苹果的新编程语言 Swift 简介

网友投稿 745 2022-08-26

苹果的新编程语言 Swift 简介

苹果的新编程语言 Swift 简介

关于

这篇文章简要介绍了苹果于WWDC 2014发布的编程语言——Swift。

前言

在这里我认为有必要提一下Brec Victor的Inventing on Principle,Swift编程环境的大部分概念都源自于Brec这个演讲。

接下来进入正题。

Swift是什么?

Swift是苹果于WWDC 2014发布的编程语言,这里引用The Swift Programming Language的原话:

Swift is a new programming language for iOS and OS X apps that builds on the best of C and Objective-C, without the constraints of C compatibility. Swift adopts safe programming patterns and adds modern features to make programming easier, more flexible and more fun. Swift’s clean slate, backed by the mature and much-loved Cocoa and Cocoa Touch frameworks, is an opportunity to imagine how software development works. Swift is the first industrial-quality systems programming language that is as expressive and enjoyable as a scripting language.

简单的说:

Swift用来写iOS和OS X程序。(估计也不会支持其它屌丝系统)

Swift吸取了C和Objective-C的优点,且更加强大易用。

Swift可以使用现有的Cocoa和Cocoa Touch框架。

Swift兼具编译语言的高性能(Performance)和脚本语言的交互性(Interactive)。

Swift语言概览

基本概念

注:这一节的代码源自The Swift Programming Language中的A Swift Tour。

Hello, world

类似于脚本语言,下面的代码即是一个完整的Swift程序。

println("Hello, world")

变量与常量

Swift使用var声明变量,let声明常量

var myVariable = 42

myVariable = 50

let myConstant = 42

类型推导

Swift支持类型推导(Type Inference),所以上面的代码不需指定类型,如果需要指定类型:

let explicitDouble : Double = 70

Swift不支持隐式类型转换(Implicitly casting),所以下面的代码需要显式类型转换(Explicitly casting):

let label = "The width is "

let width = 94

let width = label + String(width)

字符串格式化

Swift使用\(item)的形式进行字符串格式化:

let apples = 3

let oranges = 5

let appleSummary = "I have \(apples) apples."

let appleSummary = "I have \(apples + oranges) pieces of fruit."

数组和字典

Swift使用[]操作符声明数组(array)和字典(dictionary):

var shoppingList = ["catfish", "water", "tulips", "blue paint"]

shoppingList[1] = "bottle of water"

var occupations = [

"Malcolm": "Captain",

"Kaylee": "Mechanic",

]

occupations["Jayne"] = "Public Relations"

一般使用初始化器(initializer)语法创建空数组和空字典:

let emptyArray = String[]()

let emptyDictionary = Dictionary()

如果类型信息已知,则可以使用[]声明空数组,使用[:]声明空字典。

控制流

概览

Swift的条件语句包含if和switch,循环语句包含for-in、for、while和do-while,循环/判断条件不需要括号,但循环/判断体(body)必需括号:

let individualScores = [75, 43, 103, 87, 12]

var teamScore = 0

for score in individualScores {

if score > 50 {

teamScore += 3

} else {

teamScore += 1

}

}

可空类型

结合if和let,可以方便的处理可空变量(nullable variable)。对于空值,需要在类型声明后添加?显式标明该类型可空。

var optionalString: String? = "Hello"

optionalString == nil

var optionalName: String? = "John Appleseed"

var gretting = "Hello!"

if let name = optionalName {

gretting = "Hello, \(name)"

}

灵活的switch

Swift中的switch支持各种各样的比较操作:

let vegetable = "red pepper"

switch vegetable {

case "celery":

let vegetableComment = "Add some raisins and make ants on a log."

case "cucumber", "watercress":

let vegetableComment = "That would make a good tea sandwich."

case let x where x.hasSuffix("pepper"):

let vegetableComment = "Is it a spicy \(x)?"

default:

let vegetableComment = "Everything tastes good in soup."

}

其它循环

for-in除了遍历数组也可以用来遍历字典:

let interestingNumbers = [

"Prime": [2, 3, 5, 7, 11, 13],

"Fibonacci": [1, 1, 2, 3, 5, 8],

"Square": [1, 4, 9, 16, 25],

]

var largest = 0

for (kind, numbers) in interestingNumbers {

for number in numbers {

if number > largest {

largest = number

}

}

}

largest

while循环和do-while循环:

var n = 2

while n < 100 {

n = n * 2

}

n

var m = 2

do {

m = m * 2

} while m < 100

m

Swift支持传统的for循环,此外也可以通过结合..(生成一个区间)和for-in实现同样的逻辑。

var firstForLoop = 0

for i in 0..3 {

firstForLoop += i

}

firstForLoop

var secondForLoop = 0

for var i = 0; i < 3; ++i {

secondForLoop += 1

}

secondForLoop

注意:Swift除了..还有...:..生成前闭后开的区间,而...生成前闭后闭的区间。

函数和闭包

函数

Swift使用func关键字声明函数:

func greet(name: String, day: String) -> String {

return "Hello \(name), today is \(day)."

}

greet("Bob", "Tuesday")

通过元组(Tuple)返回多个值:

func getGasPrices() -> (Double, Double, Double) {

return (3.59, 3.69, 3.79)

}

getGasPrices()

支持带有变长参数的函数:

func sumOf(numbers: Int...) -> Int {

var sum = 0

for number in numbers {

sum += number

}

return sum

}

sumOf()

sumOf(42, 597, 12)

函数也可以嵌套函数:

func returnFifteen() -> Int {

var y = 10

func add() {

y += 5

}

add()

return y

}

returnFifteen()

作为头等对象,函数既可以作为返回值,也可以作为参数传递:

func makeIncrementer() -> (Int -> Int) {

func addOne(number: Int) -> Int {

return 1 + number

}

return addOne

}

var increment = makeIncrementer()

increment(7)

func hasAnyMatches(list: Int[], condition: Int -> Bool) -> Bool {

for item in list {

if condition(item) {

return true

}

}

return false

}

func lessThanTen(number: Int) -> Bool {

return number < 10

}

var numbers = [20, 19, 7, 12]

hasAnyMatches(numbers, lessThanTen)

闭包

本质来说,函数是特殊的闭包,Swift中可以利用{}声明匿名闭包:

numbers.map({

(number: Int) -> Int in

let result = 3 * number

return result

})

当闭包的类型已知时,可以使用下面的简化写法:

numbers.map({ number in 3 * number })

此外还可以通过参数的位置来使用参数,当函数最后一个参数是闭包时,可以使用下面的语法:

sort([1, 5, 3, 12, 2]) { $0 > $1 }

类和对象

创建和使用类

Swift使用class创建一个类,类可以包含字段和方法:

class Shape {

var numberOfSides = 0

func simpleDescription() -> String {

return "A shape with \(numberOfSides) sides."

}

}

创建Shape类的实例,并调用其字段和方法。

var shape = Shape()

shape.numberOfSides = 7

var shapeDescription = shape.simpleDescription()

通过init构建对象,既可以使用self显式引用成员字段(name),也可以隐式引用(numberOfSides)。

class NamedShape {

var numberOfSides: Int = 0

var name: String

init(name: String) {

self.name = name

}

func simpleDescription() -> String {

return "A shape with \(numberOfSides) sides."

}

}

使用deinit进行清理工作。

继承和多态

Swift支持继承和多态(override父类方法):

class Square: NamedShape {

var sideLength: Double

init(sideLength: Double, name: String) {

self.sideLength = sideLength

super.init(name: name)

numberOfSides = 4

}

func area() -> Double {

return sideLength * sideLength

}

override func simpleDescription() -> String {

return "A square with sides of length \(sideLength)."

}

}

let test = Square(sideLength: 5.2, name: "my test square")

test.area()

test.simpleDescription()

注意:如果这里的simpleDescription方法没有被标识为override,则会引发编译错误。

属性

为了简化代码,Swift引入了属性(property),见下面的perimeter字段:

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