A custom URLProtocol subclass to handle returning mock data for network service requests for UITests

UITestConnectionHandler.swift

final class UITestConnectionHandler: URLProtocol {

    private static var allowedResponses: [URL: MockResponse] = [:]

    static func setAllowedResponse(with response: MockResponse) {
        allowedResponses[response.url] = response
    }

    override class func canInit(with request: URLRequest) -> Bool {
        // Only return true for requests we want this provider to handle otherwise revert to default

        guard let url = request.url else { return false }
        return allowedResponses.keys.contains(url)
    }

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

    // You could also introduce an artificial delay and send the data chunks to the client gradually. It’s a great way to simulate bad network conditions.
    override func startLoading() {
        guard let client, let url = request.url else { fatalError() }

        guard let responseProvider = UITestConnectionHandler.allowedResponses[url] else {
            fatalError("Failed to set a response provider")
        }

        switch responseProvider.response {
        case .success(let content):
            let response = HTTPURLResponse(url: url, statusCode: content.statusCode, httpVersion: nil, headerFields: nil)!

            client.urlProtocol(self, didReceive: response, cacheStoragePolicy: .notAllowed)
            client.urlProtocol(self, didLoad: content.data)
            client.urlProtocolDidFinishLoading(self)
        case .error(let error):
            client.urlProtocol(self, didFailWithError: error)
            client.urlProtocolDidFinishLoading(self)
        }

    }

    override func stopLoading() { }

}