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

Nimble ("A Matcher Framework for Swift and Objective-C") got your back :

Swift Assertions

If you're using Swift, you can use the throwAssertion matcher to check if an assertion is thrown (e.g. fatalError()). This is made possible by @mattgallagher's CwlPreconditionTesting library.

// Swift

// Passes if 'somethingThatThrows()' throws an assertion, 
// such as by calling 'fatalError()' or if a precondition fails:
expect { try somethingThatThrows() }.to(throwAssertion())
expect { () -> Void in fatalError() }.to(throwAssertion())
expect { precondition(false) }.to(throwAssertion())

// Passes if throwing an NSError is not equal to throwing an assertion:
expect { throw NSError(domain: "test", code: 0, userInfo: nil) }.toNot(throwAssertion())

// Passes if the code after the precondition check is not run:
var reachedPoint1 = false
var reachedPoint2 = false
expect {
    reachedPoint1 = true
    precondition(false, "condition message")
    reachedPoint2 = true
}.to(throwAssertion())

expect(reachedPoint1) == true
expect(reachedPoint2) == false

Notes:

• This feature is only available in Swift.
• It is only supported for x86_64 binaries, meaning you cannot run this matcher on iOS devices, only simulators.
• The tvOS simulator is supported, but using a different mechanism, requiring you to turn off the Debug executable scheme setting for your tvOS scheme's Test configuration.