Customizing the source location of failures reported by XCTest

If you’re writing unit tests and use helper methods to factor out common assertions, you might have encountered situations where test failures would not show up in the correct location in your Xcode editor. The failure would be reported in the helper method, instead of in the actual test. Especially if you call the helper method multiple times, perhaps even from multiple tests, it would become hard to figure out the context of the failures.

In this situation, I can see that the assert in assertSymmetry failed twice, but I can’t see the context of the failures. I can also see that testSerialization failed, but it doesn’t show which part of the test failed.

Solution

To have the failures show up inside the testSerialization test, we need to capture the file and line number for each call to assertSymmetry and pass them to XCTAssertEqual. file and line are optional parameters of XCTAssertEquals, which default to the file path and line number of the XCTAssertEquals call site. You can pass them to override this behavior and explicitly specify the file and line to display the failure at. To capture the values for the call sites of assertSymmetry, we add file and line as optional parameters to it and use the #file and #line compiler directives as their default values. The Swift compiler fills them in, so we don’t have to.

This results in every call to assertSymmetry from testSerialization to be expanded to something like assertSymmetry(of: field, file: "/path/to/Test.swift", line: 15). The result: failures are now reported on the line where we call assertSymmetry, instead of where we call XCTAssertEqual.

This works not only for XCTAssertEquals, but also for XCTAssert, XCTAssertNil, XCTFail and any other XCTest assertion. On top of that, many third party testing libraries also support it, e.g. Nimble and iOSSnapshotTestCase.

All the right failures, in all the right places.