Mocking URLSession to return a Mocked URLSessionDataTask - swift

Question

I want to mock URLSession, and return a mocked URLSessionDataTask.
To Mock URLSession I create a protocol
protocol URLSessionProtocol {
func dataTask(with url: URL, completionHandler: #escaping (Data?, URLResponse?, Error?) -> Void) -> URLSessionDataTask
}
that URLSession can then conform to in an extension
extension URLSession: URLSessionProtocol {}
Now I want to do the same for URLSessionDataTask, and so implement a similar protocol and extension for it. I need to do this, since the way I call URLSession requires use of func dataTask(with url: URL, completionHandler: #escaping (Data?, URLResponse?, Error?) -> Void) -> URLSessionDataTask
protocol URLSessionDataTaskProtocol {
func resume()
}
extension URLSessionDataTask: URLSessionDataTaskProtocol {}
So then my URLSessionDataTask mock is set up as follows:
class URLSessionMock: URLSessionProtocol {
typealias CompletionHandler = (Data?, URLResponse?, Error?) -> Void
// data and error can be set to provide data or an error
var data: Data?
var error: Error?
func dataTask(
with url: URL,
completionHandler: #escaping CompletionHandler
) -> URLSessionDataTask {
let data = self.data
let error = self.error
return URLSessionDataTaskMock {
completionHandler(data, nil, error)
}
}
}
With my URLSessionDataTaskMock presented with:
class URLSessionDataTaskMock: URLSessionDataTaskProtocol {
private let closure: () -> Void
init(closure: #escaping () -> Void) {
self.closure = closure
}
// override resume and call the closure
func resume() {
closure()
}
}
Doesn't work since URLSessionDataTaskMock within the URLSessionProtocol isn't the correct return type - I need to return a URLSessionDataTask.
I can't cast my URLSessionDataTaskMock to URLSessionDataTask as the types are not related.
How can I return my URLSessionDataTaskMock from my URLSessionProtocol?

Answer 1

You can probably get away with something like this. The key would be your associated type in your URLSessionProtocol
protocol URLSessionProtocol {
associatedtype DataTaskType
func dataTask(with url: URL, completionHandler: #escaping (Data?, URLResponse?, Error?) -> Void) -> DataTaskType
}
extension URLSession: URLSessionProtocol {}
protocol URLSessionDataTaskProtocol {
func resume()
}
extension URLSessionDataTask: URLSessionDataTaskProtocol {}
class URLSessionDataTaskMock: URLSessionDataTaskProtocol {
typealias CompletionHandler = URLSessionMock.CompletionHandler
private let completion: CompletionHandler
init(completion: #escaping CompletionHandler) {
self.completion = completion
}
func resume() {
// create some data
completion(nil, nil, nil)
}
}
class URLSessionMock: URLSessionProtocol {
typealias CompletionHandler = (Data?, URLResponse?, Error?) -> Void
// data and error can be set to provide data or an error
var data: Data?
var error: Error?
func dataTask(
with url: URL,
completionHandler: #escaping CompletionHandler
) -> URLSessionDataTaskMock {
return URLSessionDataTaskMock(completion: completionHandler)
}
}