[Deprecated] The easy to use Swift JSON decoder





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3yrs ago











Unbox is deprecated in favor of Swift’s built-in Codable API and the Codextended project. All current users are highly encouraged to migrate to Codable as soon as possible. Click here for more information and a migration guide.


Unbox | Wrap

Travis status CocoaPods Carthage Twitter: @johnsundell

Unbox is an easy to use Swift JSON decoder. Don't spend hours writing JSON decoding code - just unbox it instead!

Unbox is lightweight, non-magical and doesn't require you to subclass, make your JSON conform to a specific schema or completely change the way you write model code. It can be used on any model with ease.

Basic example

Say you have your usual-suspect User model:

struct User {
    let name: String
    let age: Int

That can be initialized with the following JSON:

    "name": "John",
    "age": 27

To decode this JSON into a User instance, all you have to do is make User conform to Unboxable and unbox its properties:

struct User {
    let name: String
    let age: Int

extension User: Unboxable {
    init(unboxer: Unboxer) throws {
        self.name = try unboxer.unbox(key: "name")
        self.age = try unboxer.unbox(key: "age")

Unbox automatically (or, actually, Swift does) figures out what types your properties are, and decodes them accordingly. Now, we can decode a User like this:

let user: User = try unbox(dictionary: dictionary)

or even:

let user: User = try unbox(data: data)

Advanced example

The first was a pretty simple example, but Unbox can decode even the most complicated JSON structures for you, with both required and optional values, all without any extra code on your part:

struct SpaceShip {
    let type: SpaceShipType
    let weight: Double
    let engine: Engine
    let passengers: [Astronaut]
    let launchLiveStreamURL: URL?
    let lastPilot: Astronaut?
    let lastLaunchDate: Date?

extension SpaceShip: Unboxable {
    init(unboxer: Unboxer) throws {
        self.type = try unboxer.unbox(key: "type")
        self.weight = try unboxer.unbox(key: "weight")
        self.engine = try unboxer.unbox(key: "engine")
        self.passengers = try unboxer.unbox(key: "passengers")
        self.launchLiveStreamURL = try? unboxer.unbox(key: "liveStreamURL")
        self.lastPilot = try? unboxer.unbox(key: "lastPilot")

        let dateFormatter = DateFormatter()
        dateFormatter.dateFormat = "YYYY-MM-dd"
        self.lastLaunchDate = try? unboxer.unbox(key: "lastLaunchDate", formatter: dateFormatter)

enum SpaceShipType: Int, UnboxableEnum {
    case apollo
    case sputnik

struct Engine {
    let manufacturer: String
    let fuelConsumption: Float

extension Engine: Unboxable {
    init(unboxer: Unboxer) throws {
        self.manufacturer = try unboxer.unbox(key: "manufacturer")
        self.fuelConsumption = try unboxer.unbox(key: "fuelConsumption")

struct Astronaut {
    let name: String

extension Astronaut: Unboxable {
    init(unboxer: Unboxer) throws {
        self.name = try unboxer.unbox(key: "name")

Error handling

Decoding JSON is inherently a failable operation. The JSON might be in an unexpected format, or a required value might be missing. Thankfully, Unbox takes care of handling both missing and mismatched values gracefully, and uses Swift’s do, try, catch pattern to return errors to you.

You don’t have to deal with multiple error types and perform any checking yourself, and you always have the option to manually exit an unboxing process by throwing. All errors returned by Unbox are of the type UnboxError.

Supported types

Unbox supports decoding all standard JSON types, like:

  • Bool
  • Int, Double, Float
  • String
  • Array
  • Dictionary

It also supports all possible combinations of nested arrays & dictionaries. As you can see in the Advanced example above (where an array of the unboxable Astronaut struct is being unboxed), we can unbox even a complicated data structure with one simple call to unbox().

Finally, it also supports URL through the use of a transformer, and Date by using any DateFormatter.


Unbox also supports transformations that let you treat any value or object as if it was a raw JSON type.

It ships with a default String -> URL transformation, which lets you unbox any URL property from a string describing an URL without writing any transformation code.

The same is also true for String -> Int, Double, Float, CGFloat transformations. If you’re unboxing a number type and a string was found, that string will automatically be converted to that number type (if possible).

To enable your own types to be unboxable using a transformation, all you have to do is make your type conform to UnboxableByTransform and implement its protocol methods.

Here’s an example that makes a native Swift UniqueIdentifier type unboxable using a transformation:

struct UniqueIdentifier: UnboxableByTransform {
    typealias UnboxRawValueType = String

    let identifierString: String

    init?(identifierString: String) {
        if let UUID = NSUUID(uuidString: identifierString) {
            self.identifierString = UUID.uuidString
        } else {
            return nil

    static func transform(unboxedValue: String) -> UniqueIdentifier? {
        return UniqueIdentifier(identifierString: unboxedValue)


If you have values that need to be formatted before use, Unbox supports using formatters to automatically format an unboxed value. Any DateFormatter can out of the box be used to format dates, but you can also add formatters for your own custom types, like this:

enum Currency {
    case usd(Int)
    case sek(Int)
    case pln(Int)

struct CurrencyFormatter: UnboxFormatter {
    func format(unboxedValue: String) -> Currency? {
        let components = unboxedValue.components(separatedBy: ":")

        guard components.count == 2 else {
            return nil

        let identifier = components[0]

        guard let value = Int(components[1]) else {
            return nil

        switch identifier {
        case "usd":
            return .usd(value)
        case "sek":
            return .sek(value)
        case "pln":
            return .pln(value)
            return nil

You can now easily unbox any Currency using a given CurrencyFormatter:

struct Product: Unboxable {
    let name: String
    let price: Currency

    init(unboxer: Unboxer) throws {
        name = try unboxer.unbox(key: "name")
        price = try unboxer.unbox(key: "price", formatter: CurrencyFormatter())

Supports JSON with both Array and Dictionary root objects

No matter if the root object of the JSON that you want to unbox is an Array or Dictionary - you can use the same Unbox() function and Unbox will return either a single model or an array of models (based on type inference).

Built-in enum support

You can also unbox enums directly, without having to handle the case if they failed to initialize. All you have to do is make any enum type you wish to unbox conform to UnboxableEnum, like this:

enum Profession: Int, UnboxableEnum {
    case developer
    case astronaut

Now Profession can be unboxed directly in any model

struct Passenger: Unboxable {
    let profession: Profession

    init(unboxer: Unboxer) throws {
        self.profession = try unboxer.unbox(key: "profession")

Contextual objects

Sometimes you need to use data other than what's contained in a dictionary during the decoding process. For this, Unbox has support for strongly typed contextual objects that can be made available in the unboxing initializer.

To use contextual objects, make your type conform to UnboxableWithContext, which can then be unboxed using unbox(dictionary:context) where context is of the type of your choice.

Key path support

You can also use key paths (for both dictionary keys and array indexes) to unbox values from nested JSON structures. Let's expand our User model:

    "name": "John",
    "age": 27,
    "activities": {
        "running": {
            "distance": 300
    "devices": [
        "Macbook Pro",
struct User {
    let name: String
    let age: Int
    let runningDistance: Int
    let primaryDeviceName: String

extension User: Unboxable {
    init(unboxer: Unboxer) throws {
        self.name = try unboxer.unbox(key: "name")
        self.age = try unboxer.unbox(key: "age")
        self.runningDistance = try unboxer.unbox(keyPath: "activities.running.distance")
        self.primaryDeviceName = try unboxer.unbox(keyPath: "devices.0")

You can also use key paths to directly unbox nested JSON structures. This is useful when you only need to extract a specific object (or objects) out of the JSON body.

    "company": {
        "name": "Spotify",
    "jobOpenings": [
            "title": "Swift Developer",
            "salary": 120000
            "title": "UI Designer",
            "salary": 100000
struct JobOpening {
    let title: String
    let salary: Int

extension JobOpening: Unboxable {
    init(unboxer: Unboxer) throws {
        self.title = try unboxer.unbox(key: "title")
        self.salary = try unboxer.unbox(key: "salary")

struct Company {
    let name: String

extension Company: Unboxable {
    init(unboxer: Unboxer) throws {
        self.name = try unboxer.unbox(key: "name")
let company: Company = try unbox(dictionary: json, atKey: "company")
let jobOpenings: [JobOpening] = try unbox(dictionary: json, atKey: "jobOpenings")
let featuredOpening: JobOpening = try unbox(dictionary: json, atKeyPath: "jobOpenings.0")

Custom unboxing

Sometimes you need more fine grained control over the decoding process, and even though Unbox was designed for simplicity, it also features a powerful custom unboxing API that enables you to take control of how an object gets unboxed. This comes very much in handy when using Unbox together with Core Data, when using dependency injection, or when aggregating data from multiple sources. Here's an example:

let dependency = DependencyManager.loadDependency()

let model: Model = try Unboxer.performCustomUnboxing(dictionary: dictionary, closure: { unboxer in
    var model = Model(dependency: dependency)
    model.name = try? unboxer.unbox(key: "name")
    model.count = try? unboxer.unbox(key: "count")

    return model



Add the line pod "Unbox" to your Podfile


Add the line github "johnsundell/unbox" to your Cartfile


Clone the repo and drag the file Unbox.swift into your Xcode project.

Swift Package Manager:

Add the line .Package(url: "https://github.com/johnsundell/unbox.git", from: "3.0.0") to your Package.swift

Platform support

Unbox supports all current Apple platforms with the following minimum versions:

  • iOS 8
  • OS X 10.11
  • watchOS 2
  • tvOS 9

Debugging tips

In case your unboxing code isn’t working like you expect it to, here are some tips on how to debug it:

Compile time error: Ambiguous reference to member 'unbox'

Swift cannot find the appropriate overload of the unbox method to call. Make sure you have conformed to any required protocol (such as Unboxable, UnboxableEnum, etc). Note that you can only conform to one Unbox protocol for each type (that is, a type cannot be both an UnboxableEnum and UnboxableByTransform). Also remember that you can only reference concrete types (not Protocol types) in order for Swift to be able to select what overload to use.

unbox() throws

Use the do, try, catch pattern to catch and handle the error:

do {
    let model: Model = try unbox(data: data)
} catch {
    print("An error occurred: \(error)")

If you need any help in resolving any problems that you might encounter while using Unbox, feel free to open an Issue.

Community Extensions

Hope you enjoy unboxing your JSON!

For more updates on Unbox, and my other open source projects, follow me on Twitter: @johnsundell

Also make sure to check out Wrap that let’s you easily encode JSON.

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