How to unit test throwing functions in Swift?

Question

How to test wether a function in Swift 2.0 throws or not? How to assert that the correct ErrorType is thrown?

Answer 1

Given the following functions and declarations:

enum SomeError: ErrorType {
    case FifthError
    case FirstError
}

func throwingFunction(x: Int) throws {
    switch x {
    case 1:
        throw SomeError.FirstError
    case 5:
        throw SomeError.FifthError
    default:
        return
    }
}

This function will throw a FifthError if 5 is given to the function and FirstError if 1 is given.

To test, that a function successfully runs the unit test could look as follows:

func testNotError() {
    guard let _ = try? throwingFunction(2) else {
        XCTFail("Error thrown")
        return
    }
}

The let _ may also be replaced by any other name, so you can further test the output.

To assert that a function throws, no matter what ErrorType the unit test could look like this:

func testError() {
    if let _ = try? throwingFunction(5) {
        XCTFail("No error thrown")
        return
    }
}

If you want to test for a specific ErrorType it's done with a do-catch-statement. This is not the best way compared to other languages.

You have to make sure that you...

• return in the catch for the correct ErrorType
• XCTFail() and return for all other catch
• XCTFail() if no catch is executed

Given this requirements a test case could look like this:

func testFifthError() {
    do {
        try throwingFunction(5)
    } catch SomeError.FifthError {
        return
    } catch {
        XCTFail("Wrong error thrown")
        return
    }
    XCTFail("No error thrown")
}