codable-patterns

Codable Patterns Encode and decode Swift types using Codable ( Encodable & Decodable ) with JSONEncoder , JSONDecoder , and related APIs. Targets Swift 6.2 / iOS 26+. Contents 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 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 : 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: // 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 Decode arrays of mixed types using a discriminator field: // JSON: [{"type":"text","content":"Hello"},{"type":"image","url":"pic.jpg"}] enum ContentBlock : Decodable { case text ( String ) case image ( URL ) enum CodingKeys : String , CodingKey { case type , content , url } init ( from decoder : Decoder ) throws { let container = try decoder . container ( keyedBy : CodingKeys . self ) let type = try container . decode ( String . self , forKey : . type ) switch type { case "text" : let content = try container . decode ( String . self , forKey : . content ) self = . text ( content ) case "image" : let url = try container . decode ( URL . self , forKey : . url ) self = . image ( url ) default : throw DecodingError . dataCorruptedError ( forKey : . type , in : container , debugDescription : "Unknown type: ( type ) " ) } } } let blocks = try JSONDecoder ( ) . decode ( [ ContentBlock ] . self , from : jsonData ) 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 // snake_case -> camelCase // {"user_name": "Alice"} maps to var userName: String -- no CodingKeys needed let encoder = JSONEncoder ( ) encoder . dataEncodingStrategy = . base64 encoder . keyEncodingStrategy = . convertToSnakeCase Lossy Array Decoding By default, one invalid element fails the entire array. Use a wrapper to skip invalid elements: struct LossyArray < Element : Decodable

: Decodable { let elements : [ Element ] init ( from decoder : Decoder ) throws { var container = try decoder . unkeyedContainer ( ) var elements : [ Element ] = [ ] while ! container . isAtEnd { if let element = try ? container . decode ( Element . self ) { elements . append ( element ) } else { _ = try ? container . decode ( AnyCodableValue . self ) // advance past bad element } } self . elements = elements } } private struct AnyCodableValue : Decodable { } Single Value Containers Wrap primitives for type safety using singleValueContainer() : struct UserID : Codable , Hashable { let rawValue : String init ( _ rawValue : String ) { self . rawValue = rawValue } init ( from decoder : Decoder ) throws { let container = try decoder . singleValueContainer ( ) rawValue = try container . decode ( String . self ) } func encode ( to encoder : Encoder ) throws { var container = encoder . singleValueContainer ( ) try container . encode ( rawValue ) } } // JSON: "usr_abc123" decodes directly to UserID Default Values for Missing Keys Use decodeIfPresent with nil-coalescing to provide defaults: struct Settings : Decodable { let theme : String let fontSize : Int let notificationsEnabled : Bool enum CodingKeys : String , CodingKey { case theme , fontSize = "font_size" case notificationsEnabled = "notifications_enabled" } init ( from decoder : Decoder ) throws { let container = try decoder . container ( keyedBy : CodingKeys . self ) theme = try container . decodeIfPresent ( String . self , forKey : . theme ) ?? "system" fontSize = try container . decodeIfPresent ( Int . self , forKey : . fontSize ) ?? 16 notificationsEnabled = try container . decodeIfPresent ( Bool . self , forKey : . notificationsEnabled ) ?? true } } Encoder and Decoder Configuration let encoder = JSONEncoder ( ) encoder . outputFormatting = [ . prettyPrinted , . sortedKeys , . withoutEscapingSlashes ] // Non-conforming floats (NaN, Infinity are not valid JSON) encoder . nonConformingFloatEncodingStrategy = . convertToString ( positiveInfinity : "Infinity" , negativeInfinity : "-Infinity" , nan : "NaN" ) decoder . nonConformingFloatDecodingStrategy = . convertFromString ( positiveInfinity : "Infinity" , negativeInfinity : "-Infinity" , nan : "NaN" ) PropertyListEncoder / PropertyListDecoder let plistEncoder = PropertyListEncoder ( ) plistEncoder . outputFormat = . xml // or .binary let data = try plistEncoder . encode ( settings ) let decoded = try PropertyListDecoder ( ) . decode ( Settings . self , from : data ) 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 for wrapped responses struct APIResponse < T : Decodable

: Decodable { let data : T let meta : Meta ? struct Meta : Decodable { let page : Int ; let totalPages : Int } } let users = try decoder . decode ( APIResponse < [ User ]

. self , from : data ) . data Codable with SwiftData Codable structs work as composite attributes in SwiftData models. In iOS 18+, SwiftData natively supports them without explicit @Attribute(.transformable) : struct Address : Codable { var street : String var city : String var zipCode : String } @Model class Contact { var name : String var address : Address ? // Codable struct stored as composite attribute init ( name : String , address : Address ? = nil ) { self . name = name ; self . address = address } } Codable with UserDefaults Store Codable values via RawRepresentable for @AppStorage : struct UserPreferences : Codable { var showOnboarding : Bool = true var accentColor : String = "blue" } extension UserPreferences : RawRepresentable { init ? ( rawValue : String ) { guard let data = rawValue . data ( using : . utf8 ) , let decoded = try ? JSONDecoder ( ) . decode ( Self . self , from : data ) else { return nil } self = decoded } var rawValue : String { guard let data = try ? JSONEncoder ( ) . encode ( self ) , let string = String ( data : data , encoding : . utf8 ) else { return "{}" } return string } } struct SettingsView : View { @AppStorage ( "userPrefs" ) private var prefs = UserPreferences ( ) var body : some View { Toggle ( "Show Onboarding" , isOn : $prefs . showOnboarding ) } } Common Mistakes 1. Not handling missing optional keys: // DON'T -- crashes if key is absent let value = try container . decode ( String . self , forKey : . bio ) // DO -- returns nil for missing keys 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 -- use LossyArray or decode elements individually let items = try container . decode ( LossyArray < Item

. self , forKey : . items ) . elements 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 decoder . keyDecodingStrategy = . convertFromSnakeCase Review Checklist Types conform to Decodable only when encoding is not needed decodeIfPresent used with defaults for optional or missing keys keyDecodingStrategy = .convertFromSnakeCase used instead of manual CodingKeys for simple snake_case APIs 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 wrapper used for consistent API envelope handling No force-unwrapping of decoded values @AppStorage Codable types conform to RawRepresentable SwiftData composite attributes use Codable structs References 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 Encoding and Decoding Custom Types -- Apple guide on custom Codable conformance Using JSON with Custom Types -- Apple sample code for JSON patterns