Code sample #2 from https://medium.com/@johnsundell/testing-swift-code-that-uses-system-singletons-in-3-easy-steps-89f4884cd325

Testing-Swift-code-that-uses-system-singletons-in-3-easy-steps-1.swift

protocol NetworkEngine {
    typealias Handler = (Data?, URLResponse?, Error?) -> Void
    
    func performRequest(for url: URL, completionHandler: @escaping Handler)
}

extension URLSession: NetworkEngine {
    typealias Handler = NetworkEngine.Handler
    
    func performRequest(for url: URL, completionHandler: @escaping Handler) {
        let task = dataTask(with: url, completionHandler: completionHandler)
        task.resume()
    }
}