Swift Code Coverage: How to measure it

Yesterday, I talked about the merits of how code coverage can be used as a metric in your software development process. Measuring Swift code coverage in Xcode has never been easier. Apple provides a great overview in their WWDC 2015 session, “Continuous Integration and Code Coverage in Xcode”. Xcode 7 provides a integrated experience for tracking the coverage of your tests. You can literally start measuring the code coverage of your tests by clicking a single checkbox. And good news, the coverage also works for your KIF tests.

How To Turn On Code Coverage Measurement

Open your scheme editor by selecting Product -> Scheme -> Edit Scheme, or the keyboard shortcut Command-Shift-Comma.

Select Test in the left hand pane, and then check the box for Gather code coverage.

swift code coverage

That’s it! On your next test run, Xcode will measure the Swift code coverage of your tests.

Viewing Swift Code Coverage Results

Viewing the results of how your tests fare with code coverage is just as easy. Run your tests, and then open the Report Navigator. You can open this by either selecting the thing that looks like a chat bubble in the left hand pane of Xcode, or select the menu Product->Scheme->Edit Scheme…, or use the keyboard shortcut Command-8. Then select your most recent Test run in the list.

swift code coverage

From there, in the center pane of Xcode, look for the Coverage tab.

swift code coverage

On the Coverage tab, you’ll see a list of your classes, and their methods, with bar charts indicating how much of their code is covered from your tests.

You can then even jump right to the corresponding code from the coverage viewer. Just double click either the class or method name. And what’s even cooler, is that Xcode will show you the number of times the line of code was executed in the right hand gutter of the editor.

Overlapping Test Types

Keep in mind, that depending on how your test targets are configured in the scheme, that the results you are looking at may be an aggregate of more than one type of test. For example, if you’ve created both unit and UI tests, and they each live in their own targets, but both targets are included in the Test action for the scheme, that the coverage numbers will be an aggregate of both types of tests. I’ll write a post later this week with a proposal on how you can separate these metrics (hint: it involves creating separate schemes for each type of tests).

Wrap Up

Measuring your Swift code coverage really is that easy. Normally code coverage is also tracked in actual numbers, and reviewed for trends over time. With Xcode alone, it doesn’t provide this. Xcode Server helps remediate the problem while providing specific measurement numbers and also allowing you to compare coverage numbers across different devices. Have fun with code coverage. When working in a team, it’s fun to watch code coverage change over time, hopefully for the better. I suggest that even as you code review your peers’ work, peak at how the code coverage for a given piece of code changes with their change. Does it go up? Does it go down? Remember, code coverage is just a metric that indicates whether a line of code was executed or not. It doesn’t speak at all to the quality of the test. Please be a professional, and professionals don’t write code without using test driven development.

Happy cleaning.

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