来自苹果的编程语言——Swift简介

2014年6月4日 分类: swift (71 个脚步)

前言

在这里我认为有必要提一下Brec VictorInventing 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()

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ifswitch,循环语句包含for-inforwhiledo-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iflet,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

注意:赋值器(setter)中,接收的值被自动命名为newValue

willSet和didSet

EquilateralTriangle的构造器进行了如下操作:

  1. 为子类型的属性赋值。
  2. 调用父类型的构造器。
  3. 修改父类型的属性。

如果不需要计算属性的值,但需要在赋值前后进行一些操作的话,使用willSetdidSet

class TriangleAndSquare {
        var triangle: EquilateralTriangle {
        willSet {
            square.sideLength = newValue.sideLength
        }
        }
        var square: Square {
        willSet {
            triangle.sideLength = newValue.sideLength
        }
        }
        init(size: Double, name: String) {
            square = Square(sideLength: size, name: name)
            triangle = EquilateralTriangle(sideLength: size, name: name)
        }
    }
    var triangleAndSquare = TriangleAndSquare(size: 10, name: "another test shape")
    triangleAndSquare.square.sideLength
    triangleAndSquare.square = Square(sideLength: 50, name: "larger square")
    triangleAndSquare.triangle.sideLength

从而保证trianglesquare拥有相等的sideLength

调用方法

Swift中,函数的参数名称只能在函数内部使用,但方法的参数名称除了在内部使用外还可以在外部使用(第一个参数除外),例如:

class Counter {
        var count: Int = 0
        func incrementBy(amount: Int, numberOfTimes times: Int) {
            count += amount * times
        }
    }
    var counter = Counter()
    counter.incrementBy(2, numberOfTimes: 7)

注意Swift支持为方法参数取别名:在上面的代码里,numberOfTimes面向外部,times面向内部。

?的另一种用途

使用可空值时,?可以出现在方法、属性或下标前面。如果?前的值为nil,那么?后面的表达式会被忽略,而原表达式直接返回nil,例如:

let optionalSquare: Square? = Square(sideLength: 2.5, name: "optional 
    square")
    let sideLength = optionalSquare?.sideLength

optionalSquarenil时,sideLength属性调用会被忽略。

枚举和结构

枚举

使用enum创建枚举——注意Swift的枚举可以关联方法:

enum Rank: Int {
        case Ace = 1
        case Two, Three, Four, Five, Six, Seven, Eight, Nine, Ten
        case Jack, Queen, King
            func simpleDescription() -> String {
            switch self {
                case .Ace:
                    return "ace"
                case .Jack:
                    return "jack"
                case .Queen:
                    return "queen"
                case .King:
                    return "king"
                default:
                    return String(self.toRaw())
            }
        }
    }
    let ace = Rank.Ace
    let aceRawValue = ace.toRaw()

使用toRawfromRaw在原始(raw)数值和枚举值之间进行转换:

if let convertedRank = Rank.fromRaw(3) {
        let threeDescription = convertedRank.simpleDescription()
    }

注意枚举中的成员值(member value)是实际的值(actual value),和原始值(raw value)没有必然关联。

一些情况下枚举不存在有意义的原始值,这时可以直接忽略原始值:

enum Suit {
        case Spades, Hearts, Diamonds, Clubs
            func simpleDescription() -> String {
            switch self {
                case .Spades:
                    return "spades"
                case .Hearts:
                    return "hearts"
                case .Diamonds:
                    return "diamonds"
                case .Clubs:
                    return "clubs"
            }
        }
    }
    let hearts = Suit.Hearts
    let heartsDescription = hearts.simpleDescription()

除了可以关联方法,枚举还支持在其成员上关联值,同一枚举的不同成员可以有不同的关联的值:

enum ServerResponse {
        case Result(String, String)
        case Error(String)
    }
    
    let success = ServerResponse.Result("6:00 am", "8:09 pm")
    let failure = ServerResponse.Error("Out of cheese.")
    
    switch success {
        case let .Result(sunrise, sunset):
            let serverResponse = "Sunrise is at (sunrise) and sunset is at (sunset)."
        case let .Error(error):
            let serverResponse = "Failure... (error)"
    }

结构

Swift使用struct关键字创建结构。结构支持构造器和方法这些类的特性。结构和类的最大区别在于:结构的实例按值传递(passed by value),而类的实例按引用传递(passed by reference)。

struct Card {
        var rank: Rank
        var suit: Suit
        func simpleDescription() -> String {
            return "The (rank.simpleDescription()) of (suit.simpleDescription())"
        }
    }
    let threeOfSpades = Card(rank: .Three, suit: .Spades)
    let threeOfSpadesDescription = threeOfSpades.simpleDescription()

协议(protocol)和扩展(extension)

协议

Swift使用protocol定义协议:

protocol ExampleProtocol {
        var simpleDescription: String { get }
        mutating func adjust()
    }

类型、枚举和结构都可以实现(adopt)协议:

class SimpleClass: ExampleProtocol {
        var simpleDescription: String = "A very simple class."
        var anotherProperty: Int = 69105
        func adjust() {
            simpleDescription += " Now 100% adjusted."
        }
    }
    var a = SimpleClass()
    a.adjust()
    let aDescription = a.simpleDescription
    
    struct SimpleStructure: ExampleProtocol {
        var simpleDescription: String = "A simple structure"
        mutating func adjust() {
            simpleDescription += " (adjusted)"
        }
    }
    var b = SimpleStructure()
    b.adjust()
    let bDescription = b.simpleDescription

扩展

扩展用于在已有的类型上增加新的功能(比如新的方法或属性),Swift使用extension声明扩展:

extension Int: ExampleProtocol {
        var simpleDescription: String {
            return "The number (self)"
        }
        mutating func adjust() {
            self += 42
        }
    }
    7.simpleDescription

泛型(generics)

Swift使用<>来声明泛型函数或泛型类型:

func repeat(item: ItemType, times: Int) -> ItemType[] {
        var result = ItemType[]()
        for i in 0..times {
            result += item
        }
        return result
    }
    repeat("knock", 4)

Swift也支持在类、枚举和结构中使用泛型:

// Reimplement the Swift standard library's optional type
    enum OptionalValue {
        case None
        case Some(T)
    }
    var possibleInteger: OptionalValue = .None
    possibleInteger = .Some(100)

有时需要对泛型做一些需求(requirements),比如需求某个泛型类型实现某个接口或继承自某个特定类型、两个泛型类型属于同一个类型等等,Swift通过where描述这些需求:

func anyCommonElements  (lhs: T, rhs: U) -> Bool {
        for lhsItem in lhs {
            for rhsItem in rhs {
                if lhsItem == rhsItem {
                    return true
                }
            }
        }
        return false
    }
    anyCommonElements([1, 2, 3], [3])

Swift语言概览就到这里,有兴趣的朋友请进一步阅读The Swift Programming Language

以上内容来自于Lucida来自苹果的编程语言——Swift简介
转过来纯粹为了方便学习~

【说明】 本文参考自:http://zh.lucida.me/blog/an-introduction-to-swift/ , 由柠之漠然(枫之落叶)整理编辑.
本文链接地址:https://blog.shiniv.com/2014/06/an-introduction-to-swift/ , 转载请保留本说明!

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