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Swift概览

原作者: [db:作者] 来自: [db:来源] 收藏 邀请
<pre name="code" class="objc">转自:http://letsswift.com/2014/06/swift_overview/


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,以下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,假设须要指定类型:
let explicitDouble : Double = 70
Swift不支持隐式类型转换(Implicitly casting),所以以下的代码须要显式类型转换(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>()

假设类型信息已知,则能够使用[]声明空数组,使用[:]声明空字典。
控制流
概览
Swift的条件语句包括if和switch,循环语句包括for-in、for、while和do-while,循环/推断条件不须要括号,但循环/推断体(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,能够方便的处理可空变量(nullable variable)。对于空值,须要在类型声明后加入?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支持各种各样的比較操作:
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." 
}

其他循环
for-in除了遍历数组也能够用来遍历字典:
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,此外也能够通过结合..(生成一个区间)和for-in实现相同的逻辑。
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funckeyword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()

支持带有变长參数的函数:
func sumOf(numbers: Int...) -> Int { 
    var sum = 0 
    for number in numbers { 
        sum += number 
    } 
    return sum 
} 
sumOf() 
sumOf(42, 597, 12)

函数也能够嵌套函数:
func returnFifteen() -> Int { 
    var y = 10 
    func add() { 
        y += 5 
    } 
    add() 
    return y 
} 

作为头等对象,函数既能够作为返回值,也能够作为參数传递:
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中能够利用{}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,能够使用以下的简化写法:
numbers.map({ number in 3 * number })

此外还能够通过參数的位置来使用參数,当函数最后一个參数是闭包时,能够使用以下的语法:
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,类能够包括字段和方法:
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,既能够使用self显式引用成员字段(name),也能够隐式引用(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:假设这里的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),见以下的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,接收的值被自己主动命名为newValue。
willSet和didSet
EquilateralTriangle的构造器进行了例如以下操作:
1.为子类型的属性赋值。
2.调用父类型的构造器。
3.改动父类型的属性。
假设不须要计算属性的值,但须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,函数的參数名称仅仅能在函数内部使用,但方法的參数名称除了在内部使用外还能够在外部使用(第一个參数除外),比如:
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支持为方法參数取别名:在上面的代码里,numberOfTimes面向外部,times面向内部。
?的还有一种用途
使用可空值时,?能够出如今方法、属性或下标前面。假设?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,比如:
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的枚举能够关联方法:
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)没有必定关联。
一些情况下枚举不存在有意义的原始值,这时能够直接忽略原始值:
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()

除了能够关联方法,枚举还支持在其成员上关联值,同一枚举的不同成员能够有不同的关联的值:
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structkeyword创建结构。结构支持构造器和方法这些类的特性。结构和类的最大差别在于: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、枚举和结构都能够实现(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添加新的功能(比方新的方法或属性),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)

有时须要对泛型做一些需求(requirements),比方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描写叙述这些需求:
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])


本文转自Lucida的博客  感谢原作者

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