原文出处: Lucia(@peng_gong) 欢迎加入iOS小组。
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。
1println("Hello, world")变量与常量
Swift使用var声明变量,let声明常量
123var myVariable = 42myVariable = 50let 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:
1let 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):
123let label = "The width is "let width = 94let 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:
1234let apples = 3let oranges = 5let 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):
12345678var 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:
12let 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:
123456789let individualScores = [75, 43, 103, 87, 12]var teamScore = 0for 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。
12345678var optionalString: String? = "Hello"optionalString == nilvar 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:
1234567891011let 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:
1234567891011121314let interestingNumbers = [ "Prime": [2, 3, 5, 7, 11, 13], "Fibonacci": [1, 1, 2, 3, 5, 8], "Square": [1, 4, 9, 16, 25],]var largest = 0for (kind, numbers) in interestingNumbers { for number in numbers { if number > largest { largest = number } }}largest
while循环和do-while循环:
1234567891011var n = 2while n < 100 { n = n * 2}nvar m = 2do { m = m * 2} while m < 100m
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。
1234567891011var firstForLoop = 0for i in 0..3 { firstForLoop += i}firstForLoopvar secondForLoop = 0for 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:
1234func 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:
1234func getGasPrices() -> (Double, Double, Double) { return (3.59, 3.69, 3.79)}getGasPrices()
支持带有变长参数的函数:
123456789func 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:
123456789func 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:
12345678func makeIncrementer() -> (Int -> Int) { func addOne(number: Int) -> Int { return 1 + number } return addOne}var increment = makeIncrementer()increment(7)12345678910111213func 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:
12345numbers.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:
1numbers.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:
1sort([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。
123var shape = Shape()shape.numberOfSides = 7var 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)。
123456789101112class 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):
1234567891011121314151617181920class 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:
1234567891011121314151617181920212223242526class 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.perimetertriangle.perimeter = 9.9triangle.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:
为子类型的属性赋值。调用父类型的构造器。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willSet和didSet。
1234567891011121314151617181920class 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.sideLengthtriangleAndSquare.square = Square(sideLength: 50, name: "larger square")triangleAndSquare.triangle.sideLength
从而保证triangle和square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:
12345678class 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:
123let optionalSquare: Square? = Square(sideLength: 2.5, name: "optional square")let sideLength = optionalSquare?.sideLength
当optionalSquare为nil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:
123456789101112131415161718192021enum 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.Acelet aceRawValue = ace.toRaw()
使用toRaw和fromRaw在原始(raw)数值和枚举值之间进行转换:
123if 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:
1234567891011121314151617enum 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.Heartslet 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:
1234567891011121314enum 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)。
123456789struct 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:
1234protocol ExampleProtocol { var simpleDescription: String { get } mutating func adjust()}
类型、枚举和结构都可以实现(adopt)协议:
1234567891011121314151617181920class SimpleClass: ExampleProtocol { var simpleDescription: String = "A very simple class." var anotherProperty: Int = 69105 func adjust() { simpleDescription += " Now