A convenient mocking framework for Swift


Package managers License Slack

Mockingbird is a convenient mocking framework for Swift.

// Mocking
let bird = mock(Bird.self)

// Stubbing
given(bird.getName()) ~> "Ryan"

// Verification


Mockingbird uses code generation to create overridable mocks and stubs with similar semantics to Mockito.

At a high level, Mockingbird consists of two main components: the generator and the testing framework. Before each test bundle compilation, the generator creates an intermediary layer that implements protocols and subclasses classes. The testing framework provides hooks into the intermediary layer for mocking, stubbing, and verification during test runs.

Let’s start with a simple example!

protocol Bird {
  var canFly: Bool { get }
  func fly()

class Tree {
  let bird: Bird
  init(with bird: Bird) {
    self.bird = bird
  func shake() {
    guard bird.canFly else { return }

func testShakingTreeCausesBirdToFly() {
  // Given
  let bird = mock(Bird.self)
  let tree = Tree(with: bird) // a tree with a bird
  given(bird.getCanFly()) ~> true // that can fly
  // When
  tree.shake() // the tree is shaken
  // Then
  verify( // the bird flies away



Add the framework to a test target in your Podfile, making sure to include the use_frameworks! option.

target 'MyTestTarget' do
  pod 'MockingbirdFramework', '~> 0.11.0'

Initialize the pod.

$ pod install

Then install the CLI.

$ cd Pods/MockingbirdFramework
$ make install-prebuilt


Add the framework to your Cartfile.

github "birdrides/mockingbird" ~> 0.11.0

Build the framework using Carthage and link it to your test target, making sure to add the framework to a Copy Files build phase with the destination set to Frameworks.

$ carthage update --no-build
$ cp Carthage/Checkouts/mockingbird/Scripts/ ./
$ ./
Upcoming changes in Mockingbird 0.11.0
$ carthage update

Then install the CLI.

$ cd Carthage/Checkouts/mockingbird
$ make install-prebuilt

Swift Package Manager

Add as a dependency and link it to your test target.

Then download and install the latest CLI from Releases.

From Source

Clone the repository and build the MockingbirdFramework scheme for the desired platform. Drag the built Mockingbird.framework product into your project and link it to your test target.

$ git clone
$ cd mockingbird
$ open Mockingbird.xcodeproj

Then build and install the CLI.

$ make install


Use the CLI to configure a test target, listing all source targets that should generate mocks before each build. Below, mock types will be generated for the Bird app target and the BirdManagers framework target, which can then be used in BirdTests.

$ mockingbird install \
  --target BirdTests \
  --sources Bird BirdManagers

Need to set up your project manually?

System Framework Compatibility

Download the latest starter supporting source files and place the MockingbirdSupport folder in the root directory of your project. This provides basic compatibility with system frameworks such as UIKit. See Supporting Source Files for more information.

Excluding Files

You can exclude unwanted or problematic sources from being mocked by adding a .mockingbird-ignore file. Mockingbird follows the same pattern format as .gitignore and scopes ignore files to their enclosing directory.


Example projects demonstrating basic usage of Mockingbird:


Mock types can be passed in place of the original type and are suffixed with Mock. Avoid explicitly coercing mock types into their supertype, as this breaks stubbing and verification.

Protocol Mocks

Note that the initialized mock type is BirdMock instead of Bird.

let bird: BirdMock = mock(Bird.self)  // The concrete type is `BirdMock`
let inferredBird = mock(Bird.self)    // but type inference also works

Class Mocks

Initialize concrete class mocks using the initialize method. Keep in mind that class mocks rely on subclassing which has certain limitations, so consider using protocol mocks whenever possible.

class Bird {
  let name: String
  init(named name: String) { = name }
let bird = mock(Bird.self).initialize(named: "Ryan")


Stubbing allows you to define a custom value to return when a mocked method is called.

given(bird.getName()) ~> "Ryan"

You can use an argument matcher when stubbing methods with parameters. Stubs added later have precedence over those added earlier, so stubs containing specific matchers should be added last.

given(bird.canChirp(volume: any())) ~> false    // Matches any volume
given(bird.canChirp(volume: notNil())) ~> true  // Matches any non-nil volume
given(bird.canChirp(volume: 10)) ~> false       // Matches volume = 10

Stub variables with their getter and setter methods.

given(bird.getName()) ~> "Ryan"
given(bird.setName(any())) ~> { print($0) }

Getters can be stubbed to automatically save and return values.

given(bird.getName()) ~> lastSetValue(initial: "One") = "Two"
assert( == "Two")

Use a closure to implement complex stubs that contain logic or that interact with arguments.

given(bird.chirp(volume: any(), callback: any())) ~> { volume, callback in
  callback(volume * 2)

Closures also allow for stubbing methods that can throw.

given(bird.chirp(volume: any())) ~> { volume in
  if volume > 42 {
    throw BirdError.invalidVolume

It’s possible to stub multiple methods with the same return type in a single call.

) ~> "Ryan"


Verification lets you assert that a mock received a particular invocation during its lifetime.


It’s possible to verify that an invocation was called a specific number of times with a count matcher.

verify(            // n = 0
verify(      // n = 10
verify(      // n ≥ 10
verify(       // n ≤ 10
verify(  // 5 ≤ n ≤ 10

Count matchers also support chaining and negation using logical operators.

verify(           // n ≠ 10
verify(  // n = 10 || n ≤ 5

Sometimes you need to perform custom checks on received parameters by using an argument captor.

let nameCaptor = ArgumentCaptor<String>()

To enforce the relative order of invocations, use an inOrder block.

// Check that `fly` was called before `chirp`
inOrder {

You can test asynchronous code by using an eventually block which returns an XCTestExpectation.

DispatchQueue.main.async {
  Tree(with: bird).shake()
let expectation = eventually {
wait(for: [expectation], timeout: 1.0)

Verifying doesn’t remove recorded invocations, so it’s safe to call verify multiple times (even if not recommended).

verify(  // If this succeeds...
verify(  // ...this also succeeds

For methods overloaded by return type, you should help the compiler by specifying the type returned.

protocol Bird {
  func getMessage<T>() -> T
  func getMessage() -> String
  func getMessage() -> StaticString


Resetting Mocks

Occasionally it’s necessary to remove stubs or clear recorded invocations.

reset(bird)                 // Removes all stubs and recorded invocations
clearStubs(on: bird)        // Only removes stubs
clearInvocations(on: bird)  // Only removes recorded invocations

Argument Matching

Match arguments received by mocks for stubbing and verification. The parameter type must explicitly conform to Equatable or the arguments will be compared by reference.

// Many Swift standard library types such as `String` conform to `Equatable`

// Types that explicitly conform to `Equatable` work out of the box
struct Fruit: Equatable {
  let size: Int
verify( 42))).wasCalled()

// Classes can be safely compared by reference
class Tree {
  init(with bird: Bird) {
    bird.home = self
let tree = Tree(with: bird)

Argument matchers allow wildcard and custom matching of arguments.

any()                    // Matches any value
any(of: 1, 2, 3)         // Matches any value in {1, 2, 3}
any(where: { $0 > 42 })  // Matches any number greater than 42
notNil()                 // Matches any non-nil value

For methods overloaded by parameter type (such as with generics), using a matcher may cause ambiguity for the compiler. You can help the compiler by specifying an explicit type in the matcher.

any(Int.self, of: 1, 2, 3)
any(Int.self, where: { $0 > 42 })

You can also match elements or keys within collection types.

any(containing: 1, 2, 3)  // Matches any collection with values {1, 2, 3}
any(keys: "a", "b", "c")  // Matches any dictionary with keys {"a", "b", "c"}
any(count: atMost(42))    // Matches any collection with at most 42 elements
notEmpty()                // Matches any non-empty collection

Mathematical operations on floating point numbers can cause unexpected behavior, so consider using around to fuzzily match floating point arguments with some tolerance.

around(10.0, tolerance: 0.01)

Supporting Source Files

Add supporting source files to mock inherited types defined outside of your project. You should always provide supporting source files when working with system frameworks like UIKit or precompiled external dependencies.

/* MockingbirdSupport/Swift/Codable.swift */

public protocol Encodable {
  func encode(to encoder: Encoder) throws

public protocol Decodable {
  init(from decoder: Decoder) throws

public typealias Codable = Decodable & Encodable

Starter Pack

Mockingbird includes starter supporting source files for Foundation, UIKit, and other common system frameworks. Download the latest starter supporting source files and place the MockingbirdSupport folder in the root directory of your project.

If you share supporting source files between projects, you can specify a custom --support directory when running the CLI installer or generator.


Supporting source files should be contained in a directory that matches the module name. You can define submodules and transitive dependencies by nesting directories.

├── Foundation/
│   └── ObjectiveC/
│       └── NSObject.swift
└── Swift/
    ├── Codable.swift
    ├── Comparable.swift
    ├── Equatable.swift
    └── Hashable.swift

With the above file structure, NSObject can be imported from both the Foundation and ObjectiveC modules.

Mockingbird CLI


Generate mocks for a set of targets in a project.

mockingbird generate

Option Default Value Description
--project (inferred) Path to your project’s .xcodeproj file.
--targets $TARGET_NAME List of target names to generate mocks for.
--srcroot $SRCROOT The folder containing your project’s source files.
--outputs (inferred) List of mock output file paths for each target.
--support (inferred) The folder containing supporting source files.
--condition (none) Compilation condition to wrap all generated mocks in, e.g. DEBUG.
Flag Description
--disable-module-import Omit @testable import <module> from generated mocks.
--only-protocols Only generate mocks for protocols.
--disable-swiftlint Disable all SwiftLint rules in generated mocks.
--disable-cache Ignore cached mock information stored on disk.
--disable-relaxed-linking Only search explicitly imported modules.


Set up a destination (unit test) target.

mockingbird install

Option Default Value Description
--target (required) The target name where Mockingbird will be installed.
--sources (required) List of target names to generate mocks for.
--project (inferred) Your project’s .xcodeproj file.
--srcroot <project>/../ The folder containing your project’s source files.
--outputs (inferred) List of mock output file paths for each target.
--support (inferred) The folder containing supporting source files.
--condition (none) Compilation condition to wrap all generated mocks in, e.g. DEBUG.
--loglevel (none) The log level to use when generating mocks, quiet or verbose
Flag Description
--preserve-existing Don’t overwrite previously installed configurations.
--asynchronous Generate mocks asynchronously in the background when building.
--only-protocols Only generate mocks for protocols.
--disable-swiftlint Disable all SwiftLint rules in generated mocks.
--disable-cache Ignore cached mock information stored on disk.
--disable-relaxed-linking Only search explicitly imported modules.


Remove Mockingbird from a (unit test) target.

mockingbird uninstall

Option Default Value Description
--targets (required) List of target names to uninstall the Run Script Phase.
--project (inferred) Your project’s .xcodeproj file.
--srcroot <project>/../ The folder containing your project’s source files.

Global Options

Flag Description
--verbose Log all errors, warnings, and debug messages.
--quiet Only log error messages.

Inferred Paths


Mockingbird will first check if the environment variable $PROJECT_FILE_PATH was set (usually by an Xcode build context). It will then perform a shallow search of the current working directory for an .xcodeproj file. If multiple .xcodeproj files exist then you must explicitly provide a project file path.


By default Mockingbird will generate mocks into the $(SRCROOT)/MockingbirdMocks directory with the file name $(PRODUCT_MODULE_NAME)Mocks.generated.swift.


Mockingbird will recursively look for supporting source files in the $(SRCROOT)/MockingbirdSupport directory.


Mocks don’t exist or are out of date

Mocks are generated when the test target is built and run. Run tests once and check that generated mock files appear in $(SRCROOT)/MockingbirdMocks and are not empty. If nothing is generated or the files contain no mocks then something is wrong with the installation.

Generated mock does not compile

Ensure that the project has supporting source files. Common compiler errors from not having supporting source files:

  • MyTypeMock does not conform to protocol NSObjectProtocol
  • Superclass must appear first in the inheritance clause
  • Type can only be used as a generic constraint because it has Self or associated type requirements

If there are supporting source files and the error is related to inheritance, you may need to add a new supporting source file with definitions for the inherited type.

If the issue is unrelated to inheritance, you may have found a generator bug. If all else fails, exclude the problematic source file and file an issue.

Editor placeholder in source file warning

Generated mocks will contain the editor placeholder __UnknownType__ for types that could not be inferred. Help the generator by adding explicit type annotations to the definition.

Cannot call stubbing or verification functions

Ensure that Mockingbird is imported at the top of the test file.

Expression type is ambiguous without more context error

This usually happens when trying to stub or verify a mock that was explicitly coerced into its supertype. Make sure the variable storing the mock has the concrete mock type, e.g. MyTypeMock instead of MyType.

Tests crash with an unable to load framework, image not found error

Link Mockingbird and ensure that it’s included in the test bundle by adding it to the Copy Files build phase.

Unable to stub or verify methods with arguments

Ensure that all parameter types explicitly conform to Equatable or work when compared by reference. Note that struct types that implicitly conform to Equatable have undefined behavior. Use a wildcard argument matcher such as any() or any(where:) to match non-equatable or implicitly equatable types.

Debugging a configured build phase

Open the test target build phase and expand the build phase named Generate Mockingbird Mocks. If no phase exists or the listed targets seem incorrect, set up the test target again.

Debugging the generator

Open the Xcode report navigator and select the Build entry for the most recent test run. Find the log message for Run custom shell script 'Generate Mockingbird Mocks' and check for any relevant warnings or errors. To increase the log verbosity, specify --verbose in the configured build phase.

Additional Resources


  • Swift Tools


  • None
Last updated: Tue Apr 20 2021 04:55:33 GMT-0500 (GMT-05:00)