swift hero

关于Swift

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:
[green_box]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.[/green_box]
简单的说:

  1. Swift用来写iOS和OS X程序。(估计也不会支持其它屌丝系统)
  2. Swift吸取了C和Objective-C的优点,且更加强大易用。
  3. Swift可以使用现有的Cocoa和Cocoa Touch框架。
  4. Swift兼具编译语言的高性能(Performance)和脚本语言的交互性(Interactive)。
  5. 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 }
[green_box]

[/green_box]