Emcee is a tool that runs iOS tests in parallel using multiple simulators across many Macs

What's New




This release brings long awaited feature - generating a resulting xcresult file which you can open in Xcode right after running your tests. Just use runTests command and provide --result-bundle <path> to generate your result bundle.

You can also control the lifetime of xcresult attachments via TestArgFile.entries.testAttachmentLifetime - it can be deleteOnSuccess, keepAlways, keepNever. Just like xcodebuild, huh!


  • This release DOES NOT SUPPORT PLUG-INS. You won't be able to compile them against Emcee v17. Please wait until we release further update for that!
  • Worker sharing is not working between this Emcee version and previous versions. This is because we messed up key and value for HTTP field: it used to be application/json: Content-Type which swift-nio thing forcefully rejects; now it is set correctly to Content-Type: application/json.

Smaller Details

  • Less memory leaks and more speed thanks to updating HTTP server to Vapor

  • Less logs are generated! Again!

  • Emcee now kills xcodebuild with more suitable signal SIGINT which leads to less result bundle corruptions.

  • libssh freedom has arrived! Emcee does not require it to be installed on your system. Also, it is adviced to use key-based authentication.

  • We got rid of duplicate errors from junit. We got rid of duplicate errors from junit.

  • Help (e.g. Emcee runTests -h) got even better thanks to more information provided.

  • You can now configure simulator count on each worker, locale, keyboard, language via runTests command. Please refer to Emcee runTests -h.

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Emcee Banner

Welcome to Emcee project, an ultimate solution for running iOS tests in parallel locally and across many Macs.

Emcee allows you to run UI tests on many physical machines, distributing the work and getting the results of the test run faster. Shared queue manages the order of test execution. Emcee workers execute tests and maintain lifecycle of their simulators automatically. Emcee can generate the Junit and trace reports to make you see how the test run behaved on different machines.


  • Rich test plans using simple JSON file format

  • Automatic simulator lifecycle management

  • Per-test timeouts, simulator settings, environment variables

  • Single test queue to run tests from multiple parallel pull requests

  • Prioritized jobs and job groups for different kinds of test runs

  • Load balancing of worker machines to achieve optimal parallelization performance

  • On-the-go maintenance of the workers

  • Integration into existing test management systems via plugins

  • Easy to use command line interface

  • Rich test discovery mechanism

  • Swift Package for using and extending Emcee the way you want

Getting started

In this guide will demonstrate how to use Emcee. We will use two MacOS machines to run unit and UI tests from a sample project. You can also use a single machine to try out Emcee to see if it works for your project. In this case, a single machine will act as a queue and a worker simultaneously. Alternatively, you can scale this guide to as many machines as you have.

If you encounter any issues while proceeding through the guide, please open an issue or reach out via https://t.me/emcee_ios.

Table of contents

  1. Setting up machines
  2. Building the sample project
  3. Running tests using Emcee
    1. Tests without a host application
    2. Tests with a host application
    3. XCUI tests
  4. Advanced Emcee configuration

Setting up machines

You will need to grant SSH access to your machines.

Expand to see how to set up your machines.

We will be using two machines: ios-build-machine77 and ios-build-machine78.


  • ios-build-machine77 will be a worker and a queue - it will provide workers with tests to execute and execute some of those tests.
  • ios-build-machine78 will be a worker - it will only execute tests.

Both machines are set up with a standard non-administrator user emcee and a qwerty password.

Install Xcode and sudo xcode-select --switch /Applications/Xcode.app on all of your machines.

We will use Xcode 13.0 (13A233) and the iOS 15.0 simulator runtime bundled with this Xcode. If you want to use a specific version of simulator runtime, proceed to Xcode -> Preferences... -> Components -> Simulators and install the runtime on all the worker machines, where you want the tests to execute with the specific runtime version.

Emcee uses ssh to deploy itself to the machines specified as queue and workers. Enable SSH in your System Preferences -> Sharing -> Remote Login. To open this pane execute:

$ open "x-apple.systempreferences:com.apple.preferences.sharing?Services_RemoteLogin"

Remote Login SSH Settings

Now make sure that machines are accessible by ssh. For example:

ssh emcee@ios-build-machine77

If your machines are not accessible by DNS, use their IP addresses instead. You can check IP address in System Preferences -> Sharing. Please note IP addresses may change over time. To open this pane execute:

$ open "x-apple.systempreferences:com.apple.preferences.sharing"

Building the sample project

In this step, we will build a sample project that features different types of tests. Xcode and xcodebuild will produce build artifacts in derived data.

Expand to see how to build the sample project for testing purposes.

You can run this step from either machine. Clone the sample project:

cd ~
git clone https://github.com/avito-tech/Emcee.git
cd Emcee/Samples/EmceeSample

To build the project, create a simulator:

xcrun simctl create '15.0' 'iPhone X' 'iOS15.0'

Now run xcodebuild:

xcodebuild build-for-testing \
	-project EmceeSample.xcodeproj \
	-destination "platform=iOS Simulator,name=15.0,OS=15.0" \
	-scheme AllTests \
	-derivedDataPath derivedData

Xcodebuild will place the build products in:


Running tests using Emcee

Now that the machines are ready, and the project is built, download Emcee on the same machine where you built the project by running:

curl -L https://github.com/avito-tech/Emcee/releases/download/16.0.0/Emcee -o Emcee && chmod +x Emcee

If you download Emcee using a browser you will need to clear attributes and set the executable bit:

xattr -c Emcee && chmod +x Emcee

With Emcee installed it is finally time to run the tests. The sample project includes 3 test types:

  • Tests that don't require a host application
  • Tests that require a host application
  • XCUI tests

Tests without a host application

Let's first run tests that don't require a host application. We will be using the runTests command:

./Emcee runTests \
	--queue ssh://emcee:qwerty@ios-build-machine77 \
	--worker ssh://emcee:qwerty@ios-build-machine77 \
	--worker ssh://emcee:qwerty@ios-build-machine78 \
	--device "iPhone X" \
	--runtime "15.0" \
	--test-bundle derivedData/Build/Products/Debug-iphonesimulator/EmceeSampleTestsWithoutHost.xctest \
	--junit tests_without_host_junit.xml

Here is what these options stand for:

  • --queue - is a URL of a machine that will serve workers with tests
  • --worker - is a URL of a machine that will execute tests that it queries from the queue
  • --device and --runtime - are options that specify which simulators will run the tests
  • --test-bundle - is a path to the xctest bundle
  • --junit - is a path to the JUnit xml that will contain the result of the test run

You can find more about all options accepted by runTests and how to specify them using Emcee runTests -h.

After the test finishes, Emcee will create a tests_without_host_junit.xml file. The JUnit report contains four testcase entries matching the four test methods from the EmceeSampleTestsWithoutHost.xctest test bundle.

<?xml version="1.0" encoding="UTF-8"?>
<testsuites name="xctest" tests="4" failures="1">
    <testsuite name="EmceeSampleTestsWithoutHost" tests="4" failures="1">
        <testcase classname="EmceeSampleTestsWithoutHost" name="test_0___from_tests_without_host___that_always_succeeds" timestamp="2021-12-29T01:15:16+03:00" time="0.012196063995361328"></testcase>
        <testcase classname="EmceeSampleTestsWithoutHost" name="test_1___from_tests_without_host___that_always_succeeds" timestamp="2021-12-29T01:15:27+03:00" time="0.02156198024749756"></testcase>
        <testcase classname="EmceeSampleTestsWithoutHost" name="test_2___from_tests_without_host___that_always_succeeds" timestamp="2021-12-29T01:15:36+03:00" time="0.021990060806274414"></testcase>
        <testcase classname="EmceeSampleTestsWithoutHost" name="test___from_tests_without_host___that_always_fails" timestamp="2021-12-29T01:15:31+03:00" time="0.1255110502243042">
            <failure message="failed - Failure from tests without host">/Users/emcee/Emcee/SampleProject/EmceeSampleTestsWithoutHost/EmceeSampleTestsWithoutHost.swift:17</failure>
            <failure message="failed - Failure from tests without host">/Users/emcee/Emcee/SampleProject/EmceeSampleTestsWithoutHost/EmceeSampleTestsWithoutHost.swift:17</failure>

For a more sophisticated test reporting mechanism such as Allure, check out the Plugins documentation.

Tests with a host application

Now let's try running tests that require a host application. Host application path is specified using the --app option. For example:

./Emcee runTests \
    --queue ssh://emcee:qwerty@ios-build-machine77 \
    --worker ssh://emcee:qwerty@ios-build-machine77 \
    --worker ssh://emcee:qwerty@ios-build-machine78 \
    --device "iPhone X" \
    --runtime "15.0" \
    --app derivedData/Build/Products/Debug-iphonesimulator/EmceeSample.app \
    --test-bundle derivedData/Build/Products/Debug-iphonesimulator/EmceeSample.app/PlugIns/EmceeSampleHostedTests.xctest \
    --junit tests_with_host_junit.xml

To get a visual confirmation that Emcee is running the tests, you can open the Simulator app on the worker machines:

open "$(xcode-select -p)"/Applications/Simulator.app

XCUI tests

Finally, we will run XCUI tests by adding a --runner option and changing the --test-bundle option to the XCUI test bundle:

./Emcee runTests \
    --queue ssh://emcee:qwerty@ios-build-machine77 \
    --worker ssh://emcee:qwerty@ios-build-machine77 \
    --worker ssh://emcee:qwerty@ios-build-machine78 \
    --device "iPhone X" \
    --runtime "15.0" \
    --runner derivedData/Build/Products/Debug-iphonesimulator/EmceeSampleUITests-Runner.app \
    --app derivedData/Build/Products/Debug-iphonesimulator/EmceeSample.app \
    --test-bundle derivedData/Build/Products/Debug-iphonesimulator/EmceeSampleUITests-Runner.app/PlugIns/EmceeSampleUITests.xctest \
    --junit ui_tests_junit.xml

This is how the test run will look:


Advanced Emcee configuration

Complete documentation is available in our Wiki.

runTests command allows you to get Emcee up and running quickly; however, it doesn't allow for a lot of configuration. On the other hand, runTestsOnRemoteQueue command allows for fine-grained control of how your tests execute. To get started with runTestsOnRemoteQueue check out the Queue Server Configuration and Test Arg File wiki pages.


Getting Around the Code

Emcee uses Swift Package Manager for building, testing and exposing the Swift packages.

To start exploring code open Package.swift in Xcode 13 or execute make open to generate and open Xcode project.


We are happy to accept your pull requests. If something does not work for you, please let us know by submitting an issue. Read the docs and suggest improvements to them as well!

General commands that help you with a development workflow:

  • To open a package in Xcode: make open
  • To generate Package.swift: make package
  • To build the binary into .build/debug/Emcee: make build
  • To run unit tests: make test

Package.swift file is generated automatically. You must update it before submitting a pull request (run make package). CI checks will fail if you forget to do so.


  • Swift Tools 5.3.0
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Last updated: Mon Jan 30 2023 22:38:16 GMT-0500 (GMT-05:00)