अनुबंध के रूप में प्रोटोकॉल

protocol Identifiable {
    var id: String { get }
}

struct User: Identifiable {
    let id: String
}

print(User(id: "u-1").id)

protocol Shape {
    func area() -> Double
}

struct Square: Shape {
    let side: Double
    func area() -> Double { side * side }
}

print(Square(side: 3).area())

protocol Toggleable {
    mutating func toggle()
}

struct Switch: Toggleable {
    var isOn = false
    mutating func toggle() { isOn.toggle() }
}

var s = Switch()
s.toggle()
print(s.isOn)

protocol Animal { func sound() -> String }
struct Dog: Animal { func sound() -> String { "Woof" } }
struct Cat: Animal { func sound() -> String { "Meow" } }

let animals: [Animal] = [Dog(), Cat()]
for a in animals { print(a.sound()) }

protocol Describable { func describe() -> String }
struct Book: Describable { let t: String; func describe() -> String { "Book: " + t } }
struct Film: Describable { let t: String; func describe() -> String { "Film: " + t } }

func show(_ d: Describable) { print(d.describe()) }
show(Book(t: "Swift"))
show(Film(t: "Inception"))

protocol FromInt {
    init(value: Int)
}

struct Wrapper: FromInt {
    let value: Int
    init(value: Int) { self.value = value }
}

print(Wrapper(value: 42).value)

protocol Named { var name: String { get } }
protocol Aged: Named { var age: Int { get } }

struct Member: Aged {
    let name: String
    let age: Int
}

let m = Member(name: "Lin", age: 30)
print(m.name, m.age)

protocol Counter: AnyObject {
    var count: Int { get set }
}

class Box: Counter {
    var count = 0
}

let b = Box()
b.count = 5
print(b.count)

protocol Flyer { func fly() }
struct Bird: Flyer { func fly() { print("flap") } }
struct Rock {}

let things: [Any] = [Bird(), Rock()]
for t in things {
    if let f = t as? Flyer { f.fly() } else { print("grounded") }
}