Weekly Swift articles, podcasts and tips by John Sundell.

Throwing tests and LocalizedError

Published on 25 Jan 2018
Basics article available: Error Handling

Swift tests can throw, which is super useful in order to avoid complicated logic or force unwrapping. By making errors conform to LocalizedError, you can also get a nice error message in Xcode if there's a failure.

class ImageCacheTests: XCTestCase {
    func testCachingAndLoadingImage() throws {
        let bundle = Bundle(for: type(of: self))
        let cache = ImageCache(bundle: bundle)
        // Bonus tip: You can easily load images from your test
        // bundle using this UIImage initializer
        let image = try require(UIImage(named: "sample", in: bundle, compatibleWith: nil))
        try cache.cache(image, forKey: "key")
        let cachedImage = try cache.image(forKey: "key")
        XCTAssertEqual(image, cachedImage)

enum ImageCacheError {
    case emptyKey
    case dataConversionFailed

// When using throwing tests, making your errors conform to
// LocalizedError will render a much nicer error message in
// Xcode (per default only the error code is shown).
extension ImageCacheError: LocalizedError {
    var errorDescription: String? {
        switch self {
        case .emptyKey:
            return "An empty key was given"
        case .dataConversionFailed:
            return "Failed to convert the given image to Data"

For more information, and the implementation of the require method used above, check out "Avoiding force unwrapping in Swift unit tests".