Sample code for the article on Protocol Extensions at Ray Wenderlich

ProtocolExtensions-RayWenderlich-Sample.swift

// Article Name: Introducing Protocol-Oriented Programming in Swift 2
// Article URL: https://www.raywenderlich.com/109156/introducing-protocol-oriented-programming-in-swift-2
protocol Flyable {
    var speed: Double { get }
}

protocol Bird {
    var name: String { get }
    var canFly: Bool { get }
}

extension Bird where Self: Flyable {
    var canFly: Bool { return true }    // all birds that extend Flyable can fly; so there is no need of canFly in each struct
}

struct Sparrow: Bird, Flyable {
    var name = "Sparrow"
    // var canFly: Bool { return true }    // Sparrows can fly
    var speed = 100.0
}

struct Robin: Bird, Flyable {
    var name = "Robin"
    // var canFly: Bool { return true }    // Robins can fly
    var speed = 50.0
}

struct Penguin: Bird {
    var name = "Penguin"
    var canFly: Bool { return false }  // Penguins cannot fly
}

let sparrow = Sparrow()
sparrow.name
print("sparrow can fly? \(sparrow.canFly)")
sparrow.speed

let penguin = Penguin()
penguin.name
print("penguin can fly? \(penguin.canFly)")
// penguin.speed  Penguin does not have spee because it cannot fly