• 定义一个协议(protocol)，swift 中可以对protocol 进行扩展(extension)
• 通过协议的扩展可以对函数有默认的实现

swift">protocol Sleepable {
    func sleep()
}
protocol Eatable {
    func eat()
}
extension Eatable {
    func eat() {
        print("eat food")
    }
}

• 在类(class)或结构体(struct)中实现protocol

swift">//结构体实现协议
struct Cat: Eatable, Sleepable {
    func sleep() {
        print("cat sleep")
    }
}

class Animal {
    func say() {
        print("say hello")
    }
}

//类，继承和实现协议
class Dog: Animal, Eatable, Sleepable {
    func eat() {
        print("Dog eat")
    }

    func sleep() {
        print("Dog sleep")
    }
}

swift">//Cat为结构体
let cat = Cat()
cat.eat()
cat.sleep()

//Dog为class，继承了Animal的say方法,也获得了protocol的默认实现eat方法
let dog = Dog()
dog.eat()
dog.sleep()
dog.say()

//输出结果：
//eat food
//cat sleep
//eat food
//Dog sleep
//say hello

• 为现有类型（Int ,String）扩展协议功能

swift">protocol Describable {
    func describe() -> String
}
extension Describable {
    func describe() -> String {
        return "This is the number \(self)"
    }
}

//Int使用默认的协议实现
extension Int: Describable {
    
}
extension String: Describable {
    func describe() -> String {
        return "This is the string \"\(self)\""
    }
}

let number: Describable = 42
print(number.describe()) // 输出：This is the number 42

let text: Describable = "Hello"
print(text.describe()) // 输出：This is the string "Hello"

• 枚举实现协议

swift">
enum Pen:Printable{
    case normal
    
}
enum Book: Printable {
    case fiction, nonFiction, biography

    func printDetails() -> String {
        switch self {
        case .fiction:
            return "Fiction Book"
        case .nonFiction:
            return "Non-Fiction Book"
        case .biography:
            return "Biography Book"
        }
    }
}

 let pen = Pen.normal
 print(pen.printDetails())//This is a printable item.

 let book = Book.fiction
 print(book.printDetails()) // 输出：Fiction Book

• 为所有遵循 RawRepresentable 协议的枚举类型提供一个通用的 printRawValue 方法

swift">protocol RawRepresentablePrintable: RawRepresentable {
    func printRawValue()
}

extension RawRepresentablePrintable {
    func printRawValue() {
        print("Raw value: \(self.rawValue)")
    }
}

enum Direction: Int, RawRepresentablePrintable {
    case north = 1
    case south = 2
    case east = 3
    case west = 4
}

let direction = Direction.north
direction.printRawValue() // 输出：Raw value: 1