Protocol inheritance

One protocol can inherit from another in a process known as protocol inheritance. Unlike with classes, you can inherit from multiple protocols at the same time before you add your own customizations on top.

We’re going to define three protocols: Payable requires conforming types to implement a calculateWages() method, NeedsTraining requires conforming types to implement a study() method, and HasVacation requires conforming types to implement a takeVacation() method:

protocol Payable {
    func calculateWages() -> Int
}

protocol NeedsTraining {
    func study()
}

protocol HasVacation {
    func takeVacation(days: Int)
}

We can now create a single Employee protocol that brings them together in one protocol. We don’t need to add anything on top, so we’ll just write open and close braces:

protocol Employee: Payable, NeedsTraining, HasVacation { }

Now we can make new types conform to that single protocol rather than each of the three individual ones.