XCTest Target Membership Rule #1

xctest target membership

I just had to learn a lesson the hard way, and wanted to pass on to you what I’m going to call the XCTest Target Membership Rule. It took some head banging of mine to discover this rule, and I don’t want you to go through the same pain.

The XCTest Target Membership Rule

The actual implementation files for your app should not be included with the Target Membership for your XCTest target.

xctest target membership

If you were to check the box next to TestStaticTests you would start to see this error in the Console when running your tests:

objc[14049]: Class ViewController is implemented in both <path cut>/TestStatic.app/TestStatic and <path cut>/TestStaticTests.xctest/TestStaticTests. One of the two will be used. Which one is undefined. 

The runtime for XCTest is telling you that it has found the same class defined in two places, the two targets for which you specified the file should be included. And furthermore, you’re also being told that the behavior with regard to which implementation is used will be “undefined.” The funny thing was, I actually found this error message to be misleading. I found that in practice, both copies of the implementation are actually used if you can believe that!

Here’s an in-depth question I posted on StackOverflow documenting the odd behavior I noticed, and included a sample project demonstrating it as well.


These past few weeks I’ve been working on an older project, specifically AWeber Stats. If you notice, that app hasn’t been updated since April 2015, that’s a while. As a result a lot of the dependencies were out of date, so one thing the team set out to do was update the dependencies, and CocoaPods seemed like a good place to start. We were previously on version 0.34.1 and planned to update to 1.0.1 (which at the time of this writing is the latest version). We use Ruby Gems to manage the dependency versions for CocoaPods specifically. This allows us to have per-project control of which version of CocoaPods end up being used. This gives us the reassurance that we must explicitly designate the desire to update a given project’s CocoaPod’s version, and thus accordingly test and verify the update.

I move forward with the update, bumping the version of CocoaPods to 1.0.1 as defined in our Gemfile. CocoaPods 1.0 actually defines a new schema for Podfiles so I subsequently migrated the Podfile to the new format, and successfully ran pod install. Everything has gone smoothly up to this point. Then I opened my Xcode workspace and attempt to run our test suite.

100s of failures.

In addition to the failures, I saw many instances of that Console warning from earlier:

...One of the two will be used. Which one is undefined.

Now the thing is, remember, at this point I hadn’t yet discovered the XCTest Target Membership Rule. And as it turns out, nearly every implementation file in the project for the app target, also existed in membership for the test target. And in fact, as I dusted the cobwebs off of my mind, and thought back to the original work on the project, I specifically remember needing to designate those implementation files as having membership in the test target. Specifically, it was errors like this that would appear of the class under test was not part of the test target:

Undefined symbols for architecture x86_64:
  "_OBJC_CLASS_$_MyArrayController", referenced from:
      objc-class-ref in MyArrayControllerTests.o

So where did the need to include that file with the test target go? Why was Xcode now telling me that I shouldn’t include these files with both targets?

Well I never got to the bottom of it. I have two guesses at this point: something either in Xcode, or in CocoaPods changed to modify the behavior how XCTest integrates with the classes under test.

Closing Thoughts

I’m still digging to get to the bottom of this. In the meantime, I will not forget the XCTest Target Membership Rule. A couple other lessons surfaced:

  1. Don’t wait to keep your project up to date – It’s easy to launch an app on the store, or hand it off to a client, and semi-forget about it. The app is live, attracting customers, and functioning well. Just remember, at some point it will either need to be updated or die. And the longer you wait, the harder it will be to bring it up to contemporary standards. It’s much easier to make more frequent small changes, than wait 15 months, return to an unfamiliar code base, and try to bring it up to speed.
  2. Know your tool chain – I’m still not fully aware of what changed between the last app update and today such that such a fundamental piece of project functionality changed. I’m taking this as a kick in the butt to get my head wrapped around third party dependencies being used. And if you can’t do that (for whatever reason – time, complexity, interest, etc.), don’t use them. There are plenty of ways to manually install third party code, or even write it from scratch. Of course other people have already invented the wheel, just don’t blindly use the tools without a foundational understanding of how to troubleshoot them when they go wrong.

Happy cleaning.

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