A cloud run cannot build an iPhone app (see iPhone apps in a cloud run), so your Mac does, when the work comes back.
When it builds
After Pull Back or Move Back to This Mac for a run whose files hold an Xcode project, the Mac:
- checks out the run's
cloud/<name>branch in a temporary worktree of its own (after Move Back, in your checkout, when it is now on that branch), so your own branch and uncommitted work are not touched; - builds the project's scheme for the iOS Simulator, and, when the project has tests, runs them on an available iPhone simulator instead;
- removes the temporary worktree and the build products afterwards.
It runs in the background and never holds up the discussion. It stops after twenty minutes, and Cancel Build on the run's card stops it sooner.
It never signs, archives, exports or uploads anything: code signing is off for the build, and the only actions are build and test.
The result
The run's card, on the Mac and on the web and the iPhone too, shows one of:
- Built: it built, and it has no tests to run.
- Build Failed, with the first errors (file, line and message).
- Tests Passed, with how many.
- N Failed: it built, and that many tests failed, with the first failure.
- Not Built, when Xcode is not installed on that Mac: install Xcode from the App Store to build there. The command line tools alone are not enough.
The same line is added to the discussion.
Turning it off
Settings → Build Xcode Projects When Cloud Work Returns. It is on by default, and the setting is shared between the Mac, the web and the iPhone; only a Mac ever builds.