Adding methods

Now, we may want to add a method to this State enumeration. After all, it's very common to just toggle the switch on and off without thinking:

extension State {
    mutating func toggle() {
        self = self == .off ? .on : .off
    }
}

var state: State = .on
state.toggle()
state == .off // true

As in the previous section, we can just extend the State enum to add the toggle functionality. Enums follow value semantics; therefore, we have to mark the toggle method as mutating.