Assertions for XCTest which prevent fatal errors causing the process to die.

The following assertions are supported. These functions are built on top of CwlPreconditionTesting which was created by Matt Gallagher of Cocoa with Love. That package allows for intercepting the signal from fatal errors to prevent the process from dying.

  • XCTAssertThrowFatalError
  • XCTAssertNoThrowFatalError


This package includes tests which show how these functions can be used. Note that the underlying package supports iOS and macOS only currently. For tests which are running on tvOS and watchOS an XCTSkip error will be thrown to indicate that the test was skipped so that it does not simply fail. These functions take a closure is run. The first one expects second one does not. If an fatal exception is raised the first assert function will pass and if not the second assert function will pass. These assert functions mirror the non-fatal error assert functions included with XCTest.


Type safety can provide a lot of protection with support from the compiler making many bugs or invalid conditions to be impossible. Preconditions allow developers to impose strict requirements at runtime to compliment type safety. Apple prefers to have apps to be killed when it the problem is a result of programmer error. Placing preconditions in code to verify assumptions and enforce requirements can help developers understand why their code is not running properly long before it reaches production. Below is example of code which must run on the main thread and will sync to it with the requirement that it not already on the main thread. It may be necessary to do this work on the main thread before leaving the current code block and documentation would note that running this code should be off the main thread.

precondition(Thread.isMainThread, "Must not run on main thread")

DispatchQueue.main.sync {
    // run code on main threead

This precondition can also be handled by a special function from the Dispatch framework. One advantage of dispatchPrecondition is that you can also check if execution is on or off one of your own queues. The code below requires that execution is not running on the .main queue while it is running on the serialQueue.

dispatchPrecondition(condition: .notOnQueue(.main))
dispatchPrecondition(condition: .onQueue(serialQueue))

Both will not allow execution to continue if it is already operating on the main thread. If an app somehow manages to run this code on the main thread it will result in a runtime exception, crash the app and generate a crash report. That crash report will point directly to this line and show the code which lead up to the crash which is very helpful in diagnosing the crash. If the app continues running and crashes later it may be misleading and require more time to diagnose the problem.

Another common scenario is unwrapping self which was made weak with a capture list in an escaping closure. It is necessary to use self when referencing anything outside the scope of the closure and using weak self will break the potential retain cycle. Instead of risking the retain cycle many developers will do the weak self dance and immediate unwrap with a guard statement which requires returning from the else statement.

guard let self = self else { fatalError() }

Fortunately fatalError returns Never which means it will never return. It is a special type supported by the compiler which formalizes this scenario. Any function which returns Never will not return as the process should be terminated. It possible to use preconditionFailure in the same as it also returns Never. Both precondition and dispatchPrecondition do not return Never as they behave conditionally.

Assertions versus Preconditions

A key difference between an assert and precondition is that asserts are suppressed in release builds. Placing many asserts in your normal runtime code supports the development process by halting execution if any assertion fails to let a developer immediately see where the code has reached a bad state. Instead of setting breakpoints these assertions can stay in the code for ongoing development while being stripped out for release builds. A precondition can check an absolute requirement such as configuration which is necessary for the app to operate properly when that configuration should be included in the app bundle. It could also require that the Documents or Caches directory exists which it should unless something has seriously gone wrong. Triggering a crash and collecting crash reports will help with being aware of such problems in release builds. Ideally you will catch these problems with early testing before the app is available to users.

For test automation, asserts are a part of the Arrange, Act and Assert cycle.


Why Hela? Hela is a Marvel character who is a necromancer who can raise the dead. Once a fatal error is raised normally a process will die. With help from Hela, the process can be raised from the dead and continue running your tests.


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