Swift Codable

Encode and decode Swift types with Codable (Encodable & Decodable). Covers custom keys, date/data strategies, polymorphic decoding, default values, and persistence compatibility. Targets Swift 6.3 / iOS 26+.

Contents

• Basic Conformance & CodingKeys
• Decoder Strategies
• Custom Encoding & Decoding
• Polymorphic & Lossy Decoding
• SwiftData & Persistence Boundaries
• Common Mistakes
• Review Checklist
• References

Basic Conformance & CodingKeys

Map serialized JSON field names to Swift property names using CodingKey:

struct UserProfile: Codable {
    let id: UUID
    var displayName: String
    var emailAddress: String

    enum CodingKeys: String, CodingKey {
        case id
        case displayName = "display_name"
        case emailAddress = "email"
    }
}

Decoder Strategies

Configure decoder strategies rather than writing manual decoding logic:

let decoder = JSONDecoder()
decoder.keyDecodingStrategy = .convertFromSnakeCase
decoder.dateDecodingStrategy = .iso8601
decoder.dataDecodingStrategy = .base64

let profiles = try decoder.decode([UserProfile].self, from: jsonData)

Note: .convertFromSnakeCase does not handle acronyms well (e.g. server_id becomes serverId, but server_url becomes serverUrl not serverURL). Use explicit CodingKeys for non-standard mappings.

Custom Encoding & Decoding

When custom transformations are needed, implement init(from:) and encode(to:):

struct BoundedScore: Codable {
    let score: Int

    init(from decoder: Decoder) throws {
        let container = try decoder.singleValueContainer()
        let raw = try container.decode(Int.self)
        self.score = min(max(raw, 0), 100)
    }

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

Polymorphic & Lossy Decoding

• Lossy Arrays: Decode valid elements while skipping corrupted items using an unkeyed container try/catch loop.
• Polymorphic Types: Decode a discriminator key (e.g. "type") from a parent container, then decode the corresponding concrete type.

See references/codable-advanced-patterns.md for complete implementations.

SwiftData & Persistence Boundaries

SwiftData @Model classes can store complex structures that conform to Codable. When persisting Codable structs:

• Keep data types stable across app updates.
• Provide default values for newly added properties to prevent schema migration crashes.

Common Mistakes

• Omitting properties in custom init(from:): A custom decoder initializer must initialize all stored properties or compilation fails.
• Using convertFromSnakeCase with explicit CodingKeys: The key decoding strategy converts keys before matching CodingKeys, leading to double-conversion mismatches.
• Failing entire collections on one bad item: In resilient feeds, decode elements lossily rather than throwing on the entire array.
• Decoding floating point dates without explicit strategies: Default date decoding expects seconds since 2001; specify .iso8601 or .millisecondsSince1970.
• Ignoring DecodingError diagnostics: Inspect DecodingError.keyNotFound, .typeMismatch, and .valueNotFound details for exact debugging.

Review Checklist

• Automatic synthesis preferred over manual init(from:) where possible
• Appropriate dateDecodingStrategy and keyDecodingStrategy configured on decoder
• Acronyms in snake_case checked for conversion issues
• Resilient network arrays handle individual element decoding errors
• Codable models intended for SwiftData maintain stable schema types

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