Testing netwok requests with URLProtocol stubs

Apple has a powerful URL Loading System which can be used in various ways with a bunch of most common URL schemes. It also includes the URLProtocol, a class that can intercept all network requests for different purposes. You can use it for analytics, testing, and even profiling your application.

All you need to do is to subclass the URLProtocol and override nessesary methods.

class URLProtocolStub: URLProtocol {
    override class func canInit(with request: URLRequest) -> Bool {
        guard let url = request.url, let host = url.host else { return false }
            
        return host.hasSuffix("example.com")
    }

    override class func canonicalRequest(for request: URLRequest) -> URLRequest {
        return request
    }
}

After that you have to register it before any network request were made and also unregister if needed:

override func setUp() {
    super.setUp()
    
    URLProtocol.registerClass(URLProtocolStub.self)
}

override func tearDown() {
    super.tearDown()
    
    URLProtocol.unregisterClass(URLProtocolStub.self)
}

And from now you can decide how to handle every single network request made within your application.

For example you can stub a response before any paticular request will be executed and response immediately.

class URLProtocolStub: URLProtocol {
    enum URLResponseStub {
        case failure(Error)
        case success(Data)
    }
    
    static var stubs = [URL: URLResponseStub]()
    
    override func startLoading() {
        guard let url = request.url, let stub = URLProtocolStub.stubs[url] else { return }
        
        switch stub {
        case .failure(let error):
            client?.urlProtocol(self, didFailWithError: error)
        case .success(let data):
            client?.urlProtocol(self, didLoad: data)
        }
    }
    override func stopLoading() {}
}

caution

You have to implement stopLoading method even if you don't need it

This approach gives you ability to mock any response you want just writing a fiew lines of code.

let url = URL(string: "https://example.com")!
let error = NSError(domain: "", code: 0)
URLProtocolStub.stubs[url] = .failure(error)

Just make sure that the whole mock stuff happens before the actual network requests.