Protocols, like other types, can be extended. Protocol extensions can be used to provide common functionality to all types that conform to a particular protocol. This gives us the ability to add functionality to any type that conforms to a protocol rather than adding the functionality to each individual type or through a global function. Protocol extensions, like regular extensions, also give us the ability to add functionality to types for which we do not have the source code.

Protocol-oriented programming and frameworks such as GameplayKit rely heavily on protocol extensions. Without protocol extensions, if we wanted to add specific functionality to a group of types that conformed to a protocol, we would have to add the functionality to each of the types. If we were using reference types (classes), we could create a class hierarchy, but, as we mentioned...