Code Quality Rank: L3
Programming language: Swift
License: MIT License
Tags: Testing     Mock    
Latest version: v2.1.0

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Kakapo partyparrot

Language: Swift Build Status Version DocCov codecov codebeat badge License Platform Carthage compatible

Dynamically Mock server behaviors and responses.


Kakapo is a dynamic mocking library. It allows you to replicate your backend APIs and logic.
With Kakapo you can easily prototype your application based on your API specifications.

Why Kakapo?

A common approach when testing network requests is to stub them with fake network responses from local files or recorded requests. This has some disadvantages:

  • All files need to be updated when the APIs are updated.
  • Lots of files have to be generated and included in the project.
  • Are just static responses that can only be used for unit tests since they don't reflect backend behaviors and state.

While still this approach may work good, Kakapo will be a game changer in your network tests: it will give you complete control when it comes to simulating backend behaviors. Moreover, is not just unit testing: you can even take a step further and prototype your application before having a real service behind!
With Kakapo you can just create Swift structs/classes/enums that are automatically serialized to JSON.

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  • Dynamic mocking
  • Prototyping
  • Swift 3.0 compatible (from version 2.0.0, master branch)
  • Swift 2.2 and 2.3 compatible (from version 1.0.0, branch feature/legacy-swift)
  • Compatible with Platform
  • Protocol oriented and pluggable
  • Fully customizable by defining custom serialization and custom responses
  • Out-of-the-box serialization
  • JSONAPI support


Using CocoaPods:

pod 'Kakapo'

Using Carthage:

github "devlucky/Kakapo"


NOTE: The project also contains a README.playground. Check it out to see some examples of the key features of Kakapo.

Kakapo is made with an easy-to-use design in mind. To quickly get started, you can create a Router that intercepts network requests like this:

let router = Router.register("http://www.test.com")
router.get("/users") { request in
  return ["id" : 2, "name": "Kakapo"]

You might be wondering where the dynamic part is; here is when the different modules of Kakapo take place:

let store = Store()
store.create(User.self, number: 20)

router.get("/users") { request in
  return store.findAll(User.self)

Now, we've created 20 random User objects and mocked our request to return them.

Let's get a closer look to the different features:

Serializable protocol

Kakapo uses the Serializable protocol in order to serialize objects to JSON. Any type can be serialized as long as it conforms to this protocol:

struct User: Serializable {
  let name: String

let user = User(name: "Alex")
let serializedUser = user.serialized()
//  -> ["name": "Alex"]

Also, standard library types are supported: this means that Array, Dictionary or Optional can be serialized:

let serializedUserArray = [user].serialized()
// -> [["name": "Alex"]]
let serializedUserDictionary = ["test": user].serialized()
// -> ["test": ["name": "Alex"]]

Router - Register and Intercept

Kakapo uses Routers in order to keep track of the registered endpoints that have to be intercepted.
You can match any relative path from the registered base URL, as long as the components are matching the request's components. You can use wildcard components:

let router = Router.register("http://www.test.com")

// Will match http://www.test.com/users/28
router.get("/users/:id") { ... }

// Will match http://www.test.com/users/28/comments/123
router.get("/users/:id/comments/:comment_id") { ... }

The handler will have to return a Serializable object that will define the response once the URL of a request is matched. When a Router intercepts a request, it automatically serializes the Serializable object returned by the handler and converts it to Data.

router.get("/users/:id") { request in
  return ["id": request.components["id"]!, "name": "Joan"]

Now everything is ready to test your mocked API; you can perform your request as you usually would do:

let session = URLSession.shared
let url = URL(string: "http://www.test.com/users/1")!
session.dataTask(with: url) { (data, _, _) in
    // handle response

Note: query parameters are not affecting the route match http://www.test.com/users/1?foo=bar would also be matched

In the previous example the handler was returning a simple Dictionary; while this works because Dictionary is already Serializable, you can also create your own entities that conform to Serializable:

struct User: Serializable {
    let firstName: String
    let lastName: String
    let id: String

router.get("/users/:id") { request in
  return User(firstName: "Joan", lastName: "Romano", id: request.components["id"]!)

When a request is matched, the RouteHandler receives a Request object that represents your request including components, query parameters, httpBody and httpHeaders. The Request object can be useful when building dynamic responses.

Third-Party Libraries

Third-Party libraries that use the Foundation networking APIs are also supported but you might need to set a proper URLSessionConfiguration.
For example, to setup Alamofire:

let configuration = URLSessionConfiguration.default
configuration.protocolClasses = [Server.self]
let sessionManager = SessionManager(configuration: configuration)

Leverage the Store - Dynamic mocking

Kakapo gets even more powerful when using your Routers together with the Store. You can create, insert, remove, update or find objects.

This lets you mock the APIs behaviors as if you were using a real backend. This is the dynamic side of Kakapo.

To create entities that can be used with the store, your types need to conform to the Storable protocol.

struct Article: Storable, Serializable {
    let id: String
    let text: String

    init(id: String, store: Store) {
        self.id = id
        self.text = randomString() // you might use some faker library like Fakery!

An example usage could be to retrieve a specific Article:

let store = Store()
store.create(Article.self, number: 20)

router.get("/articles/:id") { request in
  let articleId = request.components["id"]!
  return store.find(Article.self, id: articleId)

Of course you can perform any logic which fits your needs:

router.post("/article/:id") { request in
    return store.insert { (id) -> Article in
        return Article(id: id, text: "text from the body")

router.del("/article/:id") { request in
  let articleId = request.components["id"]!
  let article = store.find(Article.self, id: articleId)!
  try! store.delete(article)

  return ["status": "success"]


In Serializable we described how your classes can be serialized. The serialization, by default, will Mirror (using Swift's reflection) an entity by recursively serializing its properties.

Whenever a different behavior is needed, you can instead conform to CustomSerializable to provide your custom serialization.

For instance, Array uses CustomSerializable to return an Array containing its serialized elements. Dictionary, similarly, is serialized by creating a Dictionary with the same keys and serialized values.

For other examples of CustomSerializable and how to use it to create more complex serializations, take a look at the JSONAPISerializer implementation.


Since Kakapo was built with JSONAPI support in mind, JSONAPISerializer is able to serialize your entity into JSON conforming to jsonapi.org.

Your entities, in order to be serialized conforming to JSONAPI, need to conform to JSONAPIEntity protocol.

Let's see an example:

struct Cat: JSONAPIEntity {
    let id: String
    let name: String

struct User: JSONAPIEntity {
    let id: String
    let name: String
    let cats: [Cat]

Note that JSONAPIEntity objects are already Serializable and you could just use them together with your Routers. However, to completely follow the JSONAPI structure in your responses, you should wrap them into a JSONAPISerializer struct:

router.get("/users/:id") { request in
  let cats = [Cat(id: "33", name: "Joan"), Cat(id: "44", name: "Hez")]
  let user = User(id: "11", name: "Alex", cats: cats)
  return JSONAPISerializer(user)

Expanding Null values with Property Policy

When serializing to JSON, you may want to represent a property value as null. For this, you can use the PropertyPolicy enum. It is similar to Optional, providing an additional .null case:

public enum PropertyPolicy<Wrapped>: CustomSerializable {
    case None
    case Null
    case Some(Wrapped)

It's only purpose is to be serialized in 3 different ways, to cover all possible behaviors of an Optional property. PropertyPolicy works exactly as Optional properties:

  • .none -> property not included in the serialization
  • .some(wrapped) -> serialize wrapped

The additional case ,.null, is serialized as null when converted to json.

PropertyPolicy<Int>.none.serialized() // nil
PropertyPolicy<Int>.null.serialized() // NSNull
PropertyPolicy<Int>.some(1).serialized() // 1

Key customization - Serialization Transformer

The keys of the JSON generated by the serialization are directly reflecting the property names of your entities. However, you might need different behaviors. For instance, many APIs use snake_case keys but almost everyone use camelCase properties in Swift.
To transform the keys you can use SerializationTransformer. Objects conforming to this protocol are able to transform the keys of a wrapped object at serialization time.

For a concrete implementation, check SnakecaseTransformer: a struct that implements SerializationTransformer to convert keys into snake case:

let user = User(userName: "Alex")
let serialized = SnakecaseTransformer(user).serialized()
print(serialized) // [ "user_name" : "Alex" ]

Customize responses with ResponseFieldsProvider

If your responses need to specify status code (which will be 200 by default) and/or header fields, you can take advantage of ResponseFieldsProvider to customize your responses.

Kakapo provides a default ResponseFieldsProvider implementation in the Response struct, which you can use to wrap your Serializable objects:

router.get("/users/:id"){ request in
    return Response(statusCode: 400, body: user, headerFields: ["access_token" : "094850348502"])

let url = URL(string: "http://www.test.com/users/2")!
session.dataTaskWithURL() { (data, response, _) in
    let allHeaders = response.allHeaderFields
    let statusCode = response.statusCode
    print(allHeaders["access_token"]) // 094850348502
    print(statusCode) // 400

Otherwise your Serializable object can directly implement the protocol: take a look at JSONAPIError to see another example.


Even though Kakapo is ready to use, it is not meant to be shipped to the App Store although you can also do it! In fact, you might see it in action in some Apple stores since it was used to mock some features of Runtastic's demo app; however, it's at its early stage and we would love to hear your thoughts. We encourage you to open an issue if you have any questions, feedbacks or you just want to propose new features.

  • Full JSON API support #67
  • Reverse and Recursive relationships #16
  • Custom Serializers for common json specifications


Newsfeed BuddyBuild

Make sure you check the demo app we created using Kakapo: a prototyped newsfeed app which lets the user create new posts and like/unlike them.
To quickly try it use: pod try Kakapo


@MP0w - @zzarcon - @joanromano

*Note that all licence references and agreements mentioned in the Kakapo README section above are relevant to that project's source code only.