import UIKit
// 今日目标 20220602 敲swift代码的第三天, 第一天基础到闭包 , 第二天 枚举到构造过程 第三天 析构到协议
//析构过程
//析构器只适用于类类型，当一个类的实例被释放之前，析构器会被立即调用。析构器用关键字 deinit 来标示，类似于构造器要用 init 来标示。
class Bank {
static var coinsInBank = 10_000
static func distribute(coins numberOfCoinsRequested: Int ) -> Int {
let numberOfCoinsToVend = min(numberOfCoinsRequested, coinsInBank)
coinsInBank -= numberOfCoinsToVend
return numberOfCoinsToVend
}
static func receive(coins: Int) {
coinsInBank += coins
}
}
class Player {
var coinsInPurse: Int
init(coins: Int) {
coinsInPurse = Bank.distribute(coins: coins)
}
func win(coins: Int) {
coinsInPurse += Bank.distribute(coins: coins)
}
deinit {
Bank.receive(coins: coinsInPurse)
}
}
var playerOne: Player? = Player(coins: 100)
print("A new player has joined the game with \(playerOne!.coinsInPurse) coins")
print("There are now \(Bank.coinsInBank) coins left in the bank")
playerOne = nil
//可选链
//可选链式调用是一种可以在当前值可能为 nil 的可选值上请求和调用属性、方法及下标的方法。如果可选值有值，那么调用就会成功；如果可选值是 nil，那么调用将返回 nil。多个调用可以连接在一起形成一个调用链，如果其中任何一个节点为 nil，整个调用链都会失败，即返回 nil。
class Person {
var residence: Residence?
}
class Residence {
var numberOfRooms = 1
}
let john = Person()
//let roomCount = john.residence!.numberOfRooms// // 这会引发运行时错误
if let roomCount = john.residence?.numberOfRooms {
print("John's residence has \(roomCount) room(s).")
} else {
print("Unable to retrieve the number of rooms.")
}
// 打印“Unable to retrieve the number of rooms.”
///为可选链式调用定义模型类
class Person2 {
var residence: Residence2?
}
class Residence2 {
var rooms: [Room] = []
var numberOfRooms: Int {
return rooms.count
}
subscript(i: Int) -> Room {
get {
return rooms[i]
}
set {
rooms[i] = newValue
}
}
func printNumberOfRooms() {
print("The number of rooms is \(numberOfRooms)")
}
var address: Address?
}
class Room {
let name: String
init(name: String) { self.name = name }
}
class Address {
var buildingName: String?
var buildingNumber: String?
var street: String?
func buildingIdentifier() -> String? {
if buildingName != nil {
return buildingName
} else if let buildingNumber = buildingNumber, let street = street {
return "\(buildingNumber) \(street)"//如果 buildingNumber 和 street 均有值，则返回两者拼接得到的字符串。否则，返回 nil。
} else {
return nil
}
}
}
//通过可选链式调用访问属性
let john2 = Person2()
if let roomCount = john2.residence?.numberOfRooms {
print("John's residence has \(roomCount) room(s).")
} else {
print("Unable to retrieve the number of rooms.")
}
let someAddress = Address()
someAddress.buildingNumber = "29"
someAddress.street = "road"
john2.residence?.address = someAddress //在这个例子中，通过 john.residence 来设定 address 属性也会失败，因为 john.residence 当前为 nil。
func createAddress() -> Address {
print("Function was called.")
let someAddress = Address()
someAddress.buildingNumber = "29"
someAddress.street = "Acacia Road"
return someAddress
}
john2.residence?.address = createAddress()
if john2.residence?.printNumberOfRooms() != nil {//如果在可选值上通过可选链式调用来调用这个方法，该方法的返回类型会是 Void?，而不是 Void，因为通过可选链式调用得到的返回值都是可选的。这样我们就可以使用 if 语句来判断能否成功调用 printNumberOfRooms() 方法，即使方法本身没有定义返回值。通过判断返回值是否为 nil 可以判断调用是否成功：
print("It was possible to print the number of rooms.")
} else {
print("It was not possible to print the number of rooms.")
}
///在上面的 通过可选链式调用访问属性 的例子中，我们尝试给 john.residence 中的 address 属性赋值，即使 residence 为 nil。通过可选链式调用给属性赋值会返回 Void?，通过判断返回值是否为 nil 就可以知道赋值是否成功
if (john2.residence?.address = someAddress) != nil {
print("It was possible to set the address.")
} else {
print("It was not possible to set the address.")
}
// 打印“It was not possible to set the address.”
//下面这个例子用下标访问 john.residence 属性存储的 Residence 实例的 rooms 数组中的第一个房间的名称，因为 john.residence 为 nil，所以下标调用失败了：
if let firstRoomName = john2.residence?[0].name {
print("The first room name is \(firstRoomName).")
} else {
print("Unable to retrieve the first room name.")
}
// 打印“Unable to retrieve the first room name.”
john2.residence?[0] = Room(name: "Bathroom")
let johnsHouse = Residence2()
johnsHouse.rooms.append(Room(name: "Living Room"))
johnsHouse.rooms.append(Room(name: "Kitchen"))
john2.residence = johnsHouse
if let firstRoomName = john2.residence?[0].name {
print("The first room name is \(firstRoomName).")
}else {
print("Unable to retrieve the first room name.")
}
var testScores = ["Dave": [86, 82, 84], "Bev": [79, 94, 81]]
testScores["Dave"]?[0] = 91
testScores["Bev"]?[0] += 1
testScores["Brain"]?[0] = 72
if let jognsStreat = john2.residence?.address?.street {
print("John's street name is \(jognsStreat).")
} else {
print("Unable to retrieve the address.")
}
let johnsAddress = Address()
johnsAddress.buildingName = "The Larches"
johnsAddress.street = "Laurel Street"
john2.residence?.address = johnsAddress
if let johnsStreet = john2.residence?.address?.street {
print("John's street name is \(johnsStreet).")
} else {
print("Unable to retrieve the address.")
}
if let buildingIdentifier = john2.residence?.address?.buildingIdentifier() {
print("John's building identifier is \(buildingIdentifier).")
}
// 打印“John's building identifier is The Larches.”
if let beginsWithThe = john2.residence?.address?.buildingIdentifier()?.hasPrefix("The") {
print("beginsWithThe = \(beginsWithThe)")
if beginsWithThe {
print("John's building identifier begins with \"The\".")
} else {
print("John's building identifier does not begin with \"The\".")
}
}
// 错误处理
enum VendingMachineError: Error {
case invalidSelection
case insufficientFunds(coinsNeeded: Int)
case outOfStock
}
//抛出一个错误可以让你表明有意外情况发生，导致正常的执行流程无法继续执行。抛出错误使用 throw 语句。例如，下面的代码抛出一个错误，提示贩卖机还需要 5 个硬币：
//throw VendingMachineError.insufficientFunds(coinsNeeded: 5)
//func canThrowErrors() throws -> String
//
//func cannotThrowErrors() -> String
struct Item {
var price: Int
var count: Int
}
class VendingMachine {
var inventory = [
"Candy Bar": Item(price: 12, count: 7),
"Chips": Item(price: 10, count: 4),
"Pretzels": Item(price: 7, count: 11)
]
var coinsDeposited = 0
func vend(itemNamed name: String) throws {
guard let item = inventory[name] else {
throw VendingMachineError.invalidSelection
}
guard item.count > 0 else {
throw VendingMachineError.outOfStock
}
guard item.price <= coinsDeposited else {
throw VendingMachineError.insufficientFunds(coinsNeeded: item.price - coinsDeposited)
}
coinsDeposited -= item.price
var newItem = item
newItem.count -= 1
inventory[name] = newItem
print("Dispensing \(name)")
}
}
let favoriteSnacks2 = [
"Alice": "Chips",
"Bob": "Licorice",
"Eve": "Pretzels",
]
func buyFavoriteSnack(person: String, vendingMachine: VendingMachine) throws {
let snackName = favoriteSnacks2[person] ?? "Candy Bar"
try vendingMachine.vend(itemNamed: snackName)
}
struct PurchasedSnack {
let name: String
init(name: String, vendingMachine:VendingMachine ) throws {
try vendingMachine.vend(itemNamed: name)
self.name = name
}
}
var vendingMachine = VendingMachine()
vendingMachine.coinsDeposited = 8
do {
try buyFavoriteSnack(person: "Alice", vendingMachine: vendingMachine)
print("Success! Yum")
} catch VendingMachineError.invalidSelection {
print("Invalid Selection.")
}catch VendingMachineError.outOfStock {
print("Out of Stock.")
} catch VendingMachineError.insufficientFunds(let coinsNeeded) {
print("Insufficient funds. Please insert an additional \(coinsNeeded) coins.")
} catch {
print("Unexpected error: \(error).")
}
func nourish(with item: String) throws {
do {
try vendingMachine.vend(itemNamed: item)
} catch is VendingMachineError {
print("Couldn't buy that from the vending machine.")
}
}
do {
try nourish(with: "Beet-Flavored Chips")
} catch {
print("Unexpected non-vending-machine-related error: \(error)")
}
func eat(item: String) throws {
do {
try vendingMachine.vend(itemNamed: item)
} catch VendingMachineError.invalidSelection, VendingMachineError.insufficientFunds, VendingMachineError.outOfStock {
print("Invalid selection, out of stock, or not enough money.")
}
}
//func fetchData() -> Data? {
// if let data = try? fetchDataFromDisk() { return data }
// if let data = try? fetchDataFromServer() { return data }
// return nil
//}
//禁用错误传递
//let photo = try! loadImage(atPath: "./Resources/John Appleseed.jpg")
//func processFile(filename: String) throws {
// if exists(filename) {
// let file = open(filename)
// defer {
// close(file)
// }
// while let line = try file.readline() {
// // 处理文件。
// }
// // close(file) 会在这里被调用，即作用域的最后。
// }
//}
// 并发
import Foundation
import Darwin
class MediaItem {
var name: String
init(name: String) {
self.name = name
}
}
class Movie: MediaItem {
var director: String
init(name: String, director: String) {
self.director = director
super.init(name: name)
}
}
class Song: MediaItem {
var artist: String
init(name: String, artist: String) {
self.artist = artist
super.init(name: name)
}
}
//let handle = FileHandle.standardInput
//for try await line in handle.bytes.lines {
// print(line)
//}
//类型转换
//let library = [
// Movie(name: "Casablanca", director: "Michael Curtiz"),
// Song(name: "Blue Suede Shoes", artist: "Elvis Presley"),
// Movie(name: "Citizen Kane", director: "Orson Welles"),
// Song(name: "The One And Only", artist: "Chesney Hawkes"),
// Song(name: "Never Gonna Give You Up", artist: "Rick Astley")
//]
//用类型检查操作符（is）来检查一个实例是否属于特定子类型。若实例属于那个子类型，类型检查操作符返回 true，否则返回 false。
//用类型检查操作符（is）来检查一个实例是否属于特定子类型。若实例属于那个子类型，类型检查操作符返回 true，否则返回 false。
//某类型的一个常量或变量可能在幕后实际上属于一个子类。当确定是这种情况时，你可以尝试用类型转换操作符（as? 或 as!）向下转到它的子类型。
//Any 和 AnyObject 的类型转换
var things: [Any] = []
things.append(0)
things.append(0.0)
things.append(42)
things.append(3.14159)
things.append("hello")
things.append((3.0, 5.0))
//things.append(Movie(name: "Ghostbusters", director: "Ivan Reitman"))
things.append({ (name: String) -> String in "Hello, \(name)" })
//Any 可以表示任何类型，包括函数类型。
//AnyObject 可以表示任何类类型的实例。
for thing in things {
switch thing {
case 0 as Int:
print("zero as an Int")
case 0 as Double:
print("zero as a Double")
case let someInt as Int:
print("an integer value of \(someInt)")
case let someDouble as Double where someDouble > 0:
print("a positive double value of \(someDouble)")
case is Double:
print("some other double value that I don't want to print")
case let someString as String:
print("a string value of \"\(someString)\"")
case let (x, y) as (Double, Double):
print("an (x, y) point at \(x), \(y)")
case let movie as Movie:
print("a movie called \(movie.name), dir. \(movie.director)")
case let stringConverter as (String) -> String:
print(stringConverter("Michael"))
default:
print("something else")
}
}
// zero as an Int
// zero as a Double
// an integer value of 42
// a positive double value of 3.14159
// a string value of "hello"
// an (x, y) point at 3.0, 5.0
// a movie called Ghostbusters, dir. Ivan Reitman
// Hello, Michael
//Any 类型可以表示所有类型的值，包括可选类型。Swift 会在你用 Any 类型来表示一个可选值的时候，给你一个警告。如果你确实想使用 Any 类型来承载可选值，你可以使用 as 操作符显式转换为 Any，如下所示：
let optionalNumber: Int? = 3
things.append(optionalNumber) // 警告
things.append(optionalNumber as Any) // 没有警告
struct BlackjackCard {
enum Suit: Character {
case spades = "", hearts = "", diamonds = "", clubs = ""
}
enum Rank: Int {
case two = 2, three, four, five, six, seven, eight, nine, ten
case jack, queen, king, ace
struct Values {
let first: Int, second: Int?
}
var values: Values {
switch self {
case .ace:
return Values(first: 1, second: 11)
case .jack, .queen, .king:
return Values(first: 1, second: 11)
default:
return Values(first: self.rawValue, second: nil)
}
}
}
let rank: Rank , suit: Suit
var description: String { // 计算性属性
var output = "suit is \(suit.rawValue),"
output += " value is \(rank.values.first)"
if let second = rank.values.second {
output += " or \(second)"
}
return output
}
}
// 扩展
//extension SomeType223 {
// // 在这里给 SomeType 添加新的功能
//}
//
//
//extension SomeType223: SomeProtocol, AnotherProtocol {
// // 协议所需要的实现写在这里
//}
//
extension Double {
var km: Double { return self * 1_000.0 }
var m: Double { return self }
var cm: Double { return self / 100.0 }
var mm: Double { return self / 1_000.0 }
var ft: Double { return self / 3.28084 }
}
let oneInch = 25.4.mm
let threeFeet = 3.0.ft
print("Three feet is \(threeFeet) meters")
print("One inch is \(oneInch) meters")
//扩展可以给一个类添加新的便利构造器，但是它们不能给类添加新的指定构造器或者析构器。指定构造器和析构器必须始终由类的原始实现提供。
//扩展可以给现有的类型添加新的构造器。它使你可以把自定义类型作为参数来供其他类型的构造器使用，或者在类型的原始实现上添加额外的构造选项。
//扩展可以给一个类添加新的便利构造器，但是它们不能给类添加新的指定构造器或者析构器。指定构造器和析构器必须始终由类的原始实现提供。
//如果你使用扩展给一个值类型添加构造器，而这个值类型已经为所有存储属性提供默认值，且没有定义任何自定义构造器，那么你可以在该值类型扩展的构造器中使用默认构造器和成员构造器。如果你已经将构造器写在值类型的原始实现中，则不适用于这种情况，如同 值类型的构造器代理 中所描述的那样。
//如果你使用扩展给另一个模块中定义的结构体添加构造器，那么新的构造器直到定义模块中使用一个构造器之前，不能访问 self。
struct Size {
var width = 0.0, height = 0.0
}
struct Point {
var x = 0.0, y = 0.0
}
struct Rect {
var origin = Point()
var size = Size()
}
//你可以通过扩展 Rect 结构体来提供一个允许指定 point 和 size 的构造器：
extension Rect {
init(center: Point, size: Size) {
let originX = center.x - (size.width / 2)
let originY = center.y - (size.height / 2)
self.init(origin: Point(x: originX, y: originY), size: size)
}
}
//扩展可以给现有类型添加新的实例方法和类方法。在下面的例子中，给 Int 类型添加了一个新的实例方法叫做 repetitions：
extension Int {
func repetitions(task: () -> Void) {
for _ in 0.. Int {
var decimalBase = 1
for _ in 0.. 0:
return .positive
default:
return .negative
}
}
}
func printIntegerKinds(_ numbers: [Int]) {
for number in numbers {
switch number.kind {
case .negative:
print("- ", terminator: "")
case .zero:
print("0 ", terminator: "")
case .positive:
print("+ ", terminator: "")
}
}
print("")
}
printIntegerKinds([3, 19, -27, 0, -6, 0, 7])
// 打印“+ + - 0 - 0 + ”
//方法 printIntegerKinds(_:)，使用一个 Int 类型的数组作为输入，然后依次迭代这些值。对于数组中的每一个整数，方法会检查它的 kind 计算型属性，然后打印适当的描述。
// 协议
//协议 定义了一个蓝图，规定了用来实现某一特定任务或者功能的方法、属性，以及其他需要的东西。类、结构体或枚举都可以遵循协议，并为协议定义的这些要求提供具体实现。某个类型能够满足某个协议的要求，就可以说该类型遵循这个协议。
//除了遵循协议的类型必须实现的要求外，还可以对协议进行扩展，通过扩展来实现一部分要求或者实现一些附加功能，这样遵循协议的类型就能够使用这些功能。
//实现协议中的 mutating 方法时，若是类类型，则不用写 mutating 关键字。而对于结构体和枚举，则必须写 mutating 关键字。
//
//异变方法要求
//有时需要在方法中改变（或异变）方法所属的实例。例如，在值类型（即结构体和枚举）的实例方法中，将 mutating 关键字作为方法的前缀，写在 func 关键字之前，表示可以在该方法中修改它所属的实例以及实例的任意属性的值。
protocol Togglable {
mutating func toggle()
}
enum OnOffSwitch: Togglable {
case off, on
mutating func toggle() {
switch self {
case .off:
self = .on
case .on:
self = .off
}
}
}
var lightSwitch = OnOffSwitch.off
lightSwitch.toggle()
// lightSwitch 现在的值为 .on
//构造器要求
protocol someProtocol {
init(someParameter: Int)
}
protocol RandomNumberGenerator {
func random() -> Double
}
protocol SomeProtocol {
init()
}
class SomeSuperClass {
init() {
// 这里是构造器的实现部分
}
}
class SomeSubClass: SomeSuperClass, SomeProtocol {
// 因为遵循协议，需要加上 required
// 因为继承自父类，需要加上 override
required override init() {
// 这里是构造器的实现部分
}
}
class Dice2 {
let sides: Int
let generator: RandomNumberGenerator
init(sides: Int , generator: RandomNumberGenerator) {
self.sides = sides
self.generator = generator
}
func roll() -> Int {
return Int(generator.random() * Double(sides)) + 1
}
}
var d6 = Dice2(sides: 6, generator: LinearCongruentialGenerator())
for _ in 1...5 {
print("Random dice roll is \(d6.roll())")
}
protocol DiceGame {
var dice: Dice2 { get }
func play()
}
protocol DiceGameDelegate {
func gameDidStart(_ game: DiceGame)
func game(_ game: DiceGame, didStartNewTurnWithDiceRoll diceRoll: Int)
func gameDidEnd(_ game: DiceGame)
}
class LinearCongruentialGenerator: RandomNumberGenerator {
var lastRandom = 42.0
let m = 139968.0
let a = 3877.0
let c = 29573.0
func random() -> Double {
lastRandom = ((lastRandom * a + c).truncatingRemainder(dividingBy:m))
return lastRandom / m
}
}
class SnakesAndLadders: DiceGame {
let finalSquare = 25
let dice = Dice2(sides: 6, generator: LinearCongruentialGenerator())
var square = 0
var board: [Int]
init() {
board = Array(repeating: 0, count: finalSquare + 1)
board[03] = +08; board[06] = +11; board[09] = +09; board[10] = +02
board[14] = -10; board[19] = -11; board[22] = -02; board[24] = -08
}
var delegate: DiceGameDelegate?
func play() {
square = 0
delegate?.gameDidStart(self)
gameLoop: while square != finalSquare {
let diceRoll = dice.roll()
delegate?.game(self, didStartNewTurnWithDiceRoll:diceRoll)
switch square + diceRoll {
case finalSquare:
break gameLoop
case let newSquare where newSquare > finalSquare:
continue gameLoop
default:
square += diceRoll
square += board[square]
}
}
delegate?.gameDidEnd(self)
}
}
var d63 = Dice2(sides: 6, generator: LinearCongruentialGenerator())
for _ in 1...5 {
print("Random dice roll is \(d63.roll())")
}
class DiceGameTracker: DiceGameDelegate {
var numberOfTurns = 0
func gameDidStart(_ game: DiceGame) {
numberOfTurns = 0
if game is SnakesAndLadders {
print("Started a new game of Snakes and Ladders")
}
print("The game is using a \(game.dice.sides)-sided dice")
}
func game(_ game: DiceGame, didStartNewTurnWithDiceRoll diceRoll: Int) {
numberOfTurns += 1
print("Rolled a \(diceRoll)")
}
func gameDidEnd(_ game: DiceGame) {
print("The game lasted for \(numberOfTurns) turns")
}
}
let tracker = DiceGameTracker()
let game = SnakesAndLadders()
game.delegate = tracker
game.play()
//在扩展里添加协议遵循
protocol TextRepresentable {
var textualDescription: String { get }
}
extension Dice2: TextRepresentable {
var textualDescription: String {
return "A \(sides) -slide dice"
}
}
let d12 = Dice2(sides: 12, generator: LinearCongruentialGenerator())
print(d12.textualDescription)
extension SnakesAndLadders: TextRepresentable {
var textualDescription: String {
return "A game of Snakes and Ladders with \(finalSquare) squares"
}
}
//当一个类型已经遵循了某个协议中的所有要求，却还没有声明采纳该协议时，可以通过空的扩展来让它采纳该协议：
struct Hamster {
var name: String
var textualDescription: String {
return "A hamster named \(name)"
}
}
extension Hamster: TextRepresentable {}
let simonTheHamster = Hamster(name: "Simon")
let somethingTextRepresentable: TextRepresentable = simonTheHamster
print(somethingTextRepresentable.textualDescription)
// 打印 “A hamster named Simon”
struct Vector3D: Equatable {
var x = 0.0, y = 0.0 , z = 0.0
}
let twoThreeFour = Vector3D(x: 2.0, y: 3.0, z: 4.0)
let anotherTwoThreeFour = Vector3D(x: 2.0, y: 3.0, z: 4.0)
if twoThreeFour == anotherTwoThreeFour {
print("These two vectors are also equivalent.")
}
enum SkillLevel: Comparable {
case beginner
case intermediate
case expert(starts: Int)
}
var levels = [SkillLevel.intermediate , SkillLevel.beginner, SkillLevel.expert(starts: 5), SkillLevel.expert(starts: 3)]
for level in levels.sorted() {
print(level)
}
let things2: [TextRepresentable] = [game, d12, simonTheHamster]
for thing in things2 {
print(thing.textualDescription)
}
//协议的继承
//protocol InheritingProtocol: SomeProtocol, AnotherProtocol {
//
//}
protocol PrettyTextRepresentable: TextRepresentable {
var prettyTextualDescription: String { get }
}
extension SnakesAndLadders: PrettyTextRepresentable {
var prettyTextualDescription: String {
var output = textualDescription + ":\n"
for index in 1...finalSquare {
switch board[index] {
case let ladder where ladder > 0:
output += " "
case let snake where snake < 0:
output += " "
default:
output += " "
}
}
return output
}
}
print(game.prettyTextualDescription)
//类专属的协议
// 你通过添加 AnyObject 关键字到协议的继承列表，就可以限制协议只能被类类型采纳（以及非结构体或者非枚举的类型）。
protocol SomeInheritedProtocol {
}
protocol SomeClassOnlyProtocol: AnyObject, SomeInheritedProtocol {
// 这里是类专属协议的定义部分
}
protocol Named {
var name: String { get }
}
protocol Aged {
var age: Int { get }
}
struct Person22: Named, Aged {
var name: String
var age: Int
}
func wishHappyBirthday(to celebrator: Named & Aged) {//， 这意味着“任何同时遵循 Named 和 Aged 的协议”。它不关心参数的具体类型
print("Happy birthday, \(celebrator.name), you're \(celebrator.age)!")
}
let birthdayPerson = Person22(name: "Malcolm", age: 21)
wishHappyBirthday(to: birthdayPerson)
// 打印 “Happy birthday Malcolm - you're 21!”
class Location {
var latitude: Double
var longitude: Double
init(latitude: Double, longitude: Double) {
self.latitude = latitude
self.longitude = longitude
}
}
class City: Location, Named {
var name: String
init(name: String, latitude: Double, longitude: Double) {
self.name = name
super.init(latitude: latitude, longitude: longitude)
}
}
func beginConcert(in location: Location & Named) {//函数接受一个类型为 Location & Named 的参数，这意味着“任何 Location 的子类，并且遵循 Named 协议”。在这个例子中，City 就满足这样的条件
print("Hello, \(location.name)!")
}
let seattle = City(name: "Seattle", latitude: 47.6, longitude: -122.3)
beginConcert(in: seattle)
// 打印 "Hello, Seattle!"
protocol HasArea {
var area: Double { get }
}
class Circle: HasArea {
let pi = 3.1415927
var radius: Double
var area: Double { return pi * radius * radius }
init(radius: Double) { self.radius = radius }
}
class Country: HasArea {
var area: Double
init(area: Double) { self.area = area }
}
///可选要求用在你需要和 Objective-C 打交道的代码中。协议和可选要求都必须带上 @objc 属性。标记 @objc 特性的协议只能被继承自 Objective-C 类的类或者 @objc 类遵循，其他类以及结构体和枚举均不能遵循这种协议。
@objc protocol CounterDataSource {
@objc optional func increment(forCount count: Int) -> Int
@objc optional var fixedIncrement: Int { get }
}
//，遵循协议的类型可以选择是否实现这些要求。在协议中使用 optional 关键字作为前缀来定义可选要求
//由于 dataSource 可能为 nil，因此在 dataSource 后边加上了 ?，以此表明只在 dataSource 非空时才去调用 increment(forCount:) 方法。其次，即使 dataSource 存在，也无法保证其是否实现了 increment(forCount:) 方法，因为这个方法是可选的。因此，increment(forCount:) 方法同样使用可选链式调用进行调用，只有在该方法被实现的情况下才能调用它，所以在 increment(forCount:) 方法后边也加上了 ?。
class Counter {
var count = 0
var dataSource: CounterDataSource?
func increment() {
if let amount = dataSource?.increment?(forCount: count) {
count += amount
} else if let amount = dataSource?.fixedIncrement {
count += amount
}
}
}
class ThreeSource: NSObject, CounterDataSource {
let fixedIncrement = 3
}
var counter = Counter()
counter.dataSource = ThreeSource()
for _ in 1...4 {
counter.increment()
print(counter.count)
}
class TowardsZeroSource: NSObject, CounterDataSource {
func increment(forCount count: Int) -> Int {
if count == 0 {
return 0
} else if count < 0 {
return 1
} else {
return -1
}
}
}
counter.count = -4
counter.dataSource = TowardsZeroSource()
for _ in 1...5 {
counter.increment()
print(counter.count)
}
// 协议扩展
//协议来提供 randomBool() 方法。该方法使用协议中定义的 random() 方法来返回一个随机的 Bool 值：
extension RandomNumberGenerator {
func randomBool() -> Bool {
return random() > 0.5
}
}
//通过协议扩展，所有遵循协议的类型，都能自动获得这个扩展所增加的方法实现而无需任何额外修改：
let generator = LinearCongruentialGenerator()
print("Here's a random number: \(generator.random())")
// 打印 “Here's a random number: 0.37464991998171”
print("And here's a random Boolean: \(generator.randomBool())")
// 打印 “And here's a random Boolean: true”
//协议扩展可以为遵循协议的类型增加实现，但不能声明该协议继承自另一个协议。协议的继承只能在协议声明处进行指定。
//为协议扩展添加限制条件
extension Collection where Element: Equatable {
func allEqual() -> Bool {
for element in self {
if element != self.first {
return false
}
}
return true
}
}
let equalNumbers = [100, 100, 100, 100, 100]
let differentNumbers = [100, 100, 200, 100, 200]
print(equalNumbers.allEqual())
print(differentNumbers.allEqual())
