Most apps in the App Store perform networking in one way or the other. Apple provides a great class for network requests--URLSession. Its requests are asynchronous. This means that the response is delivered on a background thread. If that wasn't the case, the UI would freeze while the app waits for a response from the server.

The main topic of this chapter is how to test an asynchronous API. There are two ways to write tests for asynchronous API calls. First, it can be done using the real server that is going to be used when the app is in the App Store. Second, it can be done using stubs, as we did in the previous chapter.

Both methods have their advantages. Tests with the real server let us additionally test whether the server is implemented as described in the documentation. Those tests are closer to the implementation of the finished app and, therefore, are more likely to find bugs that would end up in the final version.

On the flip side, stubs let us develop the network layer of our app even before the web service is implemented. We just need the documentation of the API calls and the expected responses. As the tests do not depend on communication with a server, the test execution is significantly faster.

You should have both kinds of tests in your iOS development toolbox.

This chapter covers the following topics:

• Implementing tests using a live web service
• Implementing a login request to a fake web service
• Handling errors