协议 - 符琼林的个人博客

协议(Protocol)

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)

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

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
也有办法办到只实现部分内容，以后再聊

protocol Test1 {}
protocol Test2 {}
protocol Test3 {}
class TestClass : Test1, Test2, Test3 {}

协议中的属性

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

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

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 }
    }
}

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")
    }
}

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
    }
}

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) }
}

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) {}
}

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() {}
}

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) {}

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

CustomStringConvertible

遵守CustomStringConvertible、 CustomDebugStringConvertible协议，都可以自定义实例的打印字符串
print调用的是CustomStringConvertible协议的description
debugPrint、po调用的是CustomDebugStringConvertible协议的debugDescription

class Person : CustomStringConvertible, CustomDebugStringConvertible {
    var age = 0
    var description: String { "person_\(age)" }
    var debugDescription: String { "debug_person_\(age)" }
}
var person = Person()
print(person) // person_0
debugPrint(person) // debug_person_0

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)
- 在协议后面写上: class也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<Any>()
var data = [Any]()
data.append(1)
data.append(3.14)
data.append(Student())
data.append("Jack")
data.append({ 10 })

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

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

元类型的应用

class Animal { required init() {} }
class Cat : Animal {}
class Dog : Animal {}
class Pig : Animal {}

func create(_ clses: [Animal.Type]) -> [Animal] {
    var arr = [Animal]()
    for cls in clses {
        arr.append(cls.init())
    }
    return arr
}
print(create([Cat.self, Dog.self, Pig.self]))

import Foundation
class Person {
    var age: Int = 0
}
class Student : Person {
    var no: Int = 0
}
print(class_getInstanceSize(Student.self)) // 32
print(class_getSuperclass(Student.self)!) // Person
print(class_getSuperclass(Person.self)!) // Swift._SwiftObject

从结果可以看得出来，Swift还有个隐藏的基类:Swift._SwiftObject
可以参考Swift源码

Self

Self代表当前类型

class Person {
    var age = 1
    static var count = 2
    func run() {
        print(self.age) // 1
        print(Self.count) // 2
    }
}

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())