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

What's New

Here We Go


Result Stream

This is a secret technology made by Apple allows you to get real time updates on your test execution process. All you need to do is to pass -resultStreamPath <exiting_file> to xcodebuild and it will start dumping JSON stream right into that file.

We've reverse-engineered models from that file. The results of this job are the ResultStream and ResultStreamModels modules if you want to use it in your projects.

If you want to use it in Emcee though, we've covered it under a FEATURE TOGGLE — that's right, you'll have to opt-in (for now):

  • Add EMCEE_USE_RESULT_STREAM = true to your test enviornments (via test arg file) and your tests will be executed via this new high technology.

Bugtressing and Speedbumping

  • Emcee kickstart command will now correctly start a worker even if one never started before

  • JSONStream CPU performance improved by 3.5x (or 350%). Also, we now use bytes (UInt8) instead of unicode scalars. Only UTF8 encoding is supported though.

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.

Using Emcee

Up to date documentation is available on Wiki.


  • 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 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 11 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:

  • Generating an Xcode project: make open
  • Generating Package.swift based on template and import statements in source code: make gen
  • Building the binary at .build/debug/Emcee: make build
  • Running unit tests: make test

Since Package.swift file is generated automatically, you must update it before submitting a pull request. CI checks will fail if you forget to do so.



brew install libssh2


  • Swift Tools 5.0.0


Last updated: Tue Apr 20 2021 09:08:13 GMT-0500 (GMT-05:00)