try T(from: decoder, configuration: config)
        }
    }

    func decode<T: DecodableWithConfiguration>(
        _ type: T.Type,
        from data: Data,
        configuration: T.DecodingConfiguration
    ) throws -> T {
        let decoder = JSONDecoder()
        decoder.userInfo[Self.configurationUserInfoKey] = configuration
        let wrapper = try decoder.decode(ConfigurationDecodingWrapper<T>.self, from: data)
        return wrapper.wrapped
    }
}

private let configurationUserInfoKey = CodingUserInfoKey(rawValue: "configuration")!

Partial Decoding

Decode only the fields you need:

struct ArticlePreview: Decodable {
    let id: UUID
    let title: String
    // Omit body, comments, etc.
}

// JSON has many more fields, but we only decode id and title

Part 7: Debugging

DecodingError Cases

do {
    let user = try decoder.decode(User.self, from: data)
} catch DecodingError.keyNotFound(let key, let context) {
    print("Missing key '\(key)' at path: \(context.codingPath)")
} catch DecodingError.typeMismatch(let type, let context) {
    print("Type mismatch for \(type) at path: \(context.codingPath)")
} catch DecodingError.valueNotFound(let type, let context) {
    print("Value not found for \(type) at path: \(context.codingPath)")
} catch DecodingError.dataCorrupted(let context) {
    print("Data corrupted at path: \(context.codingPath)")
} catch {
    print("Other error: \(error)")
}

Debugging Techniques

1. Pretty-print JSON

let encoder = JSONEncoder()
encoder.outputFormatting = [.prettyPrinted, .sortedKeys]
let jsonData = try encoder.encode(user)
print(String(data: jsonData, encoding: .utf8)!)

2. Inspect coding path

// In custom init(from:)
print("Decoding at path: \(decoder.codingPath)")

3. Validate JSON structure

// Quick check: Can it decode as Any?
let json = try JSONSerialization.jsonObject(with: data)
print(json)  // See actual structure

Anti-Patterns

| Anti-Pattern | Cost | Better Approach | |--------------|------|-----------------| | Manual JSON string building | Injection vulnerabilities, escaping bugs, no type safety | Use JSONEncoder | | try? swallowing DecodingError | Silent failures, debugging nightmares, data loss | Handle specific error cases | | Optional properties to avoid decode errors | Runtime crashes, nil checks everywhere, masks structural issues | Fix JSON/model mismatch or use DecodableWithConfiguration | | Duplicating partial models | 2-5 hours maintenance per change, sync issues, fragile | Use bridge types or configuration | | Ignoring date timezone | Intermittent bugs across regions, data corruption | Always use ISO8601 with timezone or explicit UTC | | JSONSerialization for Codable types | 3x more boilerplate, manual type casting, error-prone | Use JSONDecoder/JSONEncoder | | No locale on DateFormatter | Parsing fails in non-US locales | Set locale = Locale(identifier: "en_US_POSIX") |

Why try? is Dangerous

// ❌ Silent failure - production bug waiting to happen
let user = try? JSONDecoder().decode(User.self, from: data)
// If this fails, user is nil - why? No idea.

// ✅ Explicit error handling
do {
    let user = try JSONDecoder().decode(User.self, from: data)
} catch {
    logger.error("Failed to decode user: \(error)")
    // Now you know WHY it failed
}

Pressure Scenarios

Scenario 1: "Just Use try? to Make It Compile"

Context: API integration deadline tomorrow, decoder failing on some edge case.

Pressure: "We can debug it later, just make it work now."

Why You'll Rationalize:

• "It's only failing on 1% of requests"
• "We can add logging later"
• "Customers won't notice"

What Actually Happens:

• Silent data loss for that 1%
• No logs, so you can't debug in production
• Customer complaints 3 months later
• You've forgotten the context by then

Discipline Response:

"Using try? here means we'll lose data silently. Let me spend 5 minutes handling the specific error case. If it's truly rare, I'll log it so we can fix the root cause."

5-Minute Fix:

do {
    return try decoder.decode(User.self, from: data)
} catch DecodingError.keyNotFound(let key, let context) {
    logger.error("Missing key '\(key)' in API response", metadata: [
        "path": .string(context.codingPath.description),
        "rawJSON": .string(String(data: data, encoding: .utf8) ?? "")
    ])
    throw APIError.invalidResponse(reason: "Missing key: \(key)")
} catch {
    logger.error("Failed to decode User", error: error)
    throw APIError.decodingFailed(error)
}

Result: You discover the API sometimes omits the email field for deleted users. Fix: make email optional only for that case, not all users.

Scenario 2: "Dates Are Intermittent, Must Be Server Bug"

Context: Date parsing works in your timezone but fails for European QA team.

Pressure: "It works for me, QA must be doing something wrong."

Why You'll Rationalize:

• "My tests pass locally"
• "The server is probably sending bad data"
• "It's their device settings"

What Actually Happens:

• Server sends dates without timezone: "2024-12-14T10:00:00"
• Your device (PST) interprets as 10:00 PST
• QA device (CET) interprets as 10:00 CET
• Different absolute times, intermittent bugs

Discipline Response:

"Intermittent date failures are almost always timezone issues. Let me check if we're using ISO8601 with timezone offsets."

Check:

// ❌ Current (fails across timezones)
decoder.dateDecodingStrategy = .iso8601

// Server sends: "2024-12-14T10:00:00" (no timezone)
// PST device: Dec 14, 10:00 PST
// CET device: Dec 14, 10:00 CET
// Bug: Different times!

// ✅ Fix: Require server to send timezone
// "2024-12-14T10:00:00+00:00"
// OR: Explicitly parse as UTC
decoder.dateDecodingStrategy = .custom { decoder in
    let container = try decoder.singleValueContainer()
    let dateString = try container.decode(String.self)

    let formatter = ISO8601DateFormatter()
    formatter.timeZone = TimeZone(secondsFromGMT: 0)  // Force UTC

    guard let date = formatter.date(from: dateString) else {
        throw DecodingError.dataCorruptedError(
            in: container,
            debugDescription: "Invalid ISO8601 date: \(dateString)"
        )
    }

    return date
}

Result: Bug fixed, server adds timezone to API (or you parse explicitly as UTC). No more intermittent failures.

Scenario 3: "Just Make It Optional"

Context: New API field causes decoding to fail. Product manager wants a fix in 1 hour.

Pressure: "Can't you just make that field optional? We need this shipped."

Why You'll Rationalize:

• "It's faster than fixing the API"
• "We can make it non-optional later"
• "Users won't notice"

What Actually Happens:

• Field is actually required for the feature
• You add user.email ?? "" everywhere
• 3 months later: production crash because email was nil
• Now you can't remember why it was optional

Discipline Response:

"Making it optional masks the real problem. Let me check if the API is wrong or our model is wrong. This will take 10 minutes."

Investigation:

// Step 1: Print raw JSON
do {
    let json = try JSONSerialization.jsonObject(with: data)
    print(json)
} catch {
    print("Invalid JSON: \(error)")
}

// Step 2: Check if key exists but value is null
// {"email": null} vs key missing entirely

// Step 3: Check API docs - is email actually required?

Common Outcomes:

1. API is wrong: Field should be there → File bug, get hotfix
2. Model is wrong: Field is optional in some flows → Use proper optionality with clear documentation
3. Structural mismatch: Field is nested → Use nested container

Result: You discover email is nested in user.contact.email in the new API version. Fix with nested container, not optionality.

// ✅ Correct fix
struct User: Decodable {
    let id: UUID
    let email: String  // Still required

    enum CodingKeys: CodingKey {
        case id, contact
    }

    enum ContactKeys: CodingKey {
        case email
    }

    init(from decoder: Decoder) throws {
        let container = try decoder.container(keyedBy: CodingKeys.self)
        id = try container.decode(UUID.self, forKey: .id)

        let contact = try container.nestedContainer(
            keyedBy: ContactKeys.self,
            forKey: .contact
        )
        email = try contact.decode(String.self, forKey: .email)
    }
}

Related Skills

• swift-concurrency — Codable types crossing actor boundaries must be Sendable
• swiftdata — @Model types use Codable for CloudKit sync
• networking — Coder protocol wraps Codable for Network.framework
• app-intents-ref — AppEnum parameters use Codable serialization

Key Takeaways

1. Prefer automatic synthesis — Add : Codable when structure matches JSON
2. Use CodingKeys for simple mismatches — Rename or exclude without manual code
3. Manual implementation for structural differences — Nested containers, bridge types
4. Always set locale and timezone — DateFormatter requires en_US_POSIX and explicit timezone
5. Never swallow errors with try? — Handle DecodingError cases explicitly
6. Codable + Sendable — Value types (structs/enums) are ideal for async networking

Core Principle: Codable is Swift's universal serialization protocol. Master it once, use it everywhere.