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Optional requirements
There are times when we want protocols to define optional requirements. An optional requirement is a method or property that is not required to be implemented. To use optional requirements, we need to start off by marking the protocol with the @objc attribute.
It is important to note that only classes can adopt protocols that use the @objc attribute. Structures and enumerations cannot adopt these protocols.
To mark a property or method as optional, we use the optional keyword. The following example shows how we would create both an optional property and also an optional method:
@objc protocol Phone {
var phoneNumber: String {get set}
@objc optional var emailAddress: String {get set}
func dialNumber()
@objc optional func getEmail()
}
Now let's explore how protocol inheritance works.