JSON mapper for macOS, iOS, tvOS, and watchOS

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In Swift, we all deal with JSON, plists, and various forms of [String: Any]. Outlaw provides various ways to deal with these in an expressive and type safe way. Outlaw will help you write declarative, performant, error handled code using the power of Protocol Oriented Programming™.


Extracting values from [String: Any] using Outlaw is as easy as

let name: String = try json.value(for: "name")
let url: URL = try json.value(for: "user.website") // extract from nested objects!

Converting to Models

Often we want to take an extractable object (like [String: Any]) and deserialize it into one of our local models—for example we may want to take some JSON and initialize one of our local models with it:

struct User: Deserializable {
    var id: Int
    var name: String
    var email: String

    init(object: Extractable) throws {
        id = try object.value(for: "id")
        name = try object.value(for: "name")
        email = try object.value(for: "email")

Now, just by virtue of supplying a simple initializer you can pull your models directly out of [String: Any]!

let users: [User] = try json.value(for: "users")

That was easy! Thanks, Protocol Oriented Programming™!

Serializing Models

We've looked at going from our [String: Any] into our local models, but what about the other way around?

extension User: Serializable {
	func serialized() -> [String: Any] {
        return [
            "id": "id",
            "name" : name,
            "email": email

Now, you might be thinking "but couldn't I use reflection to do this for me automagically?" You could. And if you're into that, there are some other great frameworks for you to use. But Outlaw believes mirrors can lead down the road to a world of hurt. Outlaw lives in a world where What You See Is What You Get™, and you can easily adapt to APIs that snake case, camel case, or whatever case your backend developers are into. Outlaw code is explicit and declarative. But don't just take Outlaw's word for it—read the good word towards the end here on the official Swift blog.

Error Handling

Are you someone that doesn't care about errors? Use an optional data type.

let users: [User]? = json.value(for: "users")

Otherwise, wrap your code in a do-catch to get all the juicy details when things go wrong.

do {
	let users: [User] = try json.value(for: "users")
catch {

Add Your Own Values

Out of the box, Outlaw supports extracting native Swift types like String, Int, etc., as well as URL, Date and anything conforming to Deserializable, and arrays or dictionaries of all the aforementioned types.

However, Outlaw doesn't just leave you up the creek without a paddle! Adding your own Outlaw value type is as easy as extending your type with Value.

extension CGPoint: Value {
    public static func value(from object: Any) throws -> CGPoint {
        guard let properties = object as? [String: CGFloat] else {
            throw OutlawError.typeMismatch(expected: [String: CGFloat].self, actual: type(of: object))
        let x: CGFloat = properties["x"] ?? 0
        let y: CGFloat = properties["y"] ?? 0
        return CGPoint(x: x, y: y)

By simply implementing value(from:), Outlaw allows you to immediately do this:

let point: CGPoint = try json.value(for: "point")

Protocol Oriented Programming™ strikes again!

Intermediate Values

Don't like how Outlaw implements the default value extraction? Have a different date format? No problem! All you need is a transformation function when extracting the values.

let formatter = DateFormatter()
formatter.timeZone = TimeZone(abbreviation: "GMT")
formatter.dateFormat = "MM/dd/yyyy"

let date: Date? = json.value(for: "date", with: { (dateString: String) -> Date? in
	return formatter.date(from: dateString)

We can also use the power of swift and shorten the above code to:

let date: Date? = json.value(for: "date", with: formatter.date)


Outlaw is based on the same underlying code as Marshal and is just as performant. You should always take benchmarks with a grain of salt, but chew on these benchmarks for a bit anyway. Unfortunately, the JSONShootout project was built in a way so that Outlaw can't be added because of ambiguous method collisions with Marshal.


Outlaw was created by one of the main contributors to Marshal. However, Marshal was designed to be a bare bones framework that needed to be extended for additional features. Outlaw was designed to be a more feature rich framework that can handle a lot more data extraction scenarios out of the box.


  • Swift Tools 4.0.0
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Last updated: Fri Apr 12 2024 01:03:52 GMT-0900 (Hawaii-Aleutian Daylight Time)