苹果的新编程语言 Swift 简介

2018-07-20    来源:编程学习网

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 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:

  1. Swift用来写iOS和OS X程序。(估计也不会支持其它屌丝系统)
  2. Swift吸取了C和Objective-C的优点,且更加强大易用。
  3. Swift可以使用现有的Cocoa和Cocoa Touch框架。
  4. 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<String, Float>()

  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:

class EquilateralTriangle: NamedShape {
    var sideLength: Double = 0.0

    init(sideLength: Double, name: String) {
        self.sideLength = sideLength
        super.init(name: name)
        numberOfSides = 3
    }

    var perimeter: Double {
    get {
        return 3.0 * sideLength
    }
    set {
        sideLength = newValue / 3.0
    }
    }

    override func simpleDescription() -> String {
        return "An equilateral triagle with sides of length \(sideLength)."
    }
}
var triangle = EquilateralTriangle(sideLength: 3.1, name: "a triangle")
triangle.perimeter
triangle.perimeter = 9.9
triangle.sideLength

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