URL loading system

UnitTesting.playground

class MockURLProtocol: URLProtocol {
    static var requestHandler: ((URLRequest) throws -> (HTTPURLResponse, Data))?
    override func startLoading() {
        guard let handler = MockURLProtocol.requestHandler else {
            XCTFail("Received unexpected request with no handler set")
            return
        }
        do {
            let (response, data) = try handler(request)
            client?.urlProtocol(self, didReceive: response, cacheStoragePolicy: .notAllowed)
            client?.urlProtocol(self, didLoad: data)
            client?.urlProtocolDidFinishLoading(self)
        }catch {
            client?.urlProtocol(self, didFailWithError: error)
        }
    }
}