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 widthLabel = 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 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: 1 2 3 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<ItemType>(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<T> { case None case Some(T) } var possibleInteger: OptionalValue<Int> = .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])