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。
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 } 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: