WWDC 2018 What’s New in Testing Recap

In Apple’s WWDC 2018 talk on What’s New in Testing, they announced a big new feature: parallel testing. Parallel testing runs your test suite on multiple simulators at once, in parallel, so it takes significantly less time to run. And all you have to do is check a box in Xcode 10.

If you don’t already have tests, you can write your first ones today with How to write unit tests in Swift 4. And if you want to take your unit testing further, check out Unit Testing in Swift.

Below are my notes from the talk, which introduced parallel testing, code coverage improvements, and test randomization.


Smaller coverage files

Code coverage setting per target: all targets or some targets

xccov: new command line tool makes it easy to get coverage in CI

  • output: human-readable or JSON

Coverage Data

  • lives in Derived Data

Source Editor: Show/Hide code coverage


  • enable code coverage
  • Report navigator: coverage report
  • Edit scheme: gather coverage for some targets, add the targets you want
  • Report navigator: report only shows the covered targets
  • View coverage in editor to see what’s not covered

Xcode 10 features

  • test selection: 1000 unit tests, 10 UI tests

Test selection with schemes: “tests to skip” in Xcode 9

Xcode 10 adds “tests to run”

Test ordering


  • Default
  • Deterministic
  • Can hide implicit dependencies

Implicit dependencies are bad and should be avoided

Each test should set up and tear down its own state

To ensure there are no implicit dependencies, Xcode 10 has randomization

  • enable in scheme editor

Parallel Testing

Waiting for tests to finish is bad

Xcode 9 has parallel destination testing with only xcodebuild

Now, Xcode has that in the IDE

Classes execute in parallel on simulators


  • See how long tests took to run in Report Navigator
  • Edit Scheme > Test action > Options > Parallel
  • Xcode launches multiple copies of Mac app to test in parallel
  • 14s down to 5s total execution time
  • iOS: multiple clones of the simulator (iPhone X)
  • Each runner executes a different test class in the suite
  • Xcode: Report Navigator: Test Log shows runners and class that’s executing

Demo Recap

  • enabling parallelization
  • view results in test log and report
  • Instances of Mac app running tests
  • Instances of iOS Simulator running tests


  • Override number of workers
  • force parallel testing on or off

Tips and Tricks

  • split long-running test class into two classes (testing will never run faster that the slowest class)
  • Put performance tests into own bundle with parallelization disabled
  • Understand which tests are not safe for parallelization (if they access shared resource like file or database)
  • More info in Testing Tips & Tricks talk


  • Code coverage with xccov
  • Test selection and ordering
  • Parallel testing

Have a great day!