Swift语法 -- [14 - 协议]

1.协议介绍

协议可以用来定义方法、属性、下标的声明，协议可以被枚举、结构体、类遵守(多个协议之间用逗号隔开)

// 创建协议
  protocol Drawable {
    func draw()
    var x: Int { get set }
    var y: Int { get }
    subscript(index: Int) -> Int { get set }
}

// 遵守多个协议
protocol Test1 { }
protocol Test2 { }
protocol Test3 { }
class TestClass : Test1, Test2, Test3 { }

• 协议中定义方法时不能有默认参数值
• 默认情况下，协议中定义的内容必须全部都实现

2. 协议中的属性

• 协议中定义属性时必须用var关键字
• 实现协议时的属性权限要不小于协议中定义的属性权限
  • 协议定义get、set，用var存储属性或get、set计算属性去实现
  • 协议定义get，用任何属性都可以实现

protocol Drawable {
    func draw()
    var x: Int { get set }
    var y: Int { get }
    subscript(index: Int) -> Int { get set }
}

class Person : Drawable {
    var x: Int = 0
    let y: Int = 0
    func draw() {
        print("Person draw")
    }
    subscript(index: Int) -> Int { 
                  set { }
                  get { index }
     }
}

class Person : Drawable {
     var x: Int {
     get { 0 }
     set { } }
     var y: Int { 0 }
    func draw() { print("Person draw") }  
    subscript(index: Int) -> Int {
             set { }
             get { index } }
}

3. static、class

为了保证通用，协议中必须用static定义类型方法、类型属性、类型下标

 protocol Drawable {
    static func draw()
}
class Person1 : Drawable {
    class func draw() {
        print("Person1 draw")
    }
}
class Person2 : Drawable {
    static func draw() {
        print("Person2 draw")
    }
}

4.mutating

• 只有将协议中的实例方法标记为mutating
  • 才允许结构体、枚举的具体实现修改自身内存
  • 类在实现方法时不用加mutating，枚举、结构体才需要加mutating

protocol Drawable {
    mutating func draw()
}
class Size : Drawable {
    var width: Int = 0
    func draw() {
width = 10 }
}
struct Point : Drawable {
    var x: Int = 0
    mutating func draw() {
        x = 10
} }

5. init

• 协议中还可以定义初始化器init
  • 非final类实现时必须加上required

protocol Drawable {
    init(x: Int, y: Int)
}
class Point : Drawable {
    required init(x: Int, y: Int) { }
}
final class Size : Drawable {
    init(x: Int, y: Int) { }
}

• 如果从协议实现的初始化器，刚好是重写了父类的指定初始化器
  • 那么这个初始化必须同时加required、override

protocol Livable {
    init(age: Int)
}
class Person {
    init(age: Int) { }
}
class Student : Person, Livable {
    required override init(age: Int) {
        super.init(age: age)
    }
}

6.init、init?、init!

• 协议中定义的init?、init!，可以用init、init?、init!去实现
• 协议中定义的init，可以用init、init!去实现

 protocol Livable {
    init()
    init?(age: Int)
    init!(no: Int)
}

class Person : Livable {
    required init() { }
    // required init!() { }
    required init?(age: Int) { }
    // required init!(age: Int) { }
    // required init(age: Int) { }
    required init!(no: Int) { }
    // required init?(no: Int) { }
    // required init(no: Int) { }
}

7.协议的继承

一个协议可以继承其他协议

 protocol Runnable {
    func run()
}
protocol Livable : Runnable {
    func breath()
}
class Person : Livable {
    func breath() { }
    func run() { }
}

8. 协议组合

协议组合，可以包含1个类类型(最多1个)

protocol Livable { }
protocol Runnable { }
class Person { }

 // 接收Person或者其子类的实例
func fn0(obj: Person) { }
// 接收遵守Livable协议的实例
func fn1(obj: Livable) { }
// 接收同时遵守Livable、Runnable协议的实例
func fn2(obj: Livable & Runnable) { }
// 接收同时遵守Livable、Runnable协议、并且是Person或者其子类的实例 func fn3(obj: Person & Livable & Runnable) { }

 typealias RealPerson = Person & Livable & Runnable
// 接收同时遵守Livable、Runnable协议、并且是Person或者其子类的实例 func fn4(obj: RealPerson) { }

9.CaseIterable

让枚举遵守CaseIterable协议，可以实现遍历枚举值

enum Season : CaseIterable {
    case spring, summer, autumn, winter
}
let seasons = Season.allCases
print(seasons.count) // 4
for season in seasons {
    print(season)
} // spring summer autumn winter

10. CustomStringConvertible

遵守CustomStringConvertible协议，可以自定义实例的打印字符串

 class Person : CustomStringConvertible {
    var age: Int
    var name: String
    init(age: Int, name: String) {
        self.age = age
        self.name = name
    }
    var description: String {
        "age=\(age), name=\(name)"
} }
var p = Person(age: 10, name: "Jack")
print(p) // age=10, name=Jack

11. Any、AnyObject

Swift提供了2种特殊的类型:Any、AnyObject

• Any:可以代表任意类型(枚举、结构体、类，也包括函数类型)
• AnyObject:可以代表任意类类型(在协议后面写上: AnyObject代表只有类能遵守这个协议)

var stu: Any = 10
stu = "Jack"
stu = Student()

 // 创建1个能存放任意类型的数组 
 // var data = Array() 
 var data = [Any]() data.append(1) data.append(3.14) data.append(Student()) data.append("Jack") data.append({ 10 })

12. is、as?、as!、as

is用来判断是否为某种类型，as用来做强制类型转换

protocol Runnable { func run() } class Person { }
class Student : Person, Runnable {
    func run() {
        print("Student run")
    }
    func study() {
        print("Student study")
    }
}

var stu: Any = 10
print(stu is Int) // true
stu = "Jack"
print(stu is String) // true 
stu = Student()
print(stu is Person) // true 
print(stu is Student) // true 
print(stu is Runnable) // true

var stu: Any = 10
(stu as? Student)?.study() // 没有调用study stu = Student()
(stu as? Student)?.study() // Student study (stu as! Student).study() // Student study (stu as? Runnable)?.run() // Student run

 var data = [Any]() 
 data.append(Int("123") as Any)

var d = 10 as Double
print(d) // 10.0

13. X.self、X.Type、AnyClass

X.self是一个元类型(metadata)的指针，metadata存放着类型相关信息
X.self属于X.Type类型

class Person { }
class Student : Person { }
var perType: Person.Type = Person.self var stuType: Student.Type = Student.self perType = Student.self

var anyType: AnyObject.Type = Person.self anyType = Student.self
public typealias AnyClass = AnyObject.Type var anyType2: AnyClass = Person.self 
anyType2 = Student.self

var per = Person()
var perType = type(of: per) // Person.self print(Person.self == type(of: per)) // true

14. Self

Self一般用作返回值类型，限定返回值跟方法调用者必须是同一类型(也可以作为参数类型)

protocol Runnable {
    func test() -> Self
}
class Person : Runnable {
    required init() { }
func test() -> Self { type(of: self).init() } }
class Student : Person { }

var p = Person()
// Person
print(p.test())
var stu = Student()
// Student
print(stu.test())