Boilerplate-free mocking framework for Swift!

What's New


  • Merge all protocol generic parameters and remove DefaultImplCaller generic parameters.


Mock your Swift objects!

Version SwiftPM compatible Carthage compatible License Platform


Cuckoo was created due to lack of a proper Swift mocking framework. We built the DSL to be very similar to Mockito, so anyone coming from Java/Android can immediately pick it up and use it.

How does it work

Cuckoo has two parts. One is the runtime and the other one is an OS X command-line tool simply called Cuckoonator.

Unfortunately Swift does not have a proper reflection, so we decided to use a compile-time generator to go through files you specify and generate supporting structs/classes that will be used by the runtime in your test target.

The generated files contain enough information to give you the right amount of power. They work based on inheritance and protocol adoption. This means that only overridable things can be mocked. Due to the complexity of Swift it is not easy to check for all edge cases so if you find some unexpected behavior, please file an issue.


List of all changes and new features can be found here.


Cuckoo is a powerful mocking framework that supports:

  • inheritance (grandparent methods)
  • generics
  • simple type inference for instance variables (works with initializers, as TYPE notation, and can be overridden by specifying type explicitly)
  • Objective-C mocks utilizing OCMock

What will not be supported

Due to the limitations mentioned above, unoverridable code structures are not supportable by Cuckoo. This includes:

  • struct - workaround is to use a common protocol
  • everything with final or private modifier
  • global constants and functions
  • static properties and methods


Cuckoo works on the following platforms:

  • iOS 13+
  • Mac OSX 11+
  • tvOS 13+

watchOS support is not yet possible due to missing XCTest library.


1. Installation


Cuckoo runtime is available through CocoaPods. To install it, simply add the following line to your test target in your Podfile:

pod 'Cuckoo', '~> 2.0'

And add the following Run script build phase to your test target's Build Phases above the Compile Sources phase:

# Skip for indexing
if [ $ACTION == "indexbuild" ]; then
  exit 0

# Skip for preview builds
if [ "${ENABLE_PREVIEWS}" = "YES" ]; then
  exit 0


After running once, locate GeneratedMocks.swift and drag it into your Xcode test target group.

IMPORTANT: To make your mocking journey easier, make absolutely sure that the run script is above the Compile Sources phase.

NOTE: To avoid race condition errors when Xcode parallelizes build phases, add the path of the OUTPUT_FILE into the "Output Files" section of the build phase. If you find that OUTPUT_FILE still doesn't regenerate with new changes, adding mocked files to the "Input Files" section of the build phase might help.

NOTE: From Xcode 15 flag ENABLE_USER_SCRIPT_SANDBOXING in Build Settings is Yes by default. That means Xcode will sandbox the script so reading input files and writing output file will be forbidden. As a result running above script may fail to access the files. To prevent Xcode from sandboxing the script, change this option to No.

Input files can be also specified directly in Run script in Input Files form.

Note: All paths in the Run script must be absolute. Variable PROJECT_DIR automatically points to your project directory. Remember to include paths to inherited Classes and Protocols for mocking/stubbing parent and grandparents.

Swift Package Manager

  1. In Xcode, navigate in menu: File > Swift Packages > Add Package Dependency
  2. Add
  3. For the Dependency Rule, Select "Up to Next Major" with 2.0.0. Click Add Package.
  4. On the 'Choose Package Products for Cuckoo' dialog, under 'Add to Target', please ensure you select your Test target as it will not compile on the app target.
  5. Click Add Package.

When you're all set, go to your test target's Build Phases and add CuckooPluginSingleFile to the Run Build Tool Plug-ins.


To use Cuckoo with Carthage add this line to your Cartfile:

github "Brightify/Cuckoo"

Then use the Run script from above and replace




Don't forget to add the Framework into your project.

2. Cuckoofile customization

At the root of your project, create Cuckoofile.toml configuration file:

# You can define a fallback output for all modules that don't define their own.
output = "Tests/Swift/Generated/GeneratedMocks.swift"

output = "Tests/Swift/Generated/GeneratedMocks+MyProject.swift"
imports = ["Foundation"]
testableImports = ["RxSwift"]
sources = [
exclude = ["ExcludedTestClass"]
# Optionally you can use a regular expression to filter only specific classes/protocols.
regex = ""

# glob = false
# Docstrings are preserved by default, comments are omitted.
keepDocumentation = false
# enableInheritance = false
# protocolsOnly = true
# omitHeaders = true

# If specified, Cuckoo can also get sources for the module from an Xcode target.
path = "Generator"
target = "Cuckoonator"

3. Usage

Usage of Cuckoo is similar to Mockito and Hamcrest. However, there are some differences and limitations caused by generating the mocks and Swift language itself. List of all the supported features can be found below. You can find complete examples in tests.

Mock initialization

Mocks can be created with the same constructors as the mocked type. Name of mock class always corresponds to the name of the mocked class/protocol with Mock prefix (e.g. mock of protocol Greeter is called MockGreeter).

let mock = MockGreeter()


Spies are a special type of Mocks where each call is forwarded to the victim by default. When you need a spy, give Cuckoo a class, then you'll then be able to call enableSuperclassSpy() (or withEnabledSuperclassSpy()) on a mock instance and it will behave like a spy for the parent class.

let spy = MockGreeter().withEnabledSuperclassSpy()


Stubbing can be done by calling methods as a parameter of the when function. The stub call must be done on special stubbing object. You can get a reference to it with the stub function. This function takes an instance of the mock that you want to stub and a closure in which you can do the stubbing. The parameter of this closure is the stubbing object.

Note: It is currently possible for the subbing object to escape from the closure. You can still use it to stub calls but it is not recommended in practice as the behavior of this may change in the future.

After calling the when function you can specify what to do next with following methods:

/// Invokes `implementation` when invoked.
then(_ implementation: IN throws -> OUT)

/// Returns `output` when invoked.
thenReturn(_ output: OUT, _ outputs: OUT...)

/// Throws `error` when invoked.
thenThrow(_ error: ErrorType, _ errors: Error...)

/// Invokes real implementation when invoked.

/// Does nothing when invoked.

The available methods depend on the stubbed method characteristics. For example, the thenThrow method isn't available for a method that isn't throwing or rethrowing.

An example of stubbing a method looks like this:

stub(mock) { stub in
  when(stub.greetWithMessage("Hello world")).then { message in

As for a property:

stub(mock) { stub in
  when(stub.readWriteProperty.set(anyInt())).then {

Notice the get and set, these will be used in verification later.

Enabling default implementation

In addition to stubbing, you can enable default implementation using an instance of the original class that's being mocked. Every method/property that is not stubbed will behave according to the original implementation.

Enabling the default implementation is achieved by simply calling the provided method:

let original = OriginalClass<Int>(value: 12)

For passing classes into the method, nothing changes whether you're mocking a class or a protocol. However, there is a difference if you're using a struct to conform to the original protocol we are mocking:

let original = ConformingStruct<String>(value: "Hello, Cuckoo!")
// or if you need to track changes:
mock.enableDefaultImplementation(mutating: &original)

Note that this only concerns structs. enableDefaultImplementation(_:) and enableDefaultImplementation(mutating:) are different in state tracking.

The standard non-mutating method enableDefaultImplementation(_:) creates a copy of the struct for default implementation and works with that. However, the mutating method enableDefaultImplementation(mutating:) takes a reference to the struct and the changes of the original are reflected in the default implementation calls even after enabling default implementation.

We recommend using the non-mutating method for enabling default implementation unless you need to track the changes for consistency within your code.

Chain stubbing

It is possible to chain stubbing. This is useful for when you need to define different behavior for multiple calls in order. The last behavior will be used for all calls after that. The syntax goes like this:


which is equivalent to:

when(stub.readWriteProperty.get).thenReturn(10, 20)

The first call to readWriteProperty will return 10 and all calls after that will return 20.

You can combine the stubbing methods as you like.

Overriding of stubbing

When looking for stub match Cuckoo gives the highest priority to the last call of when. This means that calling when multiple times with the same function and matchers effectively overrides the previous call. Also more general parameter matchers have to be used before specific ones.


In this example calling countCharacters with a will return 1. If you reversed the order of stubbing then the output would always be 10.

Usage in real code

After previous steps the stubbed method can be called. It is up to you to inject this mock into your production code.

Note: Call on mock which wasn't stubbed will cause an error. In case of a spy, the real code will execute.


For verifying calls there is function verify. Its first parameter is the mocked object, optional second parameter is the call matcher. Then the call with its parameters follows.

verify(mock).greetWithMessage("Hello world")

Verification of properties is similar to their stubbing.

You can check if there are no more interactions on mock with function verifyNoMoreInteractions.

With Swift's generic types, it is possible to use a generic parameter as the return type. To properly verify these methods, you need to be able to specify the return type.

// Given:
func genericReturn<T: Codable>(for: String) -> T? { ... }

// Verify
verify(mock).genericReturn(for: any()).with(returnType: String?.self)
Argument capture

You can use ArgumentCaptor to capture arguments in verification of calls (doing that in stubbing is not recommended). Here is an example code:

mock.readWriteProperty = 10
mock.readWriteProperty = 20
mock.readWriteProperty = 30

let argumentCaptor = ArgumentCaptor<Int>()
verify(mock, times(3)).readWriteProperty.set(argumentCaptor.capture())
argumentCaptor.value // Returns 30
argumentCaptor.allValues // Returns [10, 20, 30]

As you can see, method capture() is used to create matcher for the call and then you can get the arguments via properties value and allValues. value returns last captured argument or nil if none. allValues returns array with all captured values.

3. Matchers

Cuckoo makes use of matchers to connect your mocks to your code under test.

A) Automatic matchers for known types

You can mock any object that conforms to the Matchable protocol. These basic values are extended to conform to Matchable:

  • Bool
  • String
  • Float
  • Double
  • Character
  • Int
  • Int8
  • Int16
  • Int32
  • Int64
  • UInt
  • UInt8
  • UInt16
  • UInt32
  • UInt64

Matchers for Array, Dictionary, and Set are automatically synthesized as long as the type of the element conforms to Matchable.

B) Custom matchers

If Cuckoo doesn't know the type you are trying to compare, you have to write your own method equal(to:) using a ParameterMatcher. Add this method to your test file:

func equal(to value: YourCustomType) -> ParameterMatcher<YourCustomType> {
  return ParameterMatcher { tested in
    // Implementation of equality test for your custom type.

⚠️ Trying to match an object with an unknown or non-Matchable type may lead to:

Command failed due to signal: Segmentation fault: 11

For details or an example (with Alamofire), see this issue.

Parameter matchers

ParameterMatcher itself also conforms to Matchable. You can create your own ParameterMatcher instances or if you want to directly use your custom types there is the Matchable protocol. Standard instances of ParameterMatcher can be obtained via these functions:

/// Returns an equality matcher.
equal<T: Equatable>(to value: T)

/// Returns an identity matcher.
equal<T: AnyObject>(to value: T)

/// Returns a matcher using the supplied function.
equal<T>(to value: T, equalWhen equalityFunction: (T, T) -> Bool)

/// Returns a matcher matching any Int value.

/// Returns a matcher matching any String value.

/// Returns a matcher matching any T value or nil.
any<T>(type: T.Type = T.self)

/// Returns a matcher matching any closure.

/// Returns a matcher matching any throwing closure.

/// Returns a matcher matching any non nil value.

Cuckoo also provides plenty of convenience matchers for sequences and dictionaries, allowing you to check if a sequence is a superset of a certain sequence, contains at least one of its elements, or is completely disjunct from it.

Matchable can be chained with methods or and and like so:

verify(mock).greetWithMessage("Hello world".or("Hallo Welt"))

Call matchers

As a second parameter of the verify function you can use instances of CallMatcher. Its primary function is to assert how many times was the call made. But the matches function has a parameter of type [StubCall] which means you can use a custom CallMatcher to inspect the stub calls or for some side effect.

Note: Call matchers are applied after the parameter matchers. So you get only stub calls of the desired method with correct arguments.

Standard call matchers are:

/// Returns a matcher ensuring a call was made `count` times.
times(_ count: Int)

/// Returns a matcher ensuring no call was made.

/// Returns a matcher ensuring at least one call was made.

/// Returns a matcher ensuring call was made at least `count` times.
atLeast(_ count: Int)

/// Returns a matcher ensuring call was made at most `count` times.
atMost(_ count: Int)

As with Matchable you can chain CallMatcher with methods or and and. However, you can't mix Matchable and CallMatcher together.

Resetting mocks

Following functions are used to reset stubbing and/or invocations on mocks.

/// Clears all invocations and stubs of given mocks.
reset<M: Mock>(_ mocks: M...)

/// Clears all stubs of given mocks.
clearStubs<M: Mock>(_ mocks: M...)

/// Clears all invocations of given mocks.
clearInvocations<M: Mock>(_ mocks: M...)

Stub objects

Stubs are used for suppressing real code. Stubs are different from Mocks in that they don't support stubbing nor verification. They can be created with the same constructors as the mocked type. Name of stub class always corresponds to name of the mocked class/protocol with Stub suffix (e.g. stub of protocol Greeter is called GreeterStub).

let stub = GreeterStub()

When a method is called or a property accessed/set on a stub, nothing happens. If a value is to be returned from a method or a property, DefaultValueRegistry provides a default value. Stubs can be used to set implicit (no) behavior to mocks without the need to use thenDoNothing() like this: MockGreeter().spy(on: GreeterStub()).


DefaultValueRegistry is used by Stubs to get default values for return types. It knows only default Swift types, sets, arrays, dictionaries, optionals, and tuples (up to 6 values). Tuples for more values can be added through extensions. Custom types must be registered before use with DefaultValueRegistry.register<T>(value: T, forType: T.Type). Furthermore, default values set by Cuckoo can also be overridden by this method. Sets, arrays, etc. do not have to be registered if their generic type is already registered.

DefaultValueRegistry.reset() returns the registry to its clean slate before the register method made any changes.

Type inference

Cuckoo does a simple type inference on all variables which allows for much cleaner source code on your side. There are a total 3 ways the inference tries to extract the type name from a variable:

// From the explicitly declared type:
let constant1: MyType

// From the initial value:
let constant2 = MyType(...)

// From the explicitly specified type `as MyType`:
let constant3 = anything as MyType

Cuckoo run script

Build Options

These options are only used for downloading or building the generator and don't interfere with the result of the generated mocks.

When the run script is executed without any parameters, it simply searches for the cuckoonator file and builds it from source code if it's missing, running the generator afterwards.

To download the generator from GitHub instead of building it, use the --download option as the first argument (i.e. run --download). If you're having issues with rather long build time (especially in CI), this might be the way to fix it.

NOTE: If you encounter Github API rate limit using the --download option, the run script refers to the environment variable GITHUB_ACCESS_TOKEN. Add this line (replacing the Xs with your GitHub token, no additional permissions are needed) to the script build phase above the run call:


The build option --clean forces either build or download of the version specified even if the generator is present. The run script also syncs the generator to the correct version (either by building from source or downloading with --download) if needed.

We recommend only using --clean when you're trying to fix a compile problem as it forces the build (or download) every time which makes the testing way longer than it needs to be.

Objective-C Support

Cuckoo subspec Cuckoo/OCMock brings support for mocking Objective-C classes and protocols.

Example usage:

let tableView = UITableView()
// stubbing the class is very similar to stubbing with Cuckoo
let mock = objcStub(for: UITableViewController.self) { stubber, mock in
  stubber.when(mock.numberOfSections(in: tableView)).thenReturn(1)
  stubber.when(mock.tableView(tableView, accessoryButtonTappedForRowWith: IndexPath(row: 14, section: 2))).then { args in
    // `args` is [Any] of the arguments passed and the closure needs to cast them manually
    let (tableView, indexPath) = (args[0] as! UITableView, args[1] as! IndexPath)
    print(tableView, indexPath)

// calling stays the same
XCTAssertEqual(mock.numberOfSections(in: tableView), 1)
mock.tableView(tableView, accessoryButtonTappedForRowWith: IndexPath(row: 14, section: 2))

// `objcVerify` is used to verify the interaction with the methods/variables
objcVerify(mock.numberOfSections(in: tableView))
objcVerify(mock.tableView(tableView, accessoryButtonTappedForRowWith: IndexPath(row: 14, section: 2)))

Detailed usage is available in Cuckoo tests along with DOs and DON'Ts of this Swift-ObjC bridge.

So far, only CocoaPods is supported. To install, simply add this line to your Podfile:

pod 'Cuckoo/OCMock'


Cuckoo is open for everyone and we'd like you to help us make the best Swift mocking library. For Cuckoo development, follow these steps:

  1. Make sure you have the latest stable version of Xcode installed.
  2. Clone the Cuckoo repository.
  3. In terminal, run make at the root of the cloned Cuckoo repository, this will generate the project, install dependencies, and open the project in Xcode.
  4. Select any scheme of Cuckoo-iOS, Cuckoo-tvOS, or Cuckoo-macOS (OCMock schemes contain Cuckoo_OCMock instead) and verify by running the tests (⌘+U).
  5. Peek around or file a pull request with your changes.
  6. Make sure to run make again whenever you checkout another branch.

The project consists of two parts - runtime and code generator. When you open the Cuckoo.xcworkspace in Xcode, you'll see these directories: - Source - runtime sources - Tests - tests for the runtime part - Generator.xcodeproj - project containing generator source code (use the Generator scheme)

Thank you for your help!


Used libraries


Cuckoo is available under the MIT License.


  • Swift Tools 5.7.0
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Last updated: Fri May 24 2024 13:03:51 GMT-0900 (Hawaii-Aleutian Daylight Time)