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Welcome to Mockolo

Mockolo is an efficient mock generator for Swift. Swift doesn't provide mocking support, and Mockolo provides a fast and easy way to autogenerate mock objects that can be tested in your code. One of the main objectives of Mockolo is fast performance. Unlike other frameworks, Mockolo provides highly performant and scalable generation of mocks via a lightweight commandline tool, so it can run as part of a linter or a build if one chooses to do so. Try Mockolo and enhance your project's test coverage in an effective, performant way.


One of the main objectives of this project is high performance. There aren't many 3rd party tools that perform fast on a large codebase containing, for example, over 2M LoC or over 10K protocols. They take several hours and even with caching enabled take several minutes. Mockolo was built to make highly performant generation of mocks possible (in the magnitude of seconds) on such large codebase. It uses a minimal set of frameworks necessary (mentioned in the Used libraries section) to keep the code lean and efficient.

Another objective is to enable flexibility in using or overriding types if needed. This allows use of some of the features that require deeper analysis such as protocols with associated types to be simpler, more straightforward, and less fragile.


This project may contain unstable APIs which may not be ready for general use. Support and/or new releases may be limited.

System Requirements

  • Swift 5.7 or later
  • Xcode 14.2 or later
  • macOS 12.0 or later and Linux
  • Support is included for the Swift Package Manager

Build / Install

Option 1: By Mint

$ mint install uber/mockolo
$ mint run uber/mockolo mockolo -h // see commandline input options below

Option 2: Homebrew

$ brew install mockolo

Option 3: Use as Build Tools Plugin with Swift Package Manager

Add binaryTarget and plugin definition to your Package.swift. Binary url and checksum can be found in the releases page.

targets: [
        name: "RunMockolo",
        capability: .buildTool(),
        dependencies: [.target(name: "mockolo")]
        name: "mockolo",
        url: "...",
        checksum: "..."

Implement the plugin and specify necessary directories in the arguments.

  • Plugins/RunMockolo/RunMockoloPlugin.swift
import PackagePlugin

@main struct RunMockoloPlugin: BuildToolPlugin {
    func createBuildCommands(context: PluginContext, target: Target) async throws -> [Command] {
        let generatedSourcePath = context.pluginWorkDirectory.appending("GeneratedMocks.swift")
        let packageRoot =

        return [
                displayName: "Run mockolo",
                executable: try context.tool(named: "mockolo").path,
                arguments: [
                    "-s", packageRoot.appending("Sources", "MyModule").string,
                    "-d", generatedSourcePath,
                outputFilesDirectory: context.pluginWorkDirectory

Finally, add the plugin to the target requiring mockolo.

    name: "MyTarget",
    dependencies: [
    plugins: [
        .plugin(name: "RunMockolo"),

Option 4: Use the binary

Go to the Release tab and download/install the binary directly.

Option 5: Clone and build/run

$ git clone
$ cd mockolo
$ swift build -c release
$ .build/release/mockolo -h  // see commandline input options below

To call mockolo from any location, copy the executable into a directory that is part of your PATH environment variable.


Mockolo is a commandline executable. To run it, pass in a list of the source file directories or file paths of a build target, and the destination filepath for the mock output. To see other arguments to the commandline, run mockolo --help.

./mockolo -s myDir -d ./OutputMocks.swift -x Images Strings

This parses all the source files in myDir directory, excluding any files ending with Images or Strings in the file name (e.g. MyImages.swift), and generates mocks to a file at OutputMocks.swift in the current directory.

Use --help to see the complete argument options.

./mockolo -h  // or --help

OVERVIEW: Mockolo: Swift mock generator.

USAGE: mockolo [<options>] --destination <destination>

  --allow-set-call-count  If set, generated *CallCount vars will be allowed to set manually.
  --annotation <annotation>
                          A custom annotation string used to indicate if a type should be mocked (default = @mockable). (default: @mockable)
  -j, --concurrency-limit <n>
                          Maximum number of threads to execute concurrently (default = number of cores on the running machine).
  --custom-imports <custom-imports>
                          If set, custom module imports (separated by a space) will be added to the final import statement list.
  --enable-args-history   Whether to enable args history for all functions (default = false). To enable history per function, use the 'history' keyword in the annotation argument.
                          Whether to disable generating Combine streams in mocks (default = false). Set this to true to control how your streams are created in your mocks.
  --exclude-imports <exclude-imports>
                          If set, listed modules (separated by a space) will be excluded from the import statements in the mock output.
  -x, --exclude-suffixes <exclude-suffixes>
                          List of filename suffix(es) without the file extensions to exclude from parsing (separated by a space).
  --header <header>       A custom header documentation to be added to the beginning of a generated mock file.
  -l, --logging-level <n> The logging level to use. Default is set to 0 (info only). Set 1 for verbose, 2 for warning, and 3 for error. (default: 0)
  --macro <macro>         If set, #if [macro] / #endif will be added to the generated mock file content to guard compilation.
  --mock-all              If set, it will mock all types (protocols and classes) with a mock annotation (default is set to false and only mocks protocols with a mock annotation).
  --mock-filelist <mock-filelist>
                          Path to a file containing a list of dependent files (separated by a new line) of modules this target depends on.
  --mock-final            If set, generated mock classes will have the 'final' attributes (default is set to false).
  -mocks, --mockfiles <mocks>
                          List of mock files (separated by a space) from modules this target depends on. If the --mock-filelist value exists, this will be ignored.
  -d, --destination <destination>
                          Output file path containing the generated Swift mock classes. If no value is given, the program will exit.
  -s, --sourcedirs <sourcedirs>
                          Paths to the directories containing source files to generate mocks for (separated by a space). If the --filelist or --sourcefiles values exist, they will be ignored.
  -f, --filelist <filelist>
                          Path to a file containing a list of source file paths (delimited by a new line). If the --sourcedirs value exists, this will be ignored.
  -srcs, --sourcefiles <srcs>
                          List of source files (separated by a space) to generate mocks for. If the --sourcedirs or --filelist value exists, this will be ignored.
  -i, --testable-imports <testable-imports>
                          If set, @testable import statements will be added for each module name in this list (separated by a space).
  --use-mock-observable   If set, a property wrapper will be used to mock RxSwift Observable variables (default is set to false).
  --use-template-func     If set, a common template function will be called from all functions in mock classes (default is set to false).
  -h, --help              Show help information.


The will build and package up the mockolo binary and other necessary resources in the same bundle.

$ ./ -h  // see input options
$ ./ -s [source dir] -t mockolo -d [destination dir] -o [output filename].tar.gz

This will create a tarball for distribution, which contains the mockolo executable.

How to use

For example, Foo.swift contains:

/// @mockable
public protocol Foo {
    var num: Int { get set }
    func bar(arg: Float) -> String

Running ./mockolo -srcs Foo.swift -d ./OutputMocks.swift will output:

public class FooMock: Foo {
    init() {}
    init(num: Int = 0) {
        self.num = num

    var numSetCallCount = 0
    var underlyingNum: Int = 0
    var num: Int {
        get {
            return underlyingNum
        set {
            underlyingNum = newValue
            numSetCallCount += 1

    var barCallCount = 0
    var barHandler: ((Float) -> (String))?
    func bar(arg: Float) -> String {
        barCallCount += 1
        if let barHandler = barHandler {
            return barHandler(arg)
        return ""

The above mock can now be used in a test as follows:

func testMock() {
    let mock = FooMock(num: 5)
    XCTAssertEqual(mock.numSetCallCount, 1)
    mock.barHandler = { arg in
        return String(arg)
    XCTAssertEqual(mock.barCallCount, 1)


A list of override arguments can be passed in (delimited by a semicolon) to the annotation to set var types, typealiases, module names, etc.


/// @mockable(module: prefix = Bar)
public protocol Foo {

This will generate:

public class FooMock: Bar.Foo {


/// @mockable(typealias: T = AnyObject; U = StringProtocol)
public protocol Foo {
    associatedtype T
    associatedtype U: Collection where U.Element == T
    associatedtype W

This will generate the following mock output:

public class FooMock: Foo {
    typealias T = AnyObject // overridden
    typealias U = StringProtocol // overridden
    typealias W = Any // default placeholder type for typealias


For a var type such as an RxSwift observable:

/// @mockable(rx: intStream = ReplaySubject; doubleStream = BehaviorSubject)
public protocol Foo {
    var intStream: Observable<Int> { get }
    var doubleStream: Observable<Double> { get }

This will generate:

public class FooMock: Foo {
    var intStreamSubject = ReplaySubject<Int>.create(bufferSize: 1)
    var intStream: Observable<Int> { /* use intStreamSubject */ }
    var doubleStreamSubject = BehaviorSubject<Int>(value: 0)
    var doubleStream: Observable<Int> { /* use doubleStreamSubject */ }

Function Argument History

To capture function arguments history:

/// @mockable(history: fooFunc = true)
public protocol Foo {
    func fooFunc(val: Int)
    func barFunc(_ val: (a: String, Float))
    func bazFunc(val1: Int, val2: String)

This will generate:

public class FooMock: Foo {
    var fooFuncCallCount = 0
    var fooFuncArgValues = [Int]() // arguments captor
    var fooFuncHandler: ((Int) -> ())?
    func fooFunc(val: Int) {
        fooFuncCallCount += 1
        fooFuncArgValues.append(val)   // capture arguments

        if fooFuncHandler = fooFuncHandler {

    var barFuncArgValues = [(a: String, Float)]() // tuple is also supported.

    var bazFuncArgValues = [(Int, String)]()

and also, enable the arguments captor for all functions if you passed --enable-args-history arg to mockolo command.

NOTE: The arguments captor only supports singular types (e.g. variable, tuple). The closure variable is not supported.

Combine's AnyPublisher

To generate mocks for Combine's AnyPublisher:

/// @mockable(combine: fooPublisher = PassthroughSubject; barPublisher = CurrentValueSubject)
public protocol Foo {
    var fooPublisher: AnyPublisher<String, Never> { get }
    var barPublisher: AnyPublisher<Int, CustomError> { get }

This will generate:

public class FooMock: Foo {
    public init() { }

    public var fooPublisher: AnyPublisher<String, Never> { return self.fooPublisherSubject.eraseToAnyPublisher() }
    public private(set) var fooPublisherSubject = PassthroughSubject<String, Never>()

    public var barPublisher: AnyPublisher<Int, CustomError> { return self.barPublisherSubject.eraseToAnyPublisher() }
    public private(set) var barPublisherSubject = CurrentValueSubject<Int, CustomError>(0)

You can also connect an AnyPublisher to a property within the protocol.

For example:

/// @mockable(combine: fooPublisher = @Published foo)
public protocol Foo {
    var foo: String { get }
    var fooPublisher: AnyPublisher<String, Never> { get }

This will generate:

public class FooMock: Foo {
    public init() { }
    public init(foo: String = "") { = foo

    public private(set) var fooSetCallCount = 0
    @Published public var foo: String = "" { didSet { fooSetCallCount += 1 } }

    public var fooPublisher: AnyPublisher<String, Never> { return self.$foo.setFailureType(to: Never.self).eraseToAnyPublisher() }


To override the generated mock name:

/// @mockable(override: name = FooMock)
public protocol FooProtocol { ... }

This will generate:

public class FooMock: FooProtocol { ... }

Used libraries

SwiftSyntax |

How to contribute to Mockolo

See CONTRIBUTING for more info.

Report any issues

If you run into any problems, please file a git issue. Please include:

  • The OS version (e.g. macOS 10.14.6)
  • The Swift version installed on your machine (from swift --version)
  • The Xcode version
  • The specific release version of this source code (you can use git tag to get a list of all the release versions or git log to get a specific commit sha)
  • Any local changes on your machine


Mockolo is licensed under Apache License 2.0. See LICENSE for more information.

Copyright (C) 2017 Uber Technologies

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
See the License for the specific language governing permissions and
limitations under the License.