Swift协议

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2023-03-14

协议(Protocols)为方法，属性和其他需求功能提供了蓝图。它为方法或属性骨架而不是实现。通过定义类，函数和枚举，可以进一步完成方法和属性的实现。协议的一致性满足了协议要求的方法或属性。

语法
协议也遵循与类，结构和枚举类似的语法 -

protocol SomeProtocol {
   // protocol definition 
}

协议在类，结构或枚举类型名称之后声明。单个和多个协议声明也是可以的。如果定义了多个协议，则必须用逗号分隔。

struct SomeStructure: Protocol1, Protocol2 {
   // structure definition 
}

当要为超类定义协议时，协议名称应使用逗号跟随超类名称。

class SomeClass: SomeSuperclass, Protocol1, Protocol2 {
   // class definition 
}

属性和方法要求

协议用于指定特定的类类型属性或实例属性。它只是单独指定类型或实例属性，而不是指定它是存储属性还是计算属性。此外，它还用于指定属性是gettable或settable。

属性需求由var关键字声明为属性变量。 {get/set}用于在类型声明后声明gettable和settable属性。参考以下一个示例代码 -

protocol classa {
   var marks: Int { get set }
   var result: Bool { get }

   func attendance() -> String
   func markssecured() -> String
}

protocol classb: classa {
   var present: Bool { get set }
   var subject: String { get set }
   var stname: String { get set }
}

class classc: classb {
   var marks = 96
   let result = true
   var present = false
   var subject = "Swift 4 Protocols"
   var stname = "Protocols"

   func attendance() -> String {
      return "The \(stname) has secured 99% attendance"
   }
   func markssecured() -> String {
      return "\(stname) has scored \(marks)"
   }
}

let studdet = classc()
studdet.stname = "Swift 4"
studdet.marks = 98
studdet.markssecured()

print(studdet.marks)
print(studdet.result)
print(studdet.present)
print(studdet.subject)
print(studdet.stname)

当使用playground 运行上述程序时，得到以下结果 -

98
true
false
Swift 4 Protocols
Swift 4

变异方法要求

protocol daysofaweek {
   mutating func print()
}

enum days: daysofaweek {
   case sun, mon, tue, wed, thurs, fri, sat 
   mutating func print() {
      switch self {
         case sun:
            self = sun
            print("Sunday")
         case mon:
            self = mon
            print("Monday")
         case tue:
            self = tue
            print("Tuesday")
         case wed:
            self = wed
            print("Wednesday")
         case mon:
            self = thurs
            print("Thursday")
         case tue:
            self = fri
            print("Friday")
         case sat:
            self = sat
            print("Saturday")
         default:
            print("NO Such Day")
      }
   }
}

var res = days.wed
res.print()

当使用playground 运行上述程序时，得到以下结果 -

Wednesday

初始化器要求

Swing允许用户初始化协议以遵循类似于普通初始化器的类型一致性。

语法

protocol SomeProtocol {
   init(someParameter: Int)
}

示例

protocol tcpprotocol {
   init(aprot: Int)
}

协议初始化程序要求的类实现

指定或便捷初始化器允许用户初始化协议以使其标准符合保留的required关键字。

class SomeClass: SomeProtocol {
   required init(someParameter: Int) {
      // initializer implementation statements
   }
}

protocol tcpprotocol {
   init(aprot: Int)
}

class tcpClass: tcpprotocol {
   required init(aprot: Int) {
   }
}

当使用playground 运行上述程序时，得到以下结果 -

Wednesday

通过required修饰符确保所有子类的协议一致性，用于显式或继承实现。当子类重写其超类初始化要求时，由override修饰符关键字指定。

protocol tcpprotocol {
   init(no1: Int)
}

class mainClass {
   var no1: Int        // local storage
   init(no1: Int) {
      self.no1 = no1  // initialization
   }
}

class subClass: mainClass, tcpprotocol {
   var no2: Int
   init(no1: Int, no2 : Int) {
      self.no2 = no2
      super.init(no1:no1)
   }
   // Requires only one parameter for convenient method
   required override convenience init(no1: Int) {
      self.init(no1:no1, no2:0)
   }
}

let res = mainClass(no1: 20)
let print = subClass(no1: 30, no2: 50)

print("res is: \(res.no1)")
print("res is: \(print.no1)")
print("res is: \(print.no2)")

当使用playground 运行上述程序时，得到以下结果 -

res is: 20
res is: 30
res is: 50

作为类型的协议

它们不是在协议中实现功能，而是用作函数，类，方法等的类型。协议可以作为以下类型访问 -

函数，方法或初始化为参数或返回类型
常数，变量或属性
数组，字典或其他容器作为项目

示例代码

protocol Generator {
   typealias members
   func next() -> members?
}

var items = [10,20,30].generate()
while let x = items.next() {
   print(x)
}

for lists in map([1,2,3], {i in i*5}) {
   print(lists)
}

print([100,200,300])
print(map([1,2,3], {i in i*10}))

当使用playground 运行上述程序时，得到以下结果 -

10
20
30
5
10
15
[100, 200, 300]
[10, 20, 30]

添加协议与扩展的一致性

通过使用扩展，可以采用现有类型并使其符合新协议。借助扩展，可以将新属性，方法和下标添加到现有类型中。

示例代码

protocol AgeClasificationProtocol {
   var age: Int { get }
   func agetype() -> String
}
class Person {
   let firstname: String
   let lastname: String
   var age: Int

   init(firstname: String, lastname: String) {
      self.firstname = firstname
      self.lastname = lastname
      self.age = 10
   }
}

extension Person : AgeClasificationProtocol {
   func fullname() -> String {
      var c: String
      c = firstname + " " + lastname
      return c
   }
   func agetype() -> String {
      switch age {
         case 0...2:
            return "Baby"
         case 2...12:
            return "Child"
         case 13...19:
            return "Teenager"
         case let x where x > 65:
            return "Elderly"
         default:
            return "Normal"
      }
   }
}

协议继承

Swift 4允许协议从其定义的属性继承属性。它类似于类继承，但可以选择列出用逗号分隔的多个继承协议。

protocol classa {
   var no1: Int { get set }
   func calc(sum: Int)
}
protocol result {
   func print(target: classa)
}
class student2: result {
   func print(target: classa) {
      target.calc(sum: 1)
   }
}
class classb: result {
   func print(target: classa) {
      target.calc(sum: 5)
   }
}

class student: classa {
   var no1: Int = 10

   func calc(sum: Int) {
      no1 -= sum
      print("Student attempted \(sum) times to pass")

      if no1 <= 0 {
         print("Student is absent for exam")
      }
   }
}

class Player {
   var stmark: result!

   init(stmark: result) {
      self.stmark = stmark
   }
   func print(target: classa) {
      stmark.print(target: target)
   }
}

var marks = Player(stmark: student2())
var marksec = student()

marks.print(target: marksec)
marks.print(target: marksec)
marks.print(target: marksec)
marks.stmark = classb()
marks.print(target: marksec)
marks.print(target: marksec)
marks.print(target: marksec)

当使用playground 运行上述程序时，得到以下结果 -

Student attempted 1 times to pass
Student attempted 1 times to pass
Student attempted 1 times to pass
Student attempted 5 times to pass
Student attempted 5 times to pass
Student is absent for exam
Student attempted 5 times to pass
Student is absent for exam

仅类协议

当定义协议并且用户想要用类定义协议时，应该首先定义类，然后是协议的继承列表来添加协议。

protocol tcpprotocol {
   init(no1: Int)
}
class mainClass {
   var no1: Int        // local storage
   init(no1: Int) {
      self.no1 = no1  // initialization
   }
}
class subClass: mainClass, tcpprotocol {
   var no2: Int
   init(no1: Int, no2 : Int) {
      self.no2 = no2
      super.init(no1:no1)
   }

   // Requires only one parameter for convenient method
   required override convenience init(no1: Int) {
      self.init(no1:no1, no2:0)
   }
}

let res = mainClass(no1: 20)
let print = subClass(no1: 30, no2: 50)

print("res is: \(res.no1)")
print("res is: \(print.no1)")
print("res is: \(print.no2)")

当使用playground 运行上述程序时，得到以下结果 -

res is: 20
res is: 30
res is: 50

协议组成

Swift 4允许使用协议组合一次调用多个协议。

语法

protocol<SomeProtocol, AnotherProtocol>

示例代码

protocol stname {
   var name: String { get }
}
protocol stage {
   var age: Int { get }
}
struct Person: stname, stage {
   var name: String
   var age: Int
}
func print(celebrator: stname & stage) {
   print("\(celebrator.name) is \(celebrator.age) years old")
}
let studname = Person(name: "Yiibai", age: 21)
print(studname)

let stud = Person(name: "Maxsu", age: 29)
print(stud)

let student = Person(name: "Roshan", age: 19)
print(student)

当使用playground 运行上述程序时，得到以下结果 -

Person(name: "Yiibai", age: 21)
Person(name: "Maxsu", age: 29)
Person(name: "Roshan", age: 19)

检查协议一致性

协议一致性由is和as运算符测试，类似于类型转换。

如果实例符合协议标准，则is运算符返回true，如果失败则返回false。
as?向下转换运算符的版本返回协议类型的可选值，如果实例不符合该协议，则此值为nil。
如果向下转换操作符的as版本强制转换为协议类型，并且如果向下转换不成功则触发运行时错误。

示例代码

import Foundation

@objc protocol rectangle {
   var area: Double { get }
}
@objc class Circle: rectangle {
   let pi = 3.1415927
   var radius: Double
   var area: Double { return pi * radius * radius }
   init(radius: Double) { self.radius = radius }
}
@objc class result: rectangle {
   var area: Double
   init(area: Double) { self.area = area }
}
class sides {
   var rectsides: Int
   init(rectsides: Int) { self.rectsides = rectsides }
}
let objects: [AnyObject] = [Circle(radius: 2.0),result(area:198),sides(rectsides: 4)]

for object in objects {
   if let objectWithArea = object as? rectangle {
      print("Area is \(objectWithArea.area)")
   } else {
      print("Rectangle area is not defined")
   }
}

当使用playground 运行上述程序时，得到以下结果 -

Area is 12.5663708
Area is 198.0
Rectangle area is not defined