Apple uses protocols extensively in the Swift standard library. The best resource that we have to see the makeup of the standard library is http://swiftdoc.org. This site shows us the types, protocols, operators, and globals that make up the standard library. To see how Apple uses protocols, let's look at the Dictionary type. This is a very commonly used type, but also one that has a pretty simple protocol hierarchy. From the http://swiftdoc.org/ main page, click on the Dictionary type. Then, scroll about halfway down the page until you see the Inheritance section, which should look similar to the following screenshot:
This section lists the protocols that the Dictionary type conforms to. If we click on the View Protocol Hierarchy → link, we will see a graphical representation of the protocol hierarchy, which will look similar to this:
As we can see from the preceding diagram, the Dictionary type conforms to five different protocols. We can also see that the Collection protocol inherits requirements from the Sequence protocol.
From the http://swiftdoc.org/ main page, we can click on each of the protocols to see their requirements. From this site, we can see that Apple uses protocols extensively within the Swift standard library. We will be looking at this site as we go through this book to see how Apple uses the various technologies that we are discussing.