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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 4.2 or later
  • Xcode 10.2 or later
  • MacOS 10.14.3 or later
  • Support is included for the Swift Package Manager

Build / Install

First, clone the project.

$ git clone https://github.com/uber/mockolo.git
$ cd mockolo

Optionally, see a list of released versions of Mockolo, and check one out by running the following.

$ git tag -l
$ git checkout [tag]

Run the following to make a release build.

$ swift build -c release

This will create a binary called mockolo in the .build/release directory.

To install, just copy this executable into a directory that is part of your PATH environment variable.

To use Xcode, run the following.

$ swift package generate-xcodeproj

Add MockoloFramework to your project

dependencies: [
    .package(url: "https://github.com/uber/mockolo.git", from: "1.1.0"),
targets: [
    .target(name: "MyTarget", dependencies: ["MockoloFramework"]),


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

.build/release/mockolo -s srcsFoo srcsBar -d ./MockResults.swift -x Images Strings

This parses all the source files in srcsFoo and srcsBar, excluding any files ending with Images or Strings in the file name (e.g. MyImages.swift), and generates mocks to a file at ./MockResults.swift.

Use --help to see the complete argument options.

.build/release/mockolo --help

OVERVIEW: Mockolo: Swift mock generator.

USAGE: mockolo <options>

                          True if mock generation should be done on types that are annotated only, thus requiring all the types that the annotated type inherits to be also annotated. If set to false, the inherited types of the annotated types will also be considered for mocking. Default is set to false.
  --annotation, -a        A custom annotation string used to indicate if a type should be mocked (default = @mockable).
  --concurrency-limit, -j
                          Maximum number of threads to execute concurrently (default = number of cores on the running machine).
  --depfilelist, -deplist
                          Path to a file containing a list of dependent files (separated by a new line) from modules this target depends on. 
  --destination, -d       Output file path containing the generated Swift mock classes. If no value is given, the program will exit.
  --exclude-suffixes, -x
                          List of filename suffix(es) without the file extensions to exclude from parsing (separated by a comma or a space).
  --filelist, -f
                          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.
  --header, -h            A custom header documentation to be added to the beginning of a generated mock file.
  --logging-level, -v     The logging level to use. Default is set to 0 (info only). Set 1 for verbose, 2 for warning, and 3 for error.
  --macro, -m             If set, #if [macro] / #endif will be added to the generated mock file content to guard compilation.
  --mockfiles, -mocks     List of mock files (separated by a comma or a space) from modules this target depends on.
  --sourcedirs, -s        Paths to the directories containing source files to generate mocks for. If the --filelist or --sourcefiles values exist, they will be ignored.
  --sourcefiles, -srcs    List of source files (separated by a comma or a space) to generate mocks for. If the --sourcedirs or --filelist value exists, this will be ignored.
  --version               Xcode version used in source files.
  --help                  Displays available options.

How to use

For example, Foo.swift contains:

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

Running the commandline .build/release/mockolo -srcs Foo.swift -d ./MockResults.swift will produce:

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)


It currently supports protocol mocking. Class mocking will be added in the future.

Used libraries


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 10.14.3)
  • 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.


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1.0.0 - Sep 3, 2019

This release includes:

Performance optimizations Support associatedtype to typealias mapping via annotation typealias argument delimited by a semicolon Relative filepath handling Documentation updates