Swiftpack.co - Package - Brightify/Cuckoo


Mock your Swift objects!

CI Status Version Carthage compatible License Platform Slack Status


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

To have a chat, join our Slack team!

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 CuckooGenerator.

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. We currently support all features which fulfill this rule except for things listed in TODO. Due to the complexity of Swift it is not easy to check for all edge cases so if you find some unexpected behavior please report it in issues.


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


We are still missing support for some important features like:

  • inheritance (grandparent methods)
  • generics
  • type inference for instance variables (you need to write it explicitly, otherwise it will be replaced with "__UnknownType")

What will not be supported

Due to the limitations mentioned above, basically all things which don't allow overriding cannot be supported. 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 8+
  • Mac OSX 10.9+
  • tvOS 9+

We plan to add a watchOS 2+ support soon.


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"

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

# Define output file. Change "$PROJECT_DIR/${PROJECT_NAME}Tests" to your test's root source folder, if it's not the default name.
echo "Generated Mocks File = $OUTPUT_FILE"

# Define input directory. Change "${PROJECT_DIR}/${PROJECT_NAME}" to your project's root source folder, if it's not the default name.
echo "Mocks Input Directory = $INPUT_DIR"

# Generate mock files, include as many input files as you'd like to create mocks for.
"${PODS_ROOT}/Cuckoo/run" generate --testable "$PROJECT_NAME" \
--output "${OUTPUT_FILE}" \
"$INPUT_DIR/FileName1.swift" \
"$INPUT_DIR/FileName2.swift" \
# ... and so forth, the last line should never end with a backslash

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

Input files can be also specified directly in Run script in Input Files form. To force run script to rebuild generator even if it already exists, use --clean as first argument.

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


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

github "SwiftKit/Cuckoo"

Then use the Run script from above and replace




Also don't forget to add the Framework into your project.

2. Usage

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

Mock initialization

Mocks can be created with the same constructors as the mocked type. Name of mock class always corresponds to name of the mocked class/protocol with Mock prefix. For example mock of protocol Greeter has a name MockGreeter.

let mock = MockGreeter()


Spies are a special case of Mocks where each call is forwarded to the victim by default. From Cuckoo version 0.11.0 we changed the way spies work. When you need a spy, give Cuckoo a class to mock instead of a protocol. 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()

NOTE: The behavior was changed due to a limitation of Swift. Since we can't create a real proxy for the spy, calls inside the spy were not catched by the Mock and it was confusing. If you rely on the old behavior (i.e. you use spies with final classes), let us know on Slack or in the issues.


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

Which methods can be used depends on the stubbed method. For example you cannot use the thenThrow method with method which cannot throw exception.

The stubbing of method can look like this:

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

And for property:

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

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

Chain stubbing

It is possible to chain stubbing. This is useful if you need to set different behavior for multiple calls in order. The last behavior will last for all other calls. Syntax goes like this:


which is equivalent to:

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

In both cases first call to readWriteProperty will return 10 and every other 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 last call of when. This means that calling when multiple times with the same function and matchers effectively overrides 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 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 error. In case of spy, the real code will execute.


For verifying calls there is function verify. Its first parameter is mocked object, optional second parameter is 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.

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 make 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

Note: Optional types (for example Int?) cannot be used directly. You need to wrap them with equal(to) function (see bellow).

B) Custom matchers

If Cuckoo doesn't know to 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 your equality test.
        // ie: (try? tested.method()) == (try? value.method())

⚠️ If you try to match an object with an unknown or non-Matchable type, it could lead to:

Command failed due to signal: Segmentation fault: 11

For details and implementation example (with Alamofire), see this issue.

Parameter matchers

ParameterMatcher 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 non nil value.

Matching of nil can be achieved with equal(to: nil).

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 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 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 wanted 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. But you cannot 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 mocks.
reset<M: Mock>(_ mocks: M...)

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

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

Stub objects

Stubs are used in case when you want to suppress real code. Stubs are different from Mocks in that they don't support stubbing and 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. For example stub of protocol Greeter has a name GreeterStub.

let stub = GreeterStub()

When method or property is called on stub nothing happens. If some type has to be returned then DefaultValueRegistry will provide 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 in 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 with extensions. Custom types must be registered before usage with DefaultValueRegistry.register<T>(value: T, forType: T.Type). Default values can be changed with the same method. Sets, arrays, etc. do not have to be registered if their generic type is already registered.

Method DefaultValueRegistry.reset() can be used to delete all value registered by user.

Cuckoo generator


For normal use you can skip this because run script in Cuckoo downloads and builds correct version of the generator automatically.


This is more complicated path. You need to clone this repository and build it yourself. You can look in the run script for more inspiration.


Generator can be called through a terminal. Each call consists of command, options and arguments. Options and arguments depends on used command. Options can have additional parameters. Names of all of them are case sensitive. The order goes like this:

cuckoo command options arguments

generate command

Generates mock files.

This command accepts arguments, in this case list (separated by spaces) of files for which you want to generate mocks. Also more options can be used to adjust behavior, these are listed below.

--output (string)

Where to put the generated mocks.

If a path to a directory is supplied, each input file will have a respective output file with mocks.

If a path to a Swift file is supplied, all mocks will be in a single file.

Default value is GeneratedMocks.swift.

--testable (string)

A comma separated list of frameworks that should be imported as @testable in the mock files.

--exclude (string)

A comma separated list of classes and protocols that should be skipped during mock generation.


Do not generate file headers.


Do not generate timestamp.


Do not mock/stub parents and grandparents.


Names of generated files in directory will start with this prefix. Only works when output path is directory.


Do not generate mocks for classes.

version command

Prints the version of this generator.

help command

Display general or command-specific help.

After the help you can write name of another command for displaying a command-specific help.


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 Xcode 9.1 installed
  2. Clone the Cuckoo repository
  3. In Terminal, run: make dev from inside the Cuckoo directory.
  4. Open Cuckoo.xcodeproj and peek around

The project is made of two parts - runtime and code generator. When you open the Cuckoo.xcodeproj in Xcode, you'll see these directories: * Source - runtime sources * Tests - tests for the runtime part * CuckoGenerator.xcodeproj - project generated by swift package generate-xcodeproj for the Generator sources

Thank you for your help!


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Cuckoo is available under the MIT License.


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0.11.3 - Apr 5, 2018

  • Make sure we keep the cuckoo_generator in the Pods/Cuckoo dir.

0.11.2 - Apr 4, 2018

  • Fix handling of access_token and username in Fastfile.
  • Make sure we bundle Swift runtime with the released generator.
  • Disable bitcode

0.11.0 - Mar 23, 2018

  • Add test for sub protocol method
  • The sort of input files occurred to soon * The files input via options are mapped to Path objects then sorted. This actually happened too soon, since the array is then converted to a Set, then converted back to an Array. The Set conversion changes the order (since it is unordered) and those we lost the effect of the sort. I move the sort to the correct place, which is on the outermost Array so that the inputPathValues array is now correctly sorted.
  • Upgrade Fastfile and podspec. run downloads the latest generator if it's deleted.
  • Make the script a bit more resilient.
  • Fix changelog creating and make commit message ignoring broader.
  • Update Gemfile.
  • Add initial deployer script for deploying release drafts to github.
  • fix(readme): fix typos and grammar
  • doc(readme): add precision on custom matchers
  • Fix CI-status badge
  • Updated readme run script
  • Make the bash script safe to use with names that contain spaces.
  • Fixes a regression introduced in 96ebbbf * There was an attempt to make the file order deterministic, but the sorting that was added only worked if all mocks were generated in a single file. If generated to a path, which creates a file per mock, the filenames and the file contents did not match. Solution was to sort the set of paths passed in as options instead, moving the sorting a little earlier than before.
  • Fix sameInstance(as:) ambiguous reference compile-time error.
  • Added test-cases for excluding Classes and Protocols from Mock stub generation
  • Added ParameterMatcher for (optional) throwing closures and optionals, e.g. anyThrowingClosure(), anyOptionalThrowingClosure and null() Added Acceptance test for inheritance stubbing and verifying
  • Adding support for overloaded method names
  • Add a few more tests.
  • Change get in property verification to method instead of a property.
  • Remove TemplateEncoder and change Spy behavior.
  • Don't test iOS for now.
  • Disable signing.
  • Remove workspace and improve project setup.

0.6.0 - Jul 2, 2016

  • Added release notes
  • Added stub resetting
  • Added thenCallRealImplementation
  • Added argument capturing
  • Added verifyNoMoreInteractions
  • Added on going stubbing
  • Added thenDoNothing

0.5.0 - Jun 15, 2016

In this release we added a run script that you can use to easily run Cuckoo generator with ease. We also changed the Cuckoo Generator so that it no longer contains all the different generator versions, which means easier updating and adding new features!