Swift是什么?

Swift是苹果于WWDC 2014发布的编程语言,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<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
 }



 

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
 varshapeDescription = 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:

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:



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:



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



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:



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toRaw和fromRaw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:



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 <T, U where T: Sequence,
 U: Sequence, T.GeneratorType.Element: Equatable,
 T.GeneratorType.Element == U.GeneratorType.Element> (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])