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Flight School Guide to Swift Codable Cover

Guide to Swift Codable Sample Code

Build Status License Swift Version

This repository contains sample code used in the Flight School Guide to Swift Codable.

Chapter 1

Chapter 1 introduces Codable by way of a round-trip journey --- going from model to JSON representation and back again.


let json = """
    "manufacturer": "Cessna",
    "model": "172 Skyhawk",
    "seats": 4,
""".data(using: .utf8)!

let decoder = JSONDecoder()
let plane = try! decoder.decode(Plane.self, from: json)

Chapter 2

Chapter 2 follows a more complicated example with nested structures, mismatched keys, and timestamps.

Flight Plan

let json = """
    "aircraft": {
        "identification": "NA12345",
        "color": "Blue/White"
    "route": ["KTTD", "KHIO"],
    "departure_time": {
        "proposed": "2018-04-20T15:07:24-07:00",
        "actual": "2018-04-20T15:07:24-07:00"
    "flight_rules": "IFR",
    "remarks": null
""".data(using: .utf8)!

let decoder = JSONDecoder()
decoder.dateDecodingStrategy = .iso8601

let plan = try! decoder.decode(FlightPlan.self, from: json)

Chapter 3

Chapter 3 shows what to do when Codable conformance can’t be synthesized by the compiler.

In the process, we share an implementation of a type-erased AnyCodable type.


struct Report: Decodable {
    var title: String
    var body: String
    var metadata: [String: AnyDecodable]


let json = """
    "coordinates": [
            "latitude": 37.332,
            "longitude": -122.011
        [-122.011, 37.332],
        "37.332, -122.011"
""".data(using: .utf8)!

let decoder = JSONDecoder()
let coordinates = try! decoder.decode([String: [Coordinate]].self, from: json)["coordinates"]


class EconomySeat: Decodable {
    var number: Int
    var letter: String
    // ...

class PremiumEconomySeat: EconomySeat {
    var mealPreference: String?
    // ...

let json = """
    "number": 7,
    "letter": "A",
    "mealPreference": "vegetarian"
""".data(using: .utf8)!

let decoder = JSONDecoder()
let seat = try! decoder.decode(PremiumEconomySeat.self, from: json)


let json = """
        "type": "bird",
        "genus": "Chaetura",
        "species": "Vauxi"
        "type": "plane",
        "identifier": "NA12345"
""".data(using: .utf8)!

let decoder = JSONDecoder()
let objects = try! decoder.decode([Either<Bird, Plane>].self, from: json)


protocol FuelPrice {
    var type: Fuel { get }
    var pricePerLiter: Double { get }
    var currency: String { get }

struct CanadianFuelPrice: Decodable {
    let type: Fuel
    let price: Double /// CAD / liter

extension CanadianFuelPrice: FuelPrice {
    var pricePerLiter: Double {
        return self.price

    var currency: String {
        return "CAD"


let encoder = JSONEncoder()
encoder.userInfo[.colorEncodingStrategy] =
    ColorEncodingStrategy.hexadecimal(hash: true)

let cyan = Pixel(red: 0, green: 255, blue: 255)
let magenta = Pixel(red: 255, green: 0, blue: 255)
let yellow = Pixel(red: 255, green: 255, blue: 0)
let black = Pixel(red: 0, green: 0, blue: 0)

let json = try! encoder.encode([cyan, magenta, yellow, black])


let json = """
    "points": ["KSQL", "KWVI"],
    "KSQL": {
        "code": "KSQL",
        "name": "San Carlos Airport"
    "KWVI": {
        "code": "KWVI",
        "name": "Watsonville Municipal Airport"
""".data(using: .utf8)!

let decoder = JSONDecoder()
let route = try decoder.decode(Route.self, from: json)

Chapter 4

Chapter 4 is a case study in which you build search functionality for a music store app using the iTunes Search API (but really, it’s a lesson about command-line tools and epistemology).

We also released AppleiTunesSearchURLComponents as a standalone component.

Music Store Music.self, with: <#artist#>)

Chapter 5

Chapter 5 shows you how to use Codable with UserDefaults by way of an example app for tabulating in-flight snack orders.

In Flight Service

guard let url = Bundle.main.url(forResource: "Inventory", withExtension: ".plist") else {
    fatalError("Inventory.plist missing from main bundle")

let inventory: [Item]
do {
    let data = try Data(contentsOf: url)

    let decoder = PropertyListDecoder()
    let plist = try decoder.decode([String: [Item]].self, from: data)
    inventory = plist["items"]!
} catch {
    fatalError("Cannot load inventory \(error)")

Chapter 6

Chapter 6 is about how Codable fits into a Core Data stack. The example app for this chapter is a luggage tag scanner that reads JSON from QR codes.

Luggage Scanner

do {
    for image in tagsAtDeparture {
        try scanner.scan(image: image, at: .origin, in: context)

} catch {

Chapter 7

Chapter 7 is a doozy. It walks through a complete implementation of a Codable-compatible encoder for the MessagePack format, from start to finish.

A complete Codable-compliant implementation is available at Flight-School/MessagePack. If you're interested in building your own Codable encoder or decoder, check out our DIY Kit.


let plane = Plane(manufacturer: "Cirrus",
                  model: "SR22",
                  seats: 4)

let encoder = MessagePackEncoder()
let data = try! encoder.encode(plane)



About Flight School

Flight School is a book series for advanced Swift developers that explores essential topics in iOS and macOS development through concise, focused guides.

If you'd like to get in touch, feel free to message us on Twitter or email us at


Xcode Playground Sample Code for the Flight School Guide to Swift Codable





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