ObjectMapper is a framework written in Swift that makes it easy for you to convert your Model objects (Classes and Structs) to and from JSON.


  • Mapping JSON to objects
  • Mapping objects to JSON
  • Nested Objects (stand alone, in Arrays or in Dictionaries)
  • Custom transformations during mapping
  • Struct support

The Basics

To support mapping, a Class or Struct just needs to implement the Mappable protocol.

public protocol Mappable {
    init?(_ map: Map)
    mutating func mapping(map: Map)

ObjectMapper uses the <- operator to define how each member variable maps to and from JSON.

class User: Mappable {
    var username: String?
    var age: Int?
    var weight: Double!
    var array: [AnyObject]?
    var dictionary: [String : AnyObject] = [:]
    var bestFriend: User?                       // Nested User object
    var friends: [User]?                        // Array of Users
    var birthday: NSDate?

    required init?(_ map: Map) {

    // Mappable
    func mapping(map: Map) {
        username    <- map["username"]
        age         <- map["age"]
        weight      <- map["weight"]
        array       <- map["arr"]
        dictionary  <- map["dict"]
        bestFriend  <- map["best_friend"]
        friends     <- map["friends"]
        birthday    <- (map["birthday"], DateTransform())

struct Temperature: Mappable {
    var celcius: Double?
    var fahrenheit: Double?


    init?(_ map: Map) {

    mutating func mapping(map: Map) {
        celcius     <- map["celcius"]
        fahrenheit  <- map["fahrenheit"]

Once your class implements Mappable, the Mapper class handles everything else for you:

Convert a JSON string to a model object:

let user = Mapper<User>().map(JSONString)

Convert a model object to a JSON string:

let JSONString = Mapper().toJSONString(user, prettyPrint: true)

Object mapper can map classes composed of the following types:

  • Int
  • Bool
  • Double
  • Float
  • String
  • RawRepresentable (Enums)
  • Array<AnyObject>
  • Dictionary<String, AnyObject>
  • Object<T: Mappable>
  • Array<T: Mappable>
  • Dictionary<String, T: Mappable>
  • Dictionary<String, Array<T: Mappable>>
  • Optionals of all the above
  • Implicitly Unwrapped Optionals of the above

Easy Mapping of Nested Objects

ObjectMapper supports dot notation within keys for easy mapping of nested objects. Given the following JSON String:

"distance" : {
     "text" : "102 ft",
     "value" : 31

You can access the nested objects as follows:

func mapping(map: Map){
    distance <- map["distance.value"]

Custom Transfoms

ObjectMapper also supports custom Transforms that convert values during the mapping process. To use a transform, simply create a tuple with map["field_name"] and the transform of choice on the right side of the <- operator:

birthday <- (map["birthday"], DateTransform())

The above transform will convert the JSON Int value to an NSDate when reading JSON and will convert the NSDate to an Int when converting objects to JSON.

You can easily create your own custom transforms by adopting and implementing the methods in the TransformType protocol:

public protocol TransformType {
    typealias Object
    typealias JSON

    func transformFromJSON(value: AnyObject?) -> Object?
    func transformToJSON(value: Object?) -> JSON?


In a lot of situations you can use the built in transform class TransformOf to quickly perform a desired transformation. TransformOf is initialized with two types and two closures. The types define what the transform is converting to and from and the closures perform the actual transformation.

For example, if you want to transform a JSON String value to an Int you could use TransformOf as follows:

let transform = TransformOf<Int, String>(fromJSON: { (value: String?) -> Int? in 
    // transform value from String? to Int?
    return value?.toInt()
}, toJSON: { (value: Int?) -> String? in
    // transform value from Int? to String?
    if let value = value {
        return String(value)
    return nil

id <- (map["id"], transform)

Here is a more condensed version of the above:

id <- (map["id"], TransformOf<Int, String>(fromJSON: { $0?.toInt() }, toJSON: { $0.map { String($0) } }))


Classes that implement the Mappable protocol can easily be subclassed. When subclassing Mappable classes, follow the structure below:

class Base: Mappable {
    var base: String?

    required init?(_ map: Map) {

    func mapping(map: Map) {
        base <- map["base"]

class Subclass: Base {
    var sub: String?

    required init?(_ map: Map) {

    override func mapping(map: Map) {

        sub <- map["sub"]

ObjectMapper + Alamofire

If you are using Alamofire for networking and you want to convert your responses to swift objects, you can use AlamofireObjectMapper. It is a simple Alamofire extension that uses ObjectMapper to automatically map JSON response data to swift objects.


Contributions are very welcomed 👍😃.

Before submitting any Pull Request, please ensure you have run the included tests and that they have passed. If you are including new functionality, please write test cases for it as well.

ObjectMapper uses Nimble to ensure test success. It is included using Carthage. Run the following command in the ObjectMapper root directory to fetch the Nimble depency and get the environment ready for running tests:

carthage checkout

From this point on, you should open the project using ObjectMapper.xcworkspace and NOT ObjectMapper.xcodeproj


ObjectMapper can be added to your project using Cocoapods 0.36 (beta) by adding the following line to your Podfile:

pod 'ObjectMapper', '~> 0.11'

Otherwise, ObjectMapper can be added as a submodule:

  1. Add ObjectMapper as a submodule by opening the Terminal, cd-ing into your top-level project directory, and entering the command git submodule add https://github.com/Hearst-DD/ObjectMapper.git
  2. Open the ObjectMapper folder, and drag ObjectMapper.xcodeproj into the file navigator of your app project.
  3. In Xcode, navigate to the target configuration window by clicking on the blue project icon, and selecting the application target under the “Targets” heading in the sidebar.
  4. Ensure that the deployment target of ObjectMapper.framework matches that of the application target.
  5. In the tab bar at the top of that window, open the “Build Phases” panel.
  6. Expand the “Target Dependencies” group, and add ObjectMapper.framework.
  7. Click on the + button at the top left of the panel and select “New Copy Files Phase”. Rename this new phase to “Copy Frameworks”, set the “Destination” to “Frameworks”, and add ObjectMapper.framework.