This is a great technique if you need to interact with singleton-based APIs but still have great testability

WrappingSingletons.swift

import UIKit

// Create a protocol that defines what APIs that you need from the singleton
protocol Application {
    func open(url: URL)
}

// Make the singleton-based class conform to your protocol
extension UIApplication: Application {
    func open(url: URL) {
        open(url, options: [:], completionHandler: nil)
    }
}

class Navigator {
    private let application: Application

    // Use a default argument to avoid increasing the complexity of the API, while still enabling dependency injection
    init(application: Application = UIApplication.shared) {
        self.application = application
    }

    func navigate(to url: URL) {
        application.open(url: url)
    }
}

// Our Navigator class can now be used like this in our production code:
let navigator = Navigator()

// And in our tests, we can inject a mocked Application
class ApplicationMock: Application {
    private(set) var openedURLs = [URL]()

    func open(url: URL) {
        openedURLs.append(url)
    }
}

let application = ApplicationMock()
let navigator = Navigator(application: application)

let url = URL(string: "https://github.com")!
navigator.navigate(to: url)

XCTAssertEqual(application.openedURLs, [url])

That seems easy for functions with no return value but what if it is something like

public func dataTask(with request: URLRequest, completionHandler: @escaping (Data?, URLResponse?, Error?) -> ()) -> URLSessionDataTask

from URLSession singleton

@kdawgwilk I think you can return anything. URLSessionDataTask is returned immediately, right? So why not pass it back to protocol method?