Mock URLSession using URLProtocol

URLProtocolMock.swift

class URLProtocolMock: URLProtocol {
    /// Dictionary maps URLs to tuples of error, data, and response
    static var mockURLs = [URL?: (error: Error?, data: Data?, response: HTTPURLResponse?)]()
    
    override class func canInit(with request: URLRequest) -> Bool {
        // Handle all types of requests
        return true
    }
    
    override class func canonicalRequest(for request: URLRequest) -> URLRequest {
        // Required to be implemented here. Just return what is passed
        return request
    }
    
    override func startLoading() {
        if let url = request.url {
            if let (error, data, response) = URLProtocolMock.mockURLs[url] {
                
                // We have a mock response specified so return it.
                if let responseStrong = response {
                    self.client?.urlProtocol(self, didReceive: responseStrong, cacheStoragePolicy: .notAllowed)
                }
                
                // We have mocked data specified so return it.
                if let dataStrong = data {
                    self.client?.urlProtocol(self, didLoad: dataStrong)
                }
                
                // We have a mocked error so return it.
                if let errorStrong = error {
                    self.client?.urlProtocol(self, didFailWithError: errorStrong)
                }
            }
        }
        
        // Send the signal that we are done returning our mock response
        self.client?.urlProtocolDidFinishLoading(self)
    }
    
    override func stopLoading() {
        // Required to be implemented. Do nothing here.
    }
}


// usage
let response = HTTPURLResponse(url: urlRequestedMock1, statusCode: 200, httpVersion: nil, headerFields: nil)
let error: Error? = nil
let data = """
  [
    {
      "someJsonKey": "someJsonData",
      "anotherJsonKey": "anotherJsonData"
    }
  ]
""".data(using: .utf8)

URLProtocolMock.mockURLs = [
  urlRequestedMock1: (error, data, response),
  //...
]

let sessionConfiguration = URLSessionConfiguration.ephemeral
sessionConfiguration.protocolClasses = [URLProtocolMock.self]
let mockedSession =  URLSession(configuration: sessionConfiguration)

let myAwesomeNetworkService = MyAwesomeNetworkService(urlSession: mockedSession)

myAwesomeNetworkService.fetchDataFromAwesomeUrl { response in
  // your awesome assertions
}