This tutorial belongs to the Swift series

A protocol is a way to have different objects, of different types, have a common set of functionality.

A protocol is defined in this way:

protocol Mammal {

}

Structs and classes can adopt a protocol in this way:

struct Dog: Mammal {

}

class Cat: Mammal {

}

A protocol can define properties and methods, without providing values and implementations, and a struct/class must implement them:

protocol Mammal {
    var age: Int { get set }
    func walk()
}

The property can be defined as get or get set. If it’s get, the property must be implemented as read only, with a getter.

Any type that adopts the protocol must conform to the protocol by implementing those methods or providing those properties:

struct Dog: Mammal {
    var age: Int = 0
    func walk() {
        print("The dog is walking")
    }
}

class Cat: Mammal {
    var age: Int = 0
    func walk() {
        print("The cat is walking")
    }
}

Structs and classes can adopt multiple protocols:

struct Dog: Mammal, Animal {

}

class Cat: Mammal, Animal {

}

Notice that for classes, this is the same syntax used to define a superclass. If there is a superclass, list it as the first item in the list, after the colon.