PRotocol Pet {        var name: String {get set}    var birthPlace: String {get}    func playWith()    func fed()        mutating func changeName(name: String)}struct Dog: Pet {        internal mutating func changeName(name: String) { // 結構體加上mutating        self.name = name    }        var name: String = "dog"    var birthPlace: String = "china"        func fed() {        print("fed me")    }    internal func playWith() {        print("play with me")    }}
協議里定義的方法和屬性要全部實現
var dog:Dog = Dog()dog.birthPlace = "beijing"var pet:Pet = Dog()pet.birthPlace = "beijing"再看上面的代碼，前兩行聲明一個Dog類型的實例，可以修改其birthPlace,后面兩行聲明的是一個Pet類型的實例，不能修改其birthPlace,因為在Pet協議里birthPlace是只讀的可以看出，雖然Dog類實現了Pet協議，但協議規定的只讀屬性發生了改變。
協議里聲明構造函數：
protocol Pet {        var name: String {get set}    var birthPlace: String {get}    func playWith()    func fed()    init(name: String)    mutating func changeName(name: String)}class Animal {    var legs: Int = 4    var name: String = "i am a animal"    init(name: String) {        self.name = name    }}class Bird: Animal, Pet {    override required init(name: String) { // override關鍵字 對應 父類animal, required 對應協議Pet        super.init(name: name)    }    internal func changeName(name: String) {            }    internal func fed() {            }    internal func playWith() {            }    internal var birthPlace: String = ""}