Swift简介 教程

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

}

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。