Testable debug assertions for Swift.
Swift Ruby Objective-C
Latest commit c642868 Jun 14, 2016 @brianpartridge brianpartridge Update README.




Swift's type system ensures that your methods are interacting with objects of the correct type, but you are left to using conditionals and assertions to validate their values.

You can test conditionals, but until now you were not able to test Swift's assertions (assert, precondition, fatalError) without crashing your test run. Ackbar allows you to write unit tests which verify that your assertions have test coverage and fire when you expect them to.

This is performed in 2 parts:

  • Add AckbarAssertions to your app/framework target. It will add methods to override Swift's assertions in DEBUG builds.
  • Add AckbarTesting to your test target and use assertHitsDebugException to test that your assertions are enforced.

For more details see the detailed design below.


In your app/framework's Podfile:

    pod 'AckbarAssertions'

In your test target's Podfile:

    pod 'AckbarTesting'

Install the pods:

    pod install

IMPORTANT: Open Ackbar/Assertions/README.swift and follow the instructions to manually integrate the assertions into your app target. For more details see the distribution method section.


In your app/framework:

    func foo(someConditionThatMustBeTrueToSucceed: Bool) {
        precondition(someConditionThatMustBeTrueToSucceed)     // Write your assertions as you normally would.

In your tests:

    func testFoo() {
        assertHitsDebugException(foo(false))                  // Use like you would use XCTAssert*.

Take a look at Endor for a larger example of how to use AckbarTesting in your tests.


Testing assertions works by throwing NSExceptions for assertion failures and using Objective-C's try/catch exception handling functionality to detect and trigger test failures.


Assertion failures typically terminate a process, but this drastically reduces testability. To make assertions testable, we need something that can be detected, and also unwinds the stack so other execution can continue to run after detection. At this time, the best option for this is to use exceptions.

Alternative Designs

Attaching a 'handler' to each thread a la NSAssertionHandler is a non-starter because this can effectively only be used for logging the failure. It does not facilitate unwinding the stack.

Swift's error handling by throwing errors is not an option because it doesn't unwind the stack and requires all callers to use the try keyword. This could interferre with legitimate uses of Swift's error handling style.

The family of Swift assertions send trap signals when an assertion fails. We could implement a signal handler, but given the hairiness of dropping down to mach, combined with the churn that the Swift language is going through, we chose not to go this route at this time. There are examples of using a signal handler for this already though.

Distribution Method

We use CocoaPods for distribution because it makes getting the code into your project easy. However, in order for the testable assertions in AckbarAssertions to override Swift's assterions you must add Assertions.swift to the app and framework targets that you want to test. Otherwise you would need to update your code to call AckbarAssertions.assert rather than just assert.

If you choose to make explicit calls to AckbarAssertions.assert be aware that your crash report tool may be affected. Many crash reporters rollup 'related' crashes bashed upon stack trace similarities, so two different crashes with AckbarAssertions.assert at the top of the stack trace will result in different crashes being rolled up into different instances of the same crash.


Apache License, Version 2.0


We’re glad you’re interested in Ackbar, and we’d love to see where you take it. Please read our contributing guidelines prior to submitting a Pull Request.