Swift 学习一

国外开发者最近发现，WWDC2014上苹果发布的新语言Swift，和古老的Scala语言在语法上存在众多的相似之处。

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本文以苹果官方教程The Swift Programming Language中的示例，比较Swift与Scala两种语言实现同一功能的代码。

Swift语言从语法上来看，几乎是Scala的一个分支，在以下功能上几乎是等同的：类型继承、闭包、元组（Tuple）、协议、扩展、泛型等。

不过Swift的运行环境和Scala的区别还是很大，这个概念才是Swift最重要的。Scala语言编译成JVM程序，使用垃圾收集机制，与Java无缝整合。但Swift最终编译到机器代码，使用引用计数机制，与Objective-C无缝整合。所以Swift和Scala在代码表象上的相似，应该并不太影响两种语言本质机理上的重大不一致。

语言基础

你好，世界。

// Swift
println("Hello,world!")
// Scala */
println()

变量与常量

// Swift
var myVariable = 42
myVariable = 50
let myConstant = 42

// Scala
var myVariable = 50
val myConstant = 42

显式类型

// Swift
let explicitDouble: Double = 70
// Scala
val explicitDouble: Double = 70

强制类型转换

// Swift
let label = "The width is "
let width = 94
let widthLabel = label + String(width)

//Scala
val label = "The width is "
val width = 94
val widthLabel = label + width

字符串数据填充

// Swift
let apples = 3
let oranges = 5
let fruitSummary = "I have \(apples + oranges) " + 
 "pieces of fruit."

// Scala
val apples = 3
val oranges = 5
val fruitSummary = s"I have ${apples + oranges} " +
 " pieces of fruit."

整数半开区间运算符

// Swift
let names = ["Anna","Alex",68)">"Brian",68)">"Jack"]
let count = names.count
for i in 0..count {
 println("Person \(i + 1) is called \(names[i])")
}
// Person 1 is called Anna
// Person 2 is called Alex
// Person 3 is called Brian
// Person 4 is called Jack

// Scala
val names = Array("Jack")
val count = names.length
for (i <- 0 until count) {
 println(s"Person ${i + 1} is called ${names(i)}")
}
// Person 4 is called Jack

整数闭区间运算符

// Swift
for index in 1...5 {
 println("\(index) times 5 is \(index * 5)")
}
// 1 times 5 is 5
// 2 times 5 is 10
// 3 times 5 is 15
// 4 times 5 is 20
// 5 times 5 is 25

// Scala
for (index <- 1 to 5) {
 println(s"$index times 5 is ${index * 5}")
}
// 5 times 5 is 25

集合

数组

// Swift
var shoppingList = ["catfish",68)">"water", "tulips",68)">"blue paint"]
shoppingList[1] = "bottle of water"

// Scala
var shoppingList = Array("blue paint")
shoppingList(1) = "bottle of water"

字典

// Swift
var occupations = [
 "Malcolm": "Captain",68)">"Kaylee": "Mechanic",]
occupations["Jayne"] = "Public Relations"

// Scala
var occupations = scala.collection.mutable.Map(
 "Malcolm" -> "Kaylee" -> "Mechanic"
)
occupations("Jayne") = "Public Relations"

空集

// Swift
let emptyArray = String[]()
let emptyDictionary = Dictionary<String,Float>()
let emptyArrayNoType = []

// Scala
val emptyArray = Array[String]()
val emptyDictionary = Map[String,Float]()
val emptyArrayNoType = Array()

函数

函数定义

// Swift
func greet(name: String,day: String) -> String {
 return "Hello \(name),today is \(day)."
}
greet("Bob",68)">"Tuesday")

// Scala
def greet(name: String,day: String): String = {
 return s"Hello $name,today is $day."
}
greet("Tuesday")

元组（Tuple）返回值

// Swift
func getGasPrices() -> (Double,Double,Double) {
 return (3.59,3.69,153)">3.79)
}

// Scala
def getGasPrices(): (Double,Double) = {
 return (3.79)
}

可变数量参数

// Swift
func sumOf(numbers: Int...) -> Int {
 var sum = 0
 for number in numbers {
  sum += number
 }
 return sum
}
sumOf(42,153)">597,153)">12)

// Scala
def sumOf(numbers: Int*): Int = {
 var sum = 0
 for (number <- numbers) {
  sum += number
 }
 return sum
}
sumOf(12)

函数作为数据类型

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

// Scala
def makeIncrementer(): Int => Int = {
 def addOne(number: Int): Int = {
  return 7)

集合迭代器（Map）

// Swift
var numbers = [20,153)">19,153)">7,153)">12]
numbers.map({ number in 3 * number })

// Scala
var numbers = Array(12)
numbers.map( number => 3 * number )

排序

// Swift
sort([1,153)">5,153)">3,153)">12,153)">2]) { $0 > $1 }

// Scala
Array(2).sortWith(_ > _)

命名参数

// Swift
def area(width: Int,height: Int) -> Int {
 return width * height
}

area(width: 10,height: 10)

// Scala
def area(width: Int,height: Int): Int = {
 return width * height
}

area(width = 10)

类

定义

// Swift
class Shape {
 var numberOfSides = 0
 func simpleDescription() -> String {
  return "A shape with \(numberOfSides) sides."
 }
}

// Scala
0
 def simpleDescription(): String = {
  return s"A shape with $numberOfSides sides."
 }
}

使用

// Swift
var shape = Shape()
shape.numberOfSides = 7
var shapeDescription = shape.simpleDescription()

// Scala
var shape = new Shape()
shape.numberOfSides = 7
var shapeDescription = shape.simpleDescription()

子类

// Swift
class NamedShape {
 var numberOfSides: Int = 0
 var name: String

 init(name: String) {
  self.name = name
 }

 func simpleDescription() -> String {
  return "A shape with \(numberOfSides) sides."
 }
}

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)
test.area()
test.simpleDescription()

// Scala
class NamedShape(var name: String) {
 var numberOfSides: Int = 0

 def simpleDescription() =
  s"A shape with $numberOfSides sides."
}

class Square(var sideLength: Double,name: String)
 extends NamedShape(name) {
 numberOfSides = 4
 def area() = sideLength * sideLength

 override def simpleDescription() =
  s"A square with sides of length $sideLength."
}

val test = new Square(5.2,"my test square")
test.area()
test.simpleDescription()

检查实例所属的类

// Swift
var movieCount = 0
var songCount = 0

for item in library {
 if item is Movie {
  ++movieCount
 } else if item is Song {
  ++songCount
 }
}

// Scala
var movieCount = 0

for (item <- library) {
 if (item.isInstanceOf[Movie]) {
  movieCount += 1
 } else if (item.isInstanceOf[Song]) {
  songCount += 1
 }
}

基类转派生类（向下转换）

// Swift
for object in someObjects {
 let movie = object as Movie
 println("Movie: '\(movie.name)',dir. \(movie.director)")
}

// Scala
for (obj <- someObjects) {
 val movie = obj.asInstanceOf[Movie]
 println(s"Movie: '${movie.name}',dir. ${movie.director}")
}

协议

// Swift
protocol Nameable {
 func name() -> String
}

func f<T: Nameable>(x: T) {
 println("Name is " + x.name())
}

// Scala
trait Nameable {
 def name(): String
}

def f[T <: Nameable](x: T) = {  println("Name is " + x.name()) } 

扩展

// Swift
extension Double {
 var km: Double { return self * 1_000.0 }
 var m: Double { return self }
 var cm: Double { return self / 100.0 }
 var mm: Double { return self / .0 }
 var ft: Double { return self / 3.28084 }
}
let oneInch = 25.4.mm
println("One inch is \(oneInch) meters")
// prints "One inch is 0.0254 meters"
let threeFeet = 3.ft
println("Three feet is \(threeFeet) meters")
// prints "Three feet is 0.914399970739201 meters"

// Scala
object Extensions {
 implicit class DoubleUnit(d: Double) {
  def km: Double = { return d * 1000.0 }
  def m: Double = { return d }
  def cm: Double = { return d / 100.0 }
  def mm: Double = { return d / 1000.0 }
  def ft: Double = { return d / 3.28084 }
 }
}

import Extensions.DoubleUnit

val oneInch = 25.4.mm
println(s"One inch is $oneInch meters")
// prints "One inch is 0.0254 meters"
val threeFeet = 3.ft
println(s"Three feet is $threeFeet meters")
// prints "Three feet is 0.914399970739201 meters"