Retrieving data from the web is something that you will often do as an iOS professional. You won't just fetch data from a web service; you'll also send data back to it. For example, you might have to make an HTTP POST request as part of a login flow or to update a user's profile information. Over time, iOS has evolved quite a bit in the web requests department, making it easier to use web services in apps.

When you want to perform a web request in iOS, you will typically use the URLSession class. The URLSession class makes asynchronous web requests on your behalf. This means that iOS loads data from the web on a background thread, ensuring that the user interface remains responsive throughout the entire request. If a web request is performed synchronously, the user interface is unresponsive for the duration of the network request because a thread can only do one thing at a time, so if it's waiting for a response from the network, it can't respond to touches or other user input.

If your user has a slow internet connection, a request could take several seconds. You don't want your interface to freeze for several seconds. Even a couple of milliseconds will create a noticeable drop in its responsiveness and frame rate. This can be easily avoided with URLSession to perform asynchronous network requests.

First, you will experiment with basic network requests in a playground. You can create a new playground or use the one provided in this book's code bundle. After you've seen the basics of URLSession, you'll implement a way to fetch movies from an open source movie database and put this implementation to use in the MustC app.