Hands-On Design Patterns with Swift

Testing Async code

A common problem every developer encounters when adding tests to their software is how to test asynchronous code. Why is it a problem? It's because the flow of a test is expecting synchronous code. Let's consider, for example, testing a file loader that loads the content of a JSON file in an asynchronous way:

class FileLoader {    func loadContent(fromFilename: String,                     onSuccess: @escaping (Any) -> Void) {        guard let path = Bundle.main.path(forResource: fromFilename,                                           ofType: "json") else {            return        }        DispatchQueue.global(qos: .background).async {            guard let data = try? Data(contentsOf: URL(fileURLWithPath: path),                                        options: .alwaysMapped) else {                print("Error: Couldn't decode data")                return            } guard let decodedJSON = try? JSONSerialization.jsonObject( ...