Unit Test fatalError in Swift

Question

How to implement unit test for a fatalError code path in Swift?

For example, I've the following swift code

func divide(x: Float, by y: Float) -> Float {

    guard y != 0 else {
        fatalError("Zero division")
    }

    return x / y
}

I want to unit test the case when y = 0.

Note, I want to use fatalError not any other assertion function.

Answer 1

The idea is to replace the built-in fatalError function with your own, which is replaced during a unit test's execution, so that you run unit test assertions in it.

However, the tricky part is that fatalError is @noreturn, so you need to override it with a function which never returns.

Override fatalError

In your app target only (don't add to the unit test target):

// overrides Swift global `fatalError`
@noreturn func fatalError(@autoclosure message: () -> String = "", file: StaticString = __FILE__, line: UInt = __LINE__) {
    FatalErrorUtil.fatalErrorClosure(message(), file, line)
    unreachable()
}

/// This is a `noreturn` function that pauses forever
@noreturn func unreachable() {
    repeat {
        NSRunLoop.currentRunLoop().run()
    } while (true)
}

/// Utility functions that can replace and restore the `fatalError` global function.
struct FatalErrorUtil {

    // Called by the custom implementation of `fatalError`.
    static var fatalErrorClosure: (String, StaticString, UInt) -> () = defaultFatalErrorClosure

    // backup of the original Swift `fatalError`
    private static let defaultFatalErrorClosure = { Swift.fatalError($0, file: $1, line: $2) }

    /// Replace the `fatalError` global function with something else.
    static func replaceFatalError(closure: (String, StaticString, UInt) -> ()) {
        fatalErrorClosure = closure
    }

    /// Restore the `fatalError` global function back to the original Swift implementation
    static func restoreFatalError() {
        fatalErrorClosure = defaultFatalErrorClosure
    }
}

Extension

Add the following extension to your unit test target:

extension XCTestCase {
    func expectFatalError(expectedMessage: String, testcase: () -> Void) {

        // arrange
        let expectation = expectationWithDescription("expectingFatalError")
        var assertionMessage: String? = nil

        // override fatalError. This will pause forever when fatalError is called.
        FatalErrorUtil.replaceFatalError { message, _, _ in
            assertionMessage = message
            expectation.fulfill()
        }

        // act, perform on separate thead because a call to fatalError pauses forever
        dispatch_async(dispatch_get_global_queue(QOS_CLASS_USER_INITIATED, 0), testcase)

        waitForExpectationsWithTimeout(0.1) { _ in
            // assert
            XCTAssertEqual(assertionMessage, expectedMessage)

            // clean up 
            FatalErrorUtil.restoreFatalError()
        }
    }
}

Testcase

class TestCase: XCTestCase {
    func testExpectPreconditionFailure() {
        expectFatalError("boom!") {
            doSomethingThatCallsFatalError()
        }
    }
}

I got the idea from this post about unit testing assert and precondition: Testing assertion in Swift