Unit Tests assertionFailure

Question

I'm unit testing:

func testParseFakeDeeplinkSuccess() {
    let fakeDeeplink = "fakeDeeplink"
    let url = touchManager?.handleTouch(["deeplink": fakeDeeplink as NSSecureCoding])
    XCTAssertNotNil(url)
}

which will result in "assertionFailure("3DTouch not trackable")"

    func handleTouch(_ userInfo: [String: NSSecureCoding]?) -> URL? {
    if let shortcut = userInfo?["deeplink"] as? String,
       let url = URL(string: shortcut) {

        switch shortcut {
        default:
            assertionFailure("3DTouch not trackable")
        }

        return url
    }

    return nil
}

What can I use in my unit tests to tests this properly?

Answer 1

You're hitting the assertion error because that's the branch your code reaches once it's evaluated.

if let shortcut = userInfo?["deeplink"] as? String, let url = URL(string: shortcut) {
  // 1. The branch is taken, unwrapped `shortcut`, it's value is "fakeDeeplink"

  switch shortcut {
  default:
    // 2. This is the only branch in the switch, so this case is always executed.
    assertionFailure("3DTouch not trackable")
  }
}

You might only be seeing a crash in your tests (and not in a production build of your app) because assertionFailure will have no effect in the default -O optimization mode used in RELEASE builds. Tests, by default, will be compiled for DEBUG which uses the -Onone optimization mode causing assertions to trigger a crash.

If you want to handle an error condition without trapping, you should either return nil or use a throwing method instead:

struct TouchNotTrackableError: Error { }

func handleTouch(_ userInfo: [String: NSSecureCoding]?) throws -> URL? {
    if let shortcut = userInfo?["deeplink"] as? String,
       let url = URL(string: shortcut) {

        switch shortcut {
        default:
            throw TouchNotTrackableError()
        }

        return url
    }

    return nil
}

Then you can test if the method throws.

func testParseFakeDeeplinkSuccess() {
    let fakeDeeplink = "fakeDeeplink"
    XCTAssertThrowsError(try touchManager?.handleTouch(["deeplink": fakeDeeplink as NSSecureCoding]))
}