Please check the previous articles on Network Layer Part 1 and Part 2.
What is Mocking?
In object-oriented programming, mock objects are simulated objects that mimic the behavior of real objects in controlled ways. (source: wiki)
In our case, we are looking to mimic a URLSession so that it will return the mock object without hitting the physical server. We can achieve this by creating MockURLProtocol that conforms to URLProtocol and implements its required methods. Now our MockURLProtocol will be capable of handling the specified request.
Now we need to add our MockURLProtocol to the URLSessionConfiguration.protocolClasses array to handle requests in a session.
Mocking in Action:
For example, when mocking the loginUser function, we will prepare a fake response object that mimics what we expect on a real server call. This response object will be returned as a response from the requestHandler of the MockURLProtocol.
Actual implementation for reference
Fake response object
The final mockLoginUser function will look like this:
Then check updates in the APIHandler.swift file, as updated code using Builder Pattern and refactored BaseRequest, AuthRequest with RequestBuilder.
Run the tests again. After refactoring the code, we observed that all the tests passed without changing the test cases. The advantage of testing is that we can guarantee that all cases are handled even after refactoring.