如何在 Swift 中存根 URLSession？

Question

I have been following this tutorial to stub out URLSession . The example was done by creating a protocol and extending the existing URLSession .

protocol URLSessionProtocol {
    typealias DataTaskResult = (Data?, URLResponse?, Error?) -> Void
    func dataTask(with request: NSURLRequest, completionHandler: @escaping DataTaskResult) -> URLSessionDataTaskProtocol
}

extension URLSession: URLSessionProtocol {
    func dataTask(with request: NSURLRequest, completionHandler: @escaping DataTaskResult) -> URLSessionDataTaskProtocol {
        return dataTask(with: request, completionHandler: completionHandler) as URLSessionDataTaskProtocol
    }
}

The unit tests work as expected. But when I try to run the real thing, the URLSession -> datatask() gets into an infinite loop and it crashes. It seems to be that datatask() is calling itself.

What am I overlooking, please?

UPDATE:

protocol URLSessionDataTaskProtocol {
    var originalRequest: URLRequest? { get }
    func resume()
}

extension URLSessionDataTask: URLSessionDataTaskProtocol {}

Answer 1

I have finally found the solution. It's fascinating as we missed the wood for the trees. There are two issues:

1) It seems that Swift 4 has changed the signature for dataTask(with: NSURLRequest) to dataTask(with: URLRequest)

Therefore the line in my opening question would only match to the Protocol's func signature, and it would never hit the dataTask inside URLSession , hence the infinite loop. To solve this issue I had to change NSURLRequest to URLRequest and refactor the code accordingly.

2) The signature remains vague, hence it is better to store the result as dataTask first with a cast to URLSessionDataTask and then return the variable.

New refactored Code for Swift 4:

typealias DataTaskResult = (Data?, URLResponse?, Error?) -> Void

protocol URLSessionProtocol {
    func dataTask(with request: URLRequest, completionHandler: @escaping DataTaskResult) -> URLSessionDataTaskProtocol
}

extension URLSession: URLSessionProtocol {
    func dataTask(with request: URLRequest, completionHandler: @escaping DataTaskResult) -> URLSessionDataTaskProtocol {
        let task:URLSessionDataTask = dataTask(with: request, completionHandler: {
            (data:Data?, response:URLResponse?, error:Error?) in completionHandler(data,response,error) }) as URLSessionDataTask
        return task
    }
}

I also found I had to inject URLSession.shared as a singleton and not as URLSession() , otherwise it could crash.

Answer 2

Came here to understand how to mock URLSession tasks such as URLSessionDataTask ?

Circa Swift 5, it is much easier to mock the URLProtocol that URLSession is using to send the request.

See Unit Testing URLSession using URLProtocol.