A smart stubbing system.

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A smart stubbing system.


Using Carthage

github "kateinoigakukun/StubKit"

Using CocoaPods

pod "StubKit", :git => ""


Getting started

You can instantiate any kind of Decodable with a single line.

import StubKit

// Codable struct
struct User: Codable {
  let id: Int
  let name: String
  let sex: Sex

let stubUser = try Stub.make(User.self)
// User(id: 1234, name: "This is Stub String", sex: .female)

Customize property

You can customize properties even if the property is defined as let.

let maleUser = try Stub.make(User.self) {
  $0.set(\.sex, value: .male)
// User(id: 1234, name: "This is Stub String", sex: .male)

Using Stubbable

If you want to customize the default stub value, please conform Stubbable.

extension String: Stubbable {
  static func stub() -> String {
    return "This is custommized Stub String"

let stubUser = try Stub.make(User.self)
// User(id: 1234, name: "This is customized Stub String", sex: .female)

Advanced Usage

struct RandomIntStubProvider: StubProvider {
    func stub<T>(of type: T.Type) -> T? {
        if type is Int.Type {
            return Int.random(in: 0..<100) as? T
        return nil

let userStub = Stub(type: User.self, provider: [RandomIntStubProvider()])
try userStub.make() // User(id: 97)
try userStub.make() // User(id: 54)
try userStub.make() // User(id: 12)

Need to conform non-final class as Stubbable?

You can make it Stubbable by defining the UnsafeStubbable.

public protocol UnsafeStubbable: Stubbable {
    associatedtype Target = Self
    static func unsafeStub() -> Target

extension UnsafeStubbable {
    public static func stub() -> Self {
        return unsafeStub() as! Self

extension UIImage: UnsafeStubbable {
    public static func unsafeStub() -> UIImage {
        return #imageLiteral(resourceName: "dummy")

How does it work

StubKit mainly uses two techniques.

  • Traverse using Decoder protocol.
  • Inject value with non-mutable KeyPath.
  • Existential type using Self.

Traverse struct using Decoder protocol

Swift has Decodable protocol and if a type conforms to Decodable, Swift compiler generates some code to decode internally. So we can decode a JSON to Swift struct without any configuration. StubKit uses this system to construct instance through Decoder. Decoder is a protocol which provide a value by key or index like JSONDecoder. If we pass the Decoder which just provide a stub value recursively, we can instantiate any kind of Decodable instance.

Inject value with non-mutable KeyPath

I know it's only natural but, Swift can't mutate let defined property. But Swift has MemoryLayout<T>.offset which provide the offset to the property from its own address. So actually in memory we can mutate let property.

Existential type using Self

A protocol that has associatedtype or uses Self type can't be existential type. But using Self for return type of method is only available in Swift4.2. (I think using Self for type of getter should be also available.) This technique makes Stubbale type-safely.


  • Swift Tools 4.2.0
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Last updated: Sat Mar 18 2023 08:43:31 GMT-0500 (GMT-05:00)