swift language

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。

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println("Hello, world")

变量与常量

Swift使用var声明变量,let声明常量

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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:

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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):

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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:

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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):

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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:

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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ifswitch,循环语句包含for-inforwhiledo-while,循环/判断条件不需要括号,但循环/判断体(body)必需括号:

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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iflet,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。

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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:

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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:

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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:

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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。

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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:

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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:

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func getGasPrices() -> (Double, Double, Double) {
return (3.59, 3.69, 3.79)
}
getGasPrices()

支持带有变长参数的函数:

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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:

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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:

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func makeIncrementer() -> (Int -> Int) {
func addOne(number: Int) -> Int {
return 1 + number
}
return addOne
}
var increment = makeIncrementer()
increment(7)
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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:

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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:

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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:

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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。

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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)。

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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):

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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:

 

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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willSetdidSet

 

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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trianglesquare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:

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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:

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let optionalSquare: Square? = Square(sideLength: 2.5, name: "optional
square")
let sideLength = optionalSquare?.sideLength

 

 

optionalSquarenil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:

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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toRawfromRaw在原始(raw)数值和枚举值之间进行转换:

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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:

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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:

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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)。

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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:

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protocol ExampleProtocol {
var simpleDescription: String { get }
mutating func adjust()
}

类型、枚举和结构都可以实现(adopt)协议:

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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:

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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:

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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:

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// 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:

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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])

 

 

Swift语言概览就到这里,有兴趣的朋友请进一步阅读The Swift Programming Language。

接下来聊聊个人对Swift的一些感受。

个人感受

注意:下面的感受纯属个人意见,仅供参考。

大杂烩

尽管我接触Swift不足两小时,但很容易看出Swift吸收了大量其它编程语言中的元素,这些元素包括但不限于:

  1. 属性(Property)、可空值(Nullable type)语法和泛型(Generic Type)语法源自C#。
  2. 格式风格与Go相仿(没有句末的分号,判断条件不需要括号)。
  3. Python风格的当前实例引用语法(使用self)和列表字典声明语法。
  4. Haskell风格的区间声明语法(比如1..31...3)。
  5. 协议和扩展源自Objective-C(自家产品随便用)。
  6. 枚举类型很像Java(可以拥有成员或方法)。
  7. classstruct的概念和C#极其相似。

注意这里不是说Swift是抄袭——实际上编程语言能玩的花样基本就这些,况且Swift选的都是在我看来相当不错的特性。

而且,这个大杂烩有一个好处——就是任何其它编程语言的开发者都不会觉得Swift很陌生——这一点很重要。

 

拒绝隐式(Refuse implicity)

Swift去除了一些隐式操作,比如隐式类型转换和隐式方法重载这两个坑,干的漂亮。

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