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Swift简介

原作者: [db:作者] 来自: [db:来源] 收藏 邀请

1. swift初始来源

    没有了解过swift可以先参考Brec VictorInventing on Principle ,Swift编程环境的大部分概念都源自于Brec这个演讲。

2. swift是啥

Swift是苹果在WWDC 2014发布的新的编程语言,这里引用The Swift Programming Language的原文:

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:

  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)。

3.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。

1 println("Hello, world")

变量与常量

Swift使用var声明变量,let声明常量。

1 var myVariable = 42
2 myVariable = 50
3 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:

1 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):

1 let label = "The width is "
2 let width = 94
3 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:

1 let apples = 3
2 let oranges = 5
3 let appleSummary = "I have \(apples) apples."
4 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):

1 var shoppingList = ["catfish", "water", "tulips", "blue paint"]
2 shoppingList[1] = "bottle of water"
3 
4 var occupations = [
5     "Malcolm": "Captain",
6     "Kaylee": "Mechanic",
7 ]
8 occupations["Jayne"] = "Public Relations"

 

一般使用初始化器(initializer)语法创建空数组和空字典:

1 let emptyArray = String[]()
2 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:

1 let individualScores = [75, 43, 103, 87, 12]
2 var teamScore = 0
3 for score in individualScores {
4     if score > 50 {
5         teamScore += 3
6     } else {
7         teamScore += 1
8     }
9 }

 

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。

1 var optionalString: String? = "Hello"
2 optionalString == nil
3 
4 var optionalName: String? = "John Appleseed"
5 var gretting = "Hello!"
6 if let name = optionalName {
7     gretting = "Hello, \(name)"
8 }

 

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:

 1 let interestingNumbers = [
 2     "Prime": [2, 3, 5, 7, 11, 13],
 3     "Fibonacci": [1, 1, 2, 3, 5, 8],
 4     "Square": [1, 4, 9, 16, 25],
 5 ]
 6 var largest = 0
 7 for (kind, numbers) in interestingNumbers {
 8     for number in numbers {
 9         if number > largest {
10             largest = number
11         }
12     }
13 }
14 largest

 

while循环和do-while循环:

 1 var n = 2
 2 while n < 100 {
 3     n = n * 2
 4 }
 5 n
 6 
 7 var m = 2
 8 do {
 9     m = m * 2
10 } while m < 100
11 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。

 1 var firstForLoop = 0
 2 for i in 0..3 {
 3     firstForLoop += i
 4 }
 5 firstForLoop
 6 
 7 var secondForLoop = 0
 8 for var i = 0; i < 3; ++i {
 9     secondForLoop += 1
10 }
11 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:

1 func greet(name: String, day: String) -> String {
2     return "Hello \(name), today is \(day)."
3 }
4 greet("Bob", "Tuesday")

 

通过元组(Tuple)返回多个值:

1 func getGasPrices() -> (Double, Double, Double) {
2     return (3.59, 3.69, 3.79)
3 }
4 getGasPrices()

 

支持带有变长参数的函数:

1 func sumOf(numbers: Int...) -> Int {
2     var sum = 0
3     for number in numbers {
4         sum += number
5     }
6     return sum
7 }
8 sumOf()
9 sumOf(42, 597, 12)

 

函数也可以嵌套函数:

1 func returnFifteen() -> Int {
2     var y = 10
3     func add() {
4         y += 5
5     }
6     add()
7     return y
8 }
9 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:

1 func makeIncrementer() -> (Int -> Int) {
2     func addOne(number: Int) -> Int {
3         return 1 + number
4     }
5     return addOne
6 }
7 var increment = makeIncrementer()
8 increment(7)

 

 1 func hasAnyMatches(list: Int[], condition: Int -> Bool) -> Bool {
 2     for item in list {
 3         if condition(item) {
 4             return true
 5         }
 6     }
 7     return false
 8 }
 9 func lessThanTen(number: Int) -> Bool {
10     return number < 10
11 }
12 var numbers = [20, 19, 7, 12]
13 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:

1 numbers.map({
2     (number: Int) -> Int in
3     let result = 3 * number
4     return result
5     })

 

当闭包的类型已知时,可以使用下面的简化写法:

1 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:

1 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:

1 class Shape {
2     var numberOfSides = 0
3     func simpleDescription() -> String {
4         return "A shape with \(numberOfSides) sides."
5     }
6 }

 

创建Shape类的实例,并调用其字段和方法。

1 var shape = Shape()
2 shape.numberOfSides = 7
3 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)。

 1 class NamedShape {
 2     var numberOfSides: Int = 0
 3     var name: String
 4 
 5     init(name: String) {
 6         self.name = name
 7     }
 8 
 9     func simpleDescription() -> String {
10         return "A shape with \(numberOfSides) sides."
11     }
12 }

 

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):

 1 class Square: NamedShape {
 2     var sideLength: Double
 3 
 4     init(sideLength: Double, name: String) {
 5         self.sideLength = sideLength
 6         super.init(name: name)
 7         numberOfSides = 4
 8     }
 9 
10     func area() -> Double {
11         return sideLength * sideLength
12     }
13 
14     override func simpleDescription() -> String {
15         return "A square with sides of length \(sideLength)."
16     }
17 }
18 let test = Square(sideLength: 5.2, name: "my test square")
19 test.area()
20 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:

 1 class EquilateralTriangle: NamedShape {
 2     var sideLength: Double = 0.0
 3 
 4     init(sideLength: Double, name: String) {
 5         self.sideLength = sideLength
 6         super.init(name: name)
 7         numberOfSides = 3
 8     }
 9 
10     var perimeter: Double {
11     get {
12         return 3.0 * sideLength
13     }
14     set {
15         sideLength = newValue / 3.0
16     }
17     }
18 
19     override func simpleDescription() -> String {
20         return "An equilateral triagle with sides of length \(sideLength)."
21     }
22 }
23 var triangle = EquilateralTriangle(sideLength: 3.1, name: "a triangle")
24 triangle.perimeter
25 triangle.perimeter = 9.9
26 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

 1 class TriangleAndSquare {
 2     var triangle: EquilateralTriangle {
 3     willSet {
 4         square.sideLength = newValue.sideLength
 5     }
 6     }
 7     var square: Square {
 8     willSet {
 9         triangle.sideLength = newValue.sideLength
10     }
11     }
12     init(size: Double, name: String) {
13         square = Square(sideLength: size, name: name)
14         triangle = EquilateralTriangle(sideLength: size, name: name)
15     }
16 }
17 var triangleAndSquare = TriangleAndSquare(size: 10, name: "another test shape")
18 triangleAndSquare.square.sideLength
19 triangleAndSquare.square = Square(sideLength: 50, name: "larger square")
20 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:

1 class Counter {
2     var count: Int = 0
3     func incrementBy(amount: Int, numberOfTimes times: Int) {
4         count += amount * times
5     }
6 }
7 var counter = Counter()
8 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:

1 let optionalSquare: Square? = Square(sideLength: 2.5, name: "optional 
2 square")
3 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:

 1 enum Rank: Int {
 2     case Ace = 1
 3     case Two, Three, Four, Five, Six, Seven, Eight, Nine, Ten
 4     case Jack, Queen, King
 5         func simpleDescription() -> String {
 6         switch self {
 7             case .Ace:
 8                 return "ace"
 9             case .Jack:
10                 return "jack"
11             case .Queen:
12                 return "queen"
13             case .King:
14                 return "king"
15             default:
16                 return String(self.toRaw())
17         }
18     }
19 }
20 let ace = Rank.Ace
21 let aceRawValue = ace.toRaw()

 

使用toRawfromRaw在原始(raw)数值和枚举值之间进行转换:

1 if let convertedRank = Rank.fromRaw(3) {
2     let threeDescription = convertedRank.simpleDescription()
3 }

 

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:

 1 enum Suit {
 2     case Spades, Hearts, Diamonds, Clubs
 3         func simpleDescription() -> String {
 4         switch self {
 5             case .Spades:
 6                 return "spades"
 7             case .Hearts:
 8                 return "hearts"
 9             case .Diamonds:
10                 return "diamonds"
11             case .Clubs:
12                 return "clubs"
13         }
14     }
15 }
16 let hearts = Suit.Hearts
17 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:

 1 enum ServerResponse {
 2     case Result(String, String)
 3     case Error(String)
 4 }
 5 
 6 let success = ServerResponse.Result("6:00 am", "8:09 pm")
 7 let failure = ServerResponse.Error("Out of cheese.")
 8 
 9 switch success {
10     case let .Result(sunrise, sunset):
11         let serverResponse = "Sunrise is at \(sunrise) and sunset is at \(sunset)."
12     case let .Error(error):
13         let serverResponse = "Failure... \(error)"
14 }

 

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)。

1 struct Card {
2     var rank: Rank
3     var suit: Suit
4     func simpleDescription() -> String {
5         return "The \(rank.simpleDescription()) of \(suit.simpleDescription())"
6     }
7 }
8 let threeOfSpades = Card(rank: .Three, suit: .Spades)
9 let threeOfSpadesDescription = threeOfSpades.simpleDescription()

 

协议(protocol)和扩展(extension)

协议

Swift使用protocol定义协议:

1 protocol ExampleProtocol {
2     var simpleDescription: String { get }
3     mutating func adjust()
4 }

 

类型、枚举和结构都可以实现(adopt)协议:

 1 class SimpleClass: ExampleProtocol {
 2     var simpleDescription: String = "A very simple class."
 3     var anotherProperty: Int = 69105
 4     func adjust() {
 5         simpleDescription += " Now 100% adjusted."
 6     }
 7 }
 8 var a = SimpleClass()
 9 a.adjust()
10 let aDescription = a.simpleDescription
11 
12 struct SimpleStructure: ExampleProtocol {
13     var simpleDescription: String = "A simple structure"
14     mutating func adjust() {
15         simpleDescription += " (adjusted)"
16     }
17 }
18 var b = SimpleStructure()
19 b.adjust()
20 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:

1 extension Int: ExampleProtocol {
2     var simpleDescription: String {
3         return "The number \(self)"
 
                       
                    
                    

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