Swift语法全面解析

let maximumNumberOfLoginAttempts = 10
var currentLoginAttempt = 0

// 类型注解
var welcomeMessage: String

// 这是一个注释

/* 这也是一个注释，
但是是多行的 */

/* 这是第一个多行注释的开头
/* 这是第二个被嵌套的多行注释 */
这是第一个多行注释的结尾 */

let cat = "🐱"; print(cat)
// 输出“🐱”

let `class` = "xiaobai"

let a= 1 + 2

let a = 1+ 2

let a = 1 + 2;  // 编码规范推荐使用这种写法
let b = 3+4 // 这样也是OK的

let minValue = UInt8.min  // minValue 为 0，是 UInt8 类型
let maxValue = UInt8.max  // maxValue 为 255，是 UInt8 类型

let meaningOfLife = 42
// meaningOfLife 会被推测为 Int 类型

let pi = 3.14159
// pi 会被推测为 Double 类型

let decimalInteger = 17
let binaryInteger = 0b10001       // 二进制的17
let octalInteger = 0o21           // 八进制的17
let hexadecimalInteger = 0x11     // 十六进制的17

typealias AudioSample = UInt16
var maxAmplitudeFound = AudioSample.min
// maxAmplitudeFound 现在是 0

let orangesAreOrange = true
let turnipsAreDelicious = false

let http404Error = (404, "Not Found")
// http404Error 的类型是 (Int, String)，值是 (404, "Not Found")

var serverResponseCode: Int? = 404
// serverResponseCode 包含一个可选的 Int 值 404
serverResponseCode = nil
// serverResponseCode 现在不包含值

func makeASandwich() throws {
    // ...
}

do {
    try makeASandwich()
    eatASandwich()
} catch SandwichError.outOfCleanDishes {
    washDishes()
} catch SandwichError.missingIngredients(let ingredients) {
    buyGroceries(ingredients)
}

let age = -3
assert(age >= 0, "A person's age cannot be less than zero")
// 因为 age < 0，所以断言会触发

let names = ["Anna", "Alex", "Brian", "Jack"]
let count = names.count
for i in 0..<count {
    print("第 \(i + 1) 个人叫 \(names[i])")
}
// 第 1 个人叫 Anna
// 第 2 个人叫 Alex
// 第 3 个人叫 Brian
// 第 4 个人叫 Jack

// 多行字符串字面量
let quotation = """
The White Rabbit put on his spectacles.  "Where shall I begin,
please your Majesty?" he asked.

"Begin at the beginning," the King said gravely, "and go on
till you come to the end; then stop."
"""

// 下面两个字符串其实是一样的
let singleLineString = "These are the same."
let multilineString = """
These are the same.
"""

// 字符串插值
let multiplier = 3
let message = "\(multiplier) times 2.5 is \(Double(multiplier) * 2.5)"
// message 是 "3 times 2.5 is 7.5"

// 计算字符数量
var word = "cafe"
print("the number of characters in \(word) is \(word.count)")
// 打印输出“the number of characters in cafe is 4”

var emptyString = ""               // 空字符串字面量
var anotherEmptyString = String()  // 初始化方法
// 两个字符串均为空并等价。

let catCharacters: [Character] = ["C", "a", "t", "!"]
let catString = String(catCharacters)
print(catString)
// 打印输出：“Cat!”

// 集合
var someInts = [Int]()
print("someInts is of type [Int] with \(someInts.count) items.")
// 打印“someInts is of type [Int] with 0 items.”

var threeDoubles = Array(repeating: 0.0, count: 3)
// threeDoubles 是一种 [Double] 数组，等价于 [0.0, 0.0, 0.0]

var anotherThreeDoubles = Array(repeating: 2.5, count: 3)
// anotherThreeDoubles 被推断为 [Double]，等价于 [2.5, 2.5, 2.5]
var sixDoubles = threeDoubles + anotherThreeDoubles
// sixDoubles 被推断为 [Double]，等价于 [0.0, 0.0, 0.0, 2.5, 2.5, 2.5]

// enumerated() 方法遍历数组
var shoppingList: [String] = ["Eggs", "Milk"]
for (index, value) in shoppingList.enumerated() {
    print("Item \(String(index + 1)): \(value)")
}

let names = ["Anna", "Alex", "Brian", "Jack"]
for name in names {
    print("Hello, \(name)!")
}

let numberOfLegs = ["spider": 8, "ant": 6, "cat": 4]
for (animalName, legCount) in numberOfLegs {
    print("\(animalName)s have \(legCount) legs")
}

//  repeat-while 循环的一般格式
repeat {
    statements
} while condition


// 提前退出
func greet(person: [String: String]) {
    guard let name = person["name"] else {
        return
    }

    print("Hello \(name)!")

    guard let location = person["location"] else {
        print("I hope the weather is nice near you.")
        return
    }

    print("I hope the weather is nice in \(location).")
}
greet(person: ["name": "John"])
// 输出“Hello John!”
// 输出“I hope the weather is nice near you.”
greet(person: ["name": "Jane", "location": "Cupertino"])
// 输出“Hello Jane!”
// 输出“I hope the weather is nice in Cupertino.”

// 函数
func greet(person: String) -> String {
    let greeting = "Hello, " + person + "!"
    return greeting
}

func greet(person: String, from hometown: String) -> String {
    return "Hello \(person)!  Glad you could visit from \(hometown)."
}
print(greet(person: "Bill", from: "Cupertino"))
// 打印“Hello Bill!  Glad you could visit from Cupertino.”

// 可选元组返回类型
func minMax(array: [Int]) -> (min: Int, max: Int)? {
    if array.isEmpty { return nil }
    var currentMin = array[0]
    var currentMax = array[0]
    for value in array[1..<array.count] {
        if value < currentMin {
            currentMin = value
        } else if value > currentMax {
            currentMax = value
        }
    }
    return (currentMin, currentMax)
}

// 隐式返回的函数
func greeting(for person: String) -> String {
    "Hello, " + person + "!"
}
print(greeting(for: "Dave"))
// 打印 "Hello, Dave!

// 参数标签
func greet(person: String, from hometown: String) -> String {
    return "Hello \(person)!  Glad you could visit from \(hometown)."
}
print(greet(person: "Bill", from: "Cupertino"))
// 打印“Hello Bill!  Glad you could visit from Cupertino.”

// 闭包表达式语法
{ (parameters) -> return type in
    statements
}

// 尾随闭包
let digitNames = [
    0: "Zero", 1: "One", 2: "Two",   3: "Three", 4: "Four",
    5: "Five", 6: "Six", 7: "Seven", 8: "Eight", 9: "Nine"
]
let numbers = [16, 58, 510]
let strings = numbers.map {
    (number) -> String in
    var number = number
    var output = ""
    repeat {
        output = digitNames[number % 10]! + output
        number /= 10
    } while number > 0
    return output
}
// strings 常量被推断为字符串类型数组，即 [String]
// 其值为 ["OneSix", "FiveEight", "FiveOneZero"]

// 值捕获
func makeIncrementer(forIncrement amount: Int) -> () -> Int {
    var runningTotal = 0
    func incrementer() -> Int {
        runningTotal += amount
        return runningTotal
    }
    return incrementer
}

// 自动闭包，延迟求值
var customersInLine = ["Chris", "Alex", "Ewa", "Barry", "Daniella"]
print(customersInLine.count)
// 打印出“5”

let customerProvider = { customersInLine.remove(at: 0) }
print(customersInLine.count)
// 打印出“5”

print("Now serving \(customerProvider())!")
// Prints "Now serving Chris!"
print(customersInLine.count)
// 打印出“4”

// 枚举语法
enum SomeEnumeration {
    // 枚举定义放在这里
}

enum CompassPoint {
    case north
    case south
    case east
    case west
}

enum Planet {
    case mercury, venus, earth, mars, jupiter, saturn, uranus, neptune
}

let somePlanet = Planet.earth
switch somePlanet {
case .earth:
    print("Mostly harmless")
default:
    print("Not a safe place for humans")
}
// 打印“Mostly harmless”

// 关联值
enum Barcode {
    case upc(Int, Int, Int, Int)
    case qrCode(String)
}
var productBarcode = Barcode.upc(8, 85909, 51226, 3)
productBarcode = .qrCode("ABCDEFGHIJKLMNOP")

switch productBarcode {
case let .upc(numberSystem, manufacturer, product, check):
    print("UPC: \(numberSystem), \(manufacturer), \(product), \(check).")
case let .qrCode(productCode):
    print("QR code: \(productCode).")
}
// 打印“QR code: ABCDEFGHIJKLMNOP.”

// 递归枚举
indirect enum ArithmeticExpression {
    case number(Int)
    case addition(ArithmeticExpression, ArithmeticExpression)
    case multiplication(ArithmeticExpression, ArithmeticExpression)
}
let five = ArithmeticExpression.number(5)
let four = ArithmeticExpression.number(4)
let sum = ArithmeticExpression.addition(five, four)
let product = ArithmeticExpression.multiplication(sum, ArithmeticExpression.number(2))
// (5 + 4) * 2

func evaluate(_ expression: ArithmeticExpression) -> Int {
    switch expression {
    case let .number(value):
        return value
    case let .addition(left, right):
        return evaluate(left) + evaluate(right)
    case let .multiplication(left, right):
        return evaluate(left) * evaluate(right)
    }
}

print(evaluate(product))
// 打印“18”

// 类和结构体
struct SomeStructure {
    // 在这里定义结构体
}
class SomeClass {
    // 在这里定义类
}

struct Resolution {
    var width = 0
    var height = 0
}
class VideoMode {
    var resolution = Resolution()
    var interlaced = false
    var frameRate = 0.0
    var name: String?
}

// 属性
struct Point {
    var x = 0.0, y = 0.0
}
struct Size {
    var width = 0.0, height = 0.0
}
struct Rect {
    var origin = Point()
    var size = Size()       //存储属性
    var center: Point {     //计算型属性
        get {
            let centerX = origin.x + (size.width / 2)
            let centerY = origin.y + (size.height / 2)
            return Point(x: centerX, y: centerY)
        }
        set(newCenter) {
            origin.x = newCenter.x - (size.width / 2)
            origin.y = newCenter.y - (size.height / 2)
        }
    }
}
var square = Rect(origin: Point(x: 0.0, y: 0.0),
    size: Size(width: 10.0, height: 10.0))
let initialSquareCenter = square.center
square.center = Point(x: 15.0, y: 15.0)
print("square.origin is now at (\(square.origin.x), \(square.origin.y))")
// 打印“square.origin is now at (10.0, 10.0)”

// 属性包装器
@propertyWrapper
struct TwelveOrLess {
    private var number = 0
    var wrappedValue: Int {
        get { return number }
        set { number = min(newValue, 12) }
    }
}

// 方法
class Counter {
    var count = 0
    func increment() {
        count += 1
    }
    func increment(by amount: Int) {
        count += amount
    }
    func reset() {
        count = 0
    }
}

subscript(index: Int) -> Int {
    get {
      // 返回一个适当的 Int 类型的值
    }
    set(newValue) {
      // 执行适当的赋值操作
    }
}

// 示例
struct TimesTable {
    let multiplier: Int
    subscript(index: Int) -> Int {
        return multiplier * index
    }
}
let threeTimesTable = TimesTable(multiplier: 3)
print("six times three is \(threeTimesTable[6])")
// 打印“six times three is 18”

var numberOfLegs = ["spider": 8, "ant": 6, "cat": 4]
numberOfLegs["bird"] = 2

// 类型下标
enum Planet: Int {
    case mercury = 1, venus, earth, mars, jupiter, saturn, uranus, neptune
    static subscript(n: Int) -> Planet {
        return Planet(rawValue: n)!
    }
}
let mars = Planet[4]
print(mars)

// 继承
class SomeClass: SomeSuperclass {
    // 这里是子类的定义
}

class Vehicle {
    var currentSpeed = 0.0
    var description: String {
        return "traveling at \(currentSpeed) miles per hour"
    }
    func makeNoise() {
        // 什么也不做——因为车辆不一定会有噪音
    }
}

class Car: Vehicle {
    var gear = 1
    override var description: String {
        return super.description + " in gear \(gear)"
    }
}

class AutomaticCar: Car {
    override var currentSpeed: Double {
        didSet {
            gear = Int(currentSpeed / 10.0) + 1
        }
    }
}

// 构造过程
init() {
    // 在此处执行构造过程
}

struct Fahrenheit {
    var temperature: Double
    init() {
        temperature = 32.0
    }
}
var f = Fahrenheit()
print("The default temperature is \(f.temperature)° Fahrenheit")
// 打印“The default temperature is 32.0° Fahrenheit”

struct Color {
    let red, green, blue: Double
    init(red: Double, green: Double, blue: Double) {
        self.red   = red
        self.green = green
        self.blue  = blue
    }
    init(white: Double) {
        red   = white
        green = white
        blue  = white
    }
}

// 析构过程
deinit {
    // 执行析构过程
}

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)
    }
}

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.”

john.residence = Residence()

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“John's residence has 1 room(s).”

// 错误处理
enum VendingMachineError: Error {
    case invalidSelection                     //选择无效
    case insufficientFunds(coinsNeeded: Int) //金额不足
    case outOfStock                             //缺货
}
throw VendingMachineError.insufficientFunds(coinsNeeded: 5)

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).")
}
// 打印“Insufficient funds. Please insert an additional 2 coins.”

// 指定清理操作
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。
    }
}

// 类型转换
// 一个基类 MediaItem
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.srtist = artist
  super.init(name: name)
 }
}

let library = [
 Movie(name: "Casablanca", director: "Micheal Curtiz"),
 Song(name: "Blue Suede Shose", artist: "Elvis Presley"),
 Movie(name: "Citizen Kane", director: "Orson Wells"),
 Song(name: "The One And Only", artist: "Chesney Hawkes"),
 Song(name: "Never Gonna Give You Up", artist: "Rick Astley")
]
var movieCount = 0
var songCount = 0

for item in library {
 if item is Movie {
  movieCount += 1
 } else if item is Song {
  songCount += 1
 }
}

print("Media library contains \(movieCount) movies and \(songCount)")
// 打印“Media library contains 2 movies and 3 songs”

for item in library {
 if let movie = item as? Movie {
  print("Movie: \(movie.name), dir. \(movie.director)")
 } else if let song = item as? Song {
  print("Song: \(song.name), by \(song.artist)")
 }
}
// Movie: Casablanca, dir. Michael Curtiz
// Song: Blue Suede Shoes, by Elvis Presley
// Movie: Citizen Kane, dir. Orson Welles
// Song: The One And Only, by Chesney Hawkes
// Song: Never Gonna Give You Up, by Rick Astley

// 嵌套类型
stuct BlackjackCard {
 // 嵌套的 Suit 枚举
 enum Suit: Character {
  case spades = "1", hearts = "2", diamonds = "3", clubs = "4"
 }
 
 // 嵌套的 Rank 枚举
 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: 10, second: nil)
   default:
    return Values(first: self.rawValue, second: nil)
   }
  }
 }
 
 // BlackjackCard 的属性和方法
 let rank: Rank, suit: Suit
 var description: String {
  var output = "suit is \(suit.rawValue),"
  output += " value is \(rank.values.first)"
  if let second = rank.values.second {
   output += " or \(second)"
  }
  return output
 }
}

let theAceOfSpades = BlackjackCard(rank: .ace, suit: .spades)
print("theAceOfSpades: \(theAceOfSpades.description)")
// 打印“theAceOfSpades: suit is 1, value is 1 or 11”

let heartsSymbol = BlackjackCard.Suit.hearts.rawValue
// 2

extension SomeType {
  // 在这里给 SomeType 添加新的功能
}

// 扩展的语法
extension SomeType {
  // 在这里给 SomeType 添加新的功能
}
// 添加一个或多个协议
extension SomeType: SomeProtocol, AnotherProtocol {
  // 协议所需要的实现写在这里
}

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()
}

extension Rect {
 init(center: Point, size: Size) {
  let originX = center.x - (size.width / 2)
  let originY = center.y - (size.height / 3)
  self.init(origin: Point(x: originX, y: originY), size: size)
 }
}
let centerRect = Rect(center: Point(x: 4.0, y: 4.0), 
 size: Size(width: 3.0, height: 3.0))
// centerRect 的 origin 是 (2.5, 2.5) 并且它的 size 是 (3.0, 3.0)

extension Int {
 func repetitions(task: () -> Void) {
  for _ in 0..<self {
   task()
  }
 }
}
3.repetitions {
 print("Hello!")
}
// Hello!
// Hello!
// Hello!

extension Int {
 mutating func square() {
  self = self * self
 }
}
var somtInt = 3
someInt.square()
// someInt 现在是9

protocol SomeProtocol {
    // 这里是协议的定义部分
}

// 协议语法
protocol SomeProtocol {
    // 这里是协议的定义部分
}

struct SomeStructure: FirstProtocol, AnotherProtocol {
 // 这里是结构体的定义部分
}

class SomeClass: SomeSuperClass, FirstProtocol, AnotherProtocol {
 // 这里是类的定义部分
}

protocol SomeProtocol {
 var mustBeSettable: Int { get set }
 var doesNotNeedToBeSettable: Int { get }
}

protocol AnotherProtocol {
 static var someTypeProperty: Int { get set }
}

protocol FullyNamed {
 var fullName: String { get }
}

struct person: FullyNamed {
 var fullName: String
}
let john = Person(fullName: "John Appleseed")
// john.fullName 为 "John Appleseed"

class Starship: FullyNamed {
 var prefix: String?
 var name: String
 init(name: String, prefix: String? = nil) {
  self.name = name
  self.prefix = prefix
 }
 
 var fullName: String {
  return (prefix != nil ? prefix! + " " : "") + name
 }
}
var ncc1701 = Starship(name: "Enterprise", prefix: "USS")
// ncc1701.fullName 为 "USS Enterprise"

func swapTwoInts(_ a: inout Int, _ b: inout Int) {
    let temporaryA = a
    a = b
    b = temporaryA
}

func swapTwoValues<T>(_ a: inout T, _ b: inout T) {
    let temporaryA = a
    a = b
    b = temporaryA
}

func swapTwoInts(_ a: inout Int, _ b: inout Int)
func swapTwoValues<T>(_ a: inout T, _ b: inout T)

var someInt = 3
var anotherInt = 107
swapTwoValues(&someInt, &anotherInt)
// someInt 现在是 107，anotherInt 现在是 3

var someString = "hello"
var anotherString = "world"
swapTwoValues(&someString, &anotherString)
// someString 现在是“world”，anotherString 现在是“hello”

protocol Shape {
    func draw() -> String
}

struct Triangle: Shape {
    var size: Int
    func draw() -> String {
        var result = [String]()
        for length in 1...size {
            result.append(String(repeating: "*", count: length))
        }
        return result.joined(separator: "\n")
    }
}
let smallTriangle = Triangle(size: 3)
print(smallTriangle.draw())
// *
// **
// ***

struct FlippedShape<T: Shape>: Shape {
    var shape: T
    func draw() -> String {
        let lines = shape.draw().split(separator: "\n")
        return lines.reversed().joined(separator: "\n")
    }
}
let flippedTriangle = FlippedShape(shape: smallTriangle)
print(flippedTriangle.draw())
// ***
// **
// *

// 自动引用计数实践
class Person {
    let name: String
    init(name: String) {
        self.name = name
        print("\(name) is being initialized")
    }
    deinit {
        print("\(name) is being deinitialized")
    }
}

var reference1: Person?
var reference2: Person?
var reference3: Person?
reference1 = Person(name: "John Appleseed")
// 打印“John Appleseed is being initialized”
reference2 = reference1
reference3 = reference1
reference1 = nil
reference2 = nil
reference3 = nil
// 打印“John Appleseed is being deinitialized”

// 循环强引用
class Person {
    let name: String
    init(name: String) { self.name = name }
    var apartment: Apartment?
    deinit { print("\(name) is being deinitialized") }
}

class Apartment {
    let unit: String
    init(unit: String) { self.unit = unit }
    var tenant: Person?
    deinit { print("Apartment \(unit) is being deinitialized") }
}
var john: Person?
var unit4A: Apartment?
john = Person(name: "John Appleseed")
unit4A = Apartment(unit: "4A")

john = nil
unit4A = nil

// 弱引用
class Person {
    let name: String
    init(name: String) { self.name = name }
    var apartment: Apartment?
    deinit { print("\(name) is being deinitialized") }
}

class Apartment {
    let unit: String
    init(unit: String) { self.unit = unit }
    weak var tenant: Person?
    deinit { print("Apartment \(unit) is being deinitialized") }
}

var john: Person?
var unit4A: Apartment?

john = Person(name: "John Appleseed")
unit4A = Apartment(unit: "4A")

john!.apartment = unit4A
unit4A!.tenant = john

john = nil
// 打印“John Appleseed is being deinitialized”

func balance(_ x: inout Int, _ y: inout Int) {
    let sum = x + y
    x = sum / 2
    y = sum - x
}
var playerOneScore = 42
var playerTwoScore = 30
balance(&playerOneScore, &playerTwoScore)  // 正常
balance(&playerOneScore, &playerOneScore)
// 错误：playerOneScore 访问冲突

public class SomePublicClass {}
internal class SomeInternalClass {}
fileprivate class SomeFilePrivateClass {}
private class SomePrivateClass {}

class SomeInternalClass {}   // 隐式 internal
var someInternalConstant = 0 // 隐式 internal

public class SomePublicClass {                  // 显式 public 类
    public var somePublicProperty = 0            // 显式 public 类成员
    var someInternalProperty = 0                 // 隐式 internal 类成员
    fileprivate func someFilePrivateMethod() {}  // 显式 fileprivate 类成员
    private func somePrivateMethod() {}          // 显式 private 类成员
}

class SomeInternalClass {                       // 隐式 internal 类
    var someInternalProperty = 0                 // 隐式 internal 类成员
    fileprivate func someFilePrivateMethod() {}  // 显式 fileprivate 类成员
    private func somePrivateMethod() {}          // 显式 private 类成员
}

fileprivate class SomeFilePrivateClass {        // 显式 fileprivate 类
    func someFilePrivateMethod() {}              // 隐式 fileprivate 类成员
    private func somePrivateMethod() {}          // 显式 private 类成员
}

private class SomePrivateClass {                // 显式 private 类
    func somePrivateMethod() {}                  // 隐式 private 类成员
}

let initialBits: UInt8 = 0b00001111
let invertedBits = ~initialBits // 等于 0b11110000

var potentialOverflow = Int16.max
// potentialOverflow 的值是 32767，这是 Int16 能容纳的最大整数
potentialOverflow += 1
// 这里会报错

struct Vector2D {
    var x = 0.0, y = 0.0
}

extension Vector2D {
    static func + (left: Vector2D, right: Vector2D) -> Vector2D {
        return Vector2D(x: left.x + right.x, y: left.y + right.y)
    }
}

let vector = Vector2D(x: 3.0, y: 1.0)
let anotherVector = Vector2D(x: 2.0, y: 4.0)
let combinedVector = vector + anotherVector
// combinedVector 是一个新的 Vector2D 实例，值为 (5.0, 5.0)