From 7e1ae9a637b6eeab29bcfd335b67c7e3d75ba9c8 Mon Sep 17 00:00:00 2001 From: stanzhai Date: Sat, 14 Jun 2014 22:00:18 +0800 Subject: [PATCH] merge 2.21 protocols --- .DS_Store | Bin 0 -> 6148 bytes source/chapter2/21_Protocols.md | 743 +++++++++++++++++--------------- 2 files changed, 406 insertions(+), 337 deletions(-) create mode 100644 .DS_Store diff --git a/.DS_Store b/.DS_Store new file mode 100644 index 0000000000000000000000000000000000000000..5008ddfcf53c02e82d7eee2e57c38e5672ef89f6 GIT binary patch literal 6148 zcmeH~Jr2S!425mzP>H1@V-^m;4Wg<&0T*E43hX&L&p$$qDprKhvt+--jT7}7np#A3 zem<@ulZcFPQ@L2!n>{z**++&mCkOWA81W14cNZlEfg7;MkzE(HCqgga^y>{tEnwC%0;vJ&^%eQ zLs35+`xjp>T0 翻译：geek5nan - -> 校对：dabing1022 +> 翻译：geek5nan +> 校对：dabing1022 # 协议 ----------------- @@ -30,22 +29,27 @@ `协议`的定义与类，结构体，枚举的定义非常相似，如下所示： - protocol SomeProtocol { - // 协议内容 - } - +```swift +protocol SomeProtocol { + // 协议内容 +} +``` 在类，结构体，枚举的名称后加上`协议名称`，中间以冒号`:`分隔即可实现协议；实现多个协议时，各协议之间用逗号`,`分隔，如下所示： - struct SomeStructure: FirstProtocol, AnotherProtocol { - // 结构体内容 - } +```swift +struct SomeStructure: FirstProtocol, AnotherProtocol { + // 结构体内容 +} +``` 当某个类含有父类的同时并实现了协议，应当把父类放在所有的协议之前，如下所示： - class SomeClass: SomeSuperClass, FirstProtocol, AnotherProtocol { - // 类的内容 - } +```swift +class SomeClass: SomeSuperClass, FirstProtocol, AnotherProtocol { + // 类的内容 +} +``` ## 属性要求 @@ -54,46 +58,54 @@ 通常前置`var`关键字将属性声明为变量。在属性声明后写上`{ get set }`表示属性为可读写的。`{ get }`用来表示属性为可读的。即使你为可读的属性实现了`setter`方法，它也不会出错。 - protocol SomeProtocol { - var musBeSettable : Int { get set } - var doesNotNeedToBeSettable: Int { get } - } +```swift +protocol SomeProtocol { + var musBeSettable : Int { get set } + var doesNotNeedToBeSettable: Int { get } +} +``` 用类来实现协议时，使用`class`关键字来表示该属性为类成员；用结构体或枚举实现协议时，则使用`static`关键字来表示： - protocol AnotherProtocol { - class var someTypeProperty: Int { get set } - } +```swift +protocol AnotherProtocol { + class var someTypeProperty: Int { get set } +} - protocol FullyNamed { - var fullName: String { get } - } +protocol FullyNamed { + var fullName: String { get } +} +``` `FullyNamed`协议含有`fullName`属性。因此其`遵循者`必须含有一个名为`fullName`，类型为`String`的可读属性。 - struct Person: FullyNamed{ - var fullName: String - } - let john = Person(fullName: "John Appleseed") - //john.fullName 为 "John Appleseed" +```swift +struct Person: FullyNamed{ + var fullName: String +} +let john = Person(fullName: "John Appleseed") +//john.fullName 为 "John Appleseed" +``` `Person`结构体含有一个名为`fullName`的`存储型属性`，完整的`遵循`了协议。(*若协议未被完整遵循，编译时则会报错*)。 如下所示，`Startship`类`遵循`了`FullyNamed`协议： - class Starship: FullyNamed { - var prefix: String? - var name: String - init(name: String, prefix: String? = nil ) { - self.anme = name - self.prefix = prefix - } - var fullName: String { - return (prefix ? prefix ! + " " : " ") + name - } +```swift +class Starship: FullyNamed { + var prefix: String? + var name: String + init(name: String, prefix: String? = nil ) { + self.anme = name + self.prefix = prefix } - var ncc1701 = Starship(name: "Enterprise", prefix: "USS") - // ncc1701.fullName == "USS Enterprise" + var fullName: String { + return (prefix ? prefix ! + " " : " ") + name + } +} +var ncc1701 = Starship(name: "Enterprise", prefix: "USS") +// ncc1701.fullName == "USS Enterprise" +``` `Starship`类将`fullName`实现为可读的`计算型属性`。它的每一个实例都有一个名为`name`的必备属性和一个名为`prefix`的可选属性。 当`prefix`存在时，将`prefix`插入到`name`之前来为`Starship`构建`fullName`。 @@ -102,39 +114,42 @@ `协议`能够要求其`遵循者`必备某些特定的`实例方法`和`类方法`。协议方法的声明与普通方法声明相似，但它不需要`方法`内容。 -> 注意： -> +> 注意： 协议方法支持`变长参数(variadic parameter)`，不支持`默认参数(default parameter)`。 前置`class`关键字表示协议中的成员为`类成员`；当协议用于被`枚举`或`结构体`遵循时，则使用`static`关键字。如下所示： - protocol SomeProtocol { - class func someTypeMethod() - } +```swift +protocol SomeProtocol { + class func someTypeMethod() +} - protocol RandomNumberGenerator { - func random() -> Double - } +protocol RandomNumberGenerator { + func random() -> Double +} +``` `RandomNumberGenerator`协议要求其`遵循者`必须拥有一个名为`random`， 返回值类型为`Double`的实例方法。(我们假设随机数在[0，1]区间内)。 `LinearCongruentialGenerator`类`遵循`了`RandomNumberGenerator`协议，并提供了一个叫做*线性同余生成器(linear congruential generator)*的伪随机数算法。 - class LinearCongruentialGenerator: RandomNumberGenerator { - var lastRandom = 42.0 - let m = 139968.0 - let a = 3877.0 - let c = 29573.0 - func random() -> Double { - lastRandom = ((lastRandom * a + c) % m) - return lastRandom / m - } +```swift +class LinearCongruentialGenerator: RandomNumberGenerator { + var lastRandom = 42.0 + let m = 139968.0 + let a = 3877.0 + let c = 29573.0 + func random() -> Double { + lastRandom = ((lastRandom * a + c) % m) + return lastRandom / m } - let generator = LinearCongruentialGenerator() - println("Here's a random number: \(generator.random())") - // 输出 : "Here's a random number: 0.37464991998171" - println("And another one: \(generator.random())") - // 输出 : "And another one: 0.729023776863283" +} +let generator = LinearCongruentialGenerator() +println("Here's a random number: \(generator.random())") +// 输出 : "Here's a random number: 0.37464991998171" +println("And another one: \(generator.random())") +// 输出 : "And another one: 0.729023776863283" +``` ## 突变方法要求 @@ -143,34 +158,37 @@ (*译者注：类中的成员为`引用类型(Reference Type)`，可以方便的修改实例及其属性的值而无需改变类型；而`结构体`和`枚举`中的成员均为`值类型(Value Type)`，修改变量的值就相当于修改变量的类型，而`Swift`默认不允许修改类型，因此需要前置`mutating`关键字用来表示该`函数`中能够修改类型*) -> 注意： -> +> 注意： 用`class`实现协议中的`mutating`方法时，不用写`mutating`关键字；用`结构体`，`枚举`实现协议中的`mutating`方法时，必须写`mutating`关键字。 如下所示，`Togglable`协议含有`toggle`函数。根据函数名称推测，`toggle`可能用于**切换或恢复**某个属性的状态。`mutating`关键字表示它为`突变方法`： - protocol Togglable { - mutating func toggle() - } +```swift +protocol Togglable { + mutating func toggle() +} +``` 当使用`枚举`或`结构体`来实现`Togglabl`协议时，必须在`toggle`方法前加上`mutating`关键字。 如下所示，`OnOffSwitch`枚举`遵循`了`Togglable`协议，`On`，`Off`两个成员用于表示当前状态 - enum OnOffSwitch: Togglable { - case Off, On - mutating func toggle() { - switch self { - case Off: - self = On - case On: - self = Off - } +```swift +enum OnOffSwitch: Togglable { + case Off, On + mutating func toggle() { + switch self { + case Off: + self = On + case On: + self = Off } } - var lightSwitch = OnOffSwitch.Off - lightSwitch.toggle() - //lightSwitch 现在的值为 .On +} +var lightSwitch = OnOffSwitch.Off +lightSwitch.toggle() +//lightSwitch 现在的值为 .On +``` ## 协议类型 @@ -183,25 +201,26 @@ * 作为常量，变量，属性的类型 * 作为数组，字典或其他容器中的元素类型 -> 注意： -> +> 注意： 协议类型应与其他类型(Int，Double，String)的写法相同，使用驼峰式 - class Dice { - let sides: Int - let generator: RandomNumberGenerator - init(sides: Int, generator: RandomNumberGenerator) { - self.sides = sides - self.generator = generator - } - func roll() -> Int { - return Int(generator.random() * Double(sides)) +1 - } +```swift +class Dice { + let sides: Int + let generator: RandomNumberGenerator + init(sides: Int, generator: RandomNumberGenerator) { + self.sides = sides + self.generator = generator } + func roll() -> Int { + return Int(generator.random() * Double(sides)) +1 + } +} +``` 这里定义了一个名为 `Dice`的类，用来代表桌游中的N个面的骰子。 - `Dice`含有`sides`和`generator`两个属性，前者用来表示骰子有几个面，后者为骰子提供一个随机数生成器。由于后者为`RandomNumberGenerator`的协议类型。所以它能够被赋值为任意`遵循`该协议的类型。 +`Dice`含有`sides`和`generator`两个属性，前者用来表示骰子有几个面，后者为骰子提供一个随机数生成器。由于后者为`RandomNumberGenerator`的协议类型。所以它能够被赋值为任意`遵循`该协议的类型。 此外，使用`构造器(init)`来代替之前版本中的`setup`操作。构造器中含有一个名为`generator`，类型为`RandomNumberGenerator`的形参，使得它可以接收任意遵循`RandomNumberGenerator`协议的类型。 @@ -209,16 +228,18 @@ 如下所示，`LinearCongruentialGenerator`的实例作为随机数生成器传入`Dice`的`构造器` - var d6 = Dice(sides: 6,generator: LinearCongruentialGenerator()) - for _ in 1...5 { - println("Random dice roll is \(d6.roll())") - } - //输出结果 - //Random dice roll is 3 - //Random dice roll is 5 - //Random dice roll is 4 - //Random dice roll is 5 - //Random dice roll is 4 +```swift +var d6 = Dice(sides: 6,generator: LinearCongruentialGenerator()) +for _ in 1...5 { + println("Random dice roll is \(d6.roll())") +} +//输出结果 +//Random dice roll is 3 +//Random dice roll is 5 +//Random dice roll is 4 +//Random dice roll is 5 +//Random dice roll is 4 +``` ## 委托(代理)模式 @@ -231,55 +252,59 @@ 下文是两个基于骰子游戏的协议： - protocol DiceGame { - var dice: Dice { get } - func play() - } - protocol DiceGameDelegate { - func gameDidStart(game: DiceGame) - func game(game: DiceGame, didStartNewTurnWithDiceRoll diceRoll:Int) - func gameDidEnd(game: DiceGame) - } +```swift +protocol DiceGame { + var dice: Dice { get } + func play() +} + +protocol DiceGameDelegate { + func gameDidStart(game: DiceGame) + func game(game: DiceGame, didStartNewTurnWithDiceRoll diceRoll:Int) + func gameDidEnd(game: DiceGame) +} +``` `DiceGame`协议可以在任意含有骰子的游戏中实现，`DiceGameDelegate`协议可以用来追踪`DiceGame`的游戏过程。 如下所示，`SnakesAndLadders`是`Snakes and Ladders`(译者注：[控制流](05_Control_Flow.html)章节有该游戏的详细介绍)游戏的新版本。新版本使用`Dice`作为骰子，并且实现了`DiceGame`和`DiceGameDelegate`协议 - class SnakesAndLadders: DiceGame { - let finalSquare = 25 - let dic = Dice(sides: 6, generator: LinearCongruentialGenerator()) - var square = 0 - var board: Int[] - init() { - board = Int[](count: finalSquare + 1, repeatedValue: 0) - board[03] = +08; board[06] = +11; borad[09] = +09; board[10] = +02 - borad[14] = -10; board[19] = -11; borad[22] = -02; board[24] = -08 - } - var delegate: DiceGameDelegate? - func play() { - square = 0 - delegate?.gameDidStart(self) - gameLoop: while square != finalSquare { - let diceRoll = dice.roll() - delegate?.game(self,didStartNewTurnWithDiceRoll: diceRoll) - switch square + diceRoll { - case finalSquare: - break gameLoop - case let newSquare where newSquare > finalSquare: - continue gameLoop - default: - square += diceRoll - square += board[square] - } - } - delegate?.gameDIdEnd(self) - } +```swift +class SnakesAndLadders: DiceGame { + let finalSquare = 25 + let dic = Dice(sides: 6, generator: LinearCongruentialGenerator()) + var square = 0 + var board: Int[] + init() { + board = Int[](count: finalSquare + 1, repeatedValue: 0) + board[03] = +08; board[06] = +11; borad[09] = +09; board[10] = +02 + borad[14] = -10; board[19] = -11; borad[22] = -02; board[24] = -08 } + var delegate: DiceGameDelegate? + func play() { + square = 0 + delegate?.gameDidStart(self) + gameLoop: while square != finalSquare { + let diceRoll = dice.roll() + delegate?.game(self,didStartNewTurnWithDiceRoll: diceRoll) + switch square + diceRoll { + case finalSquare: + break gameLoop + case let newSquare where newSquare > finalSquare: + continue gameLoop + default: + square += diceRoll + square += board[square] + } + } + delegate?.gameDIdEnd(self) + } +} +``` 游戏的`初始化设置(setup)`被`SnakesAndLadders`类的`构造器(initializer)`实现。所有的游戏逻辑被转移到了`play`方法中。 -> 注意： -> +> 注意： 因为`delegate`并不是该游戏的必备条件，`delegate`被定义为遵循`DiceGameDelegate`协议的可选属性 `DicegameDelegate`协议提供了三个方法用来追踪游戏过程。被放置于游戏的逻辑中，即`play()`方法内。分别在游戏开始时，新一轮开始时，游戏结束时被调用。 @@ -288,23 +313,25 @@ 如下所示，`DiceGameTracker`遵循了`DiceGameDelegate`协议 - class DiceGameTracker: DiceGameDelegate { - var numberOfTurns = 0 - func gameDidStart(game: DiceGame) { - numberOfTurns = 0 - if game is SnakesAndLadders { - println("Started a new game of Snakes and Ladders") - } - println("The game is using a \(game.dice.sides)-sided dice") - } - func game(game: DiceGame, didStartNewTurnWithDiceRoll diceRoll: Int) { - ++numberOfTurns - println("Rolled a \(diceRoll)") - } - func gameDidEnd(game: DiceGame) { - println("The game lasted for \(numberOfTurns) turns") - } - } +```swift +class DiceGameTracker: DiceGameDelegate { + var numberOfTurns = 0 + func gameDidStart(game: DiceGame) { + numberOfTurns = 0 + if game is SnakesAndLadders { + println("Started a new game of Snakes and Ladders") + } + println("The game is using a \(game.dice.sides)-sided dice") + } + func game(game: DiceGame, didStartNewTurnWithDiceRoll diceRoll: Int) { + ++numberOfTurns + println("Rolled a \(diceRoll)") + } + func gameDidEnd(game: DiceGame) { + println("The game lasted for \(numberOfTurns) turns") + } +} +``` `DiceGameTracker`实现了`DiceGameDelegate`协议的方法要求，用来记录游戏已经进行的轮数。 当游戏开始时，`numberOfTurns`属性被赋值为0；在每新一轮中递加；游戏结束后，输出打印游戏的总轮数。 @@ -312,79 +339,91 @@ `DiceGameTracker`的运行情况，如下所示： - “let tracker = DiceGameTracker() - let game = SnakesAndLadders() - game.delegate = tracker - game.play() - // Started a new game of Snakes and Ladders - // The game is using a 6-sided dice - // Rolled a 3 - // Rolled a 5 - // Rolled a 4 - // Rolled a 5 - // The game lasted for 4 turns” +```swift +“let tracker = DiceGameTracker() +let game = SnakesAndLadders() +game.delegate = tracker +game.play() +// Started a new game of Snakes and Ladders +// The game is using a 6-sided dice +// Rolled a 3 +// Rolled a 5 +// Rolled a 4 +// Rolled a 5 +// The game lasted for 4 turns” +``` ## 在扩展中添加协议成员 即便无法修改源代码，依然可以通过`扩展(Extension)`来扩充已存在类型(*译者注： 类，结构体，枚举等*)。`扩展`可以为已存在的类型添加`属性`，`方法`，`下标`，`协议`等成员。详情请在[扩展](20_Extensions.html)章节中查看。 -> 注意： -> +> 注意： 通过`扩展`为已存在的类型`遵循`协议时，该类型的所有实例也会随之添加协议中的方法 `TextRepresentable`协议含有一个`asText`，如下所示： - protocol TextRepresentable { - func asText() -> String - } +```swift +protocol TextRepresentable { + func asText() -> String +} +``` 通过`扩展`为上一节中提到的`Dice`类遵循`TextRepresentable`协议 - extension Dice: TextRepresentable { - func asText() -> String { - return "A \(sides)-sided dice" - } +```swift +extension Dice: TextRepresentable { + cun asText() -> String { + return "A \(sides)-sided dice" } +} +``` 从现在起，`Dice`类型的实例可被当作`TextRepresentable`类型： - let d12 = Dice(sides: 12,generator: LinearCongruentialGenerator()) - println(d12.asText()) - // 输出 "A 12-sided dice" +```swift +let d12 = Dice(sides: 12,generator: LinearCongruentialGenerator()) +println(d12.asText()) +// 输出 "A 12-sided dice" +``` `SnakesAndLadders`类也可以通过`扩展`的方式来遵循协议： - extension SnakeAndLadders: TextRepresentable { - func asText() -> String { - return "A game of Snakes and Ladders with \(finalSquare) squares" - } +```swift +extension SnakeAndLadders: TextRepresentable { + func asText() -> String { + return "A game of Snakes and Ladders with \(finalSquare) squares" } - println(game.asText()) - // 输出 "A game of Snakes and Ladders with 25 squares" +} +println(game.asText()) +// 输出 "A game of Snakes and Ladders with 25 squares" +``` ## 通过延展补充协议声明 当一个类型已经实现了协议中的所有要求，却没有声明时，可以通过`扩展`来补充协议声明： - struct Hamster { - var name: String - func asText() -> String { - return "A hamster named \(name)" - } +```swift +struct Hamster { + var name: String + func asText() -> String { + return "A hamster named \(name)" } - extension Hamster: TextRepresentabl {} +} +extension Hamster: TextRepresentabl {} +``` 从现在起，`Hamster`的实例可以作为`TextRepresentable`类型使用 - let simonTheHamster = Hamster(name: "Simon") - let somethingTextRepresentable: TextRepresentabl = simonTheHamester - println(somethingTextRepresentable.asText()) - // 输出 "A hamster named Simon" +```swift +let simonTheHamster = Hamster(name: "Simon") +let somethingTextRepresentable: TextRepresentabl = simonTheHamester +println(somethingTextRepresentable.asText()) +// 输出 "A hamster named Simon" +``` -> 注意： -> +> 注意： 即时满足了协议的所有要求，类型也不会自动转变，因此你必须为它做出明显的协议声明 @@ -392,16 +431,20 @@ 协议类型可以被集合使用，表示集合中的元素均为协议类型： - let things: TextRepresentable[] = [game,d12,simoTheHamster] +```swift +let things: TextRepresentable[] = [game,d12,simoTheHamster] +``` 如下所示，`things`数组可以被直接遍历，并调用其中元素的`asText()`函数： - for thing in things { - println(thing.asText()) - } - // A game of Snakes and Ladders with 25 squares - // A 12-sided dice - // A hamster named Simon +```swift +for thing in things { + println(thing.asText()) +} +// A game of Snakes and Ladders with 25 squares +// A 12-sided dice +// A hamster named Simon +``` `thing`被当做是`TextRepresentable`类型而不是`Dice`，`DiceGame`，`Hamster`等类型。因此能且仅能调用`asText`方法 @@ -410,36 +453,42 @@ 协议能够*继承*一到多个其他协议。语法与类的继承相似，多个协议间用逗号`,`分隔 - protocol InheritingProtocol: SomeProtocol, AnotherProtocol { - // 协议定义 - } +```swift +protocol InheritingProtocol: SomeProtocol, AnotherProtocol { + // 协议定义 +} +``` 如下所示，`PrettyTextRepresentable`协议继承了`TextRepresentable`协议 - protocol PrettyTextRepresentable: TextRepresentable { - func asPrettyText() -> String - } +```swift +protocol PrettyTextRepresentable: TextRepresentable { + func asPrettyText() -> String +} +``` `遵循``PrettyTextRepresentable`协议的同时，也需要`遵循`TextRepresentable`协议。 如下所示，用`扩展`为`SnakesAndLadders`遵循`PrettyTextRepresentable`协议： - extension SnakesAndLadders: PrettyTextRepresentable { - func asPrettyText() -> String { - var output = asText() + ":\n" - for index in 1...finalSquare { - switch board[index] { - case let ladder where ladder > 0: - output += "▲ " - case let snake where snake < 0: - output += "▼ " - default: - output += "○ " - } - } - return output - } - } +```swift +extension SnakesAndLadders: PrettyTextRepresentable { + func asPrettyText() -> String { + var output = asText() + ":\n" + for index in 1...finalSquare { + switch board[index] { + case let ladder where ladder > 0: + output += "▲ " + case let snake where snake < 0: + output += "▼ " + default: + output += "○ " + } + } + return output + } +} +``` 在`for in`中迭代出了`board`数组中的每一个元素： @@ -449,9 +498,11 @@ 任意`SankesAndLadders`的实例都可以使用`asPrettyText()`方法。 - println(game.asPrettyText()) - // A game of Snakes and Ladders with 25 squares: - // ○ ○ ▲ ○ ○ ▲ ○ ○ ▲ ▲ ○ ○ ○ ▼ ○ ○ ○ ○ ▼ ○ ○ ▼ ○ ▼ ○ +```swift +println(game.asPrettyText()) +// A game of Snakes and Ladders with 25 squares: +// ○ ○ ▲ ○ ○ ▲ ○ ○ ▲ ▲ ○ ○ ○ ▼ ○ ○ ○ ○ ▼ ○ ○ ▼ ○ ▼ ○ +``` ## 协议合成 @@ -460,29 +511,30 @@ 举个例子： - protocol Named { - var name: String { get } - } - protocol Aged { - var age: Int { get } - } - struct Person: Named, Aged { - var name: String - var age: Int - } - func wishHappyBirthday(celebrator: protocol) { - println("Happy birthday \(celebrator.name) - you're \(celebrator.age)!") - } - let birthdayPerson = Person(name: "Malcolm", age: 21) - wishHappyBirthday(birthdayPerson) - // 输出 "Happy birthday Malcolm - you're 21! +```swift +protocol Named { + var name: String { get } +} +protocol Aged { + var age: Int { get } +} +struct Person: Named, Aged { + var name: String + var age: Int +} +func wishHappyBirthday(celebrator: protocol) { + println("Happy birthday \(celebrator.name) - you're \(celebrator.age)!") +} +let birthdayPerson = Person(name: "Malcolm", age: 21) +wishHappyBirthday(birthdayPerson) +// 输出 "Happy birthday Malcolm - you're 21! +``` `Named`协议包含`String`类型的`name`属性；`Aged`协议包含`Int`类型的`age`属性。`Person`结构体`遵循`了这两个协议。 `wishHappyBirthday`函数的形参`celebrator`的类型为`protocol`。可以传入任意`遵循`这两个协议的类型的实例 -> 注意： -> +> 注意： `协议合成`并不会生成一个新协议类型，而是将多个协议合成为一个临时的协议，超出范围后立即失效。 @@ -494,57 +546,64 @@ * `as?`返回一个可选值，当实例`遵循`协议时，返回该协议类型；否则返回`nil` * `as`用以强制向下转换型。 -``` +```swift @objc protocol HasArea { - var area: Double { get } + var area: Double { get } } ``` -> 注意： -> +> 注意： `@objc`用来表示协议是可选的，也可以用来表示暴露给`Objective-C`的代码，此外，`@objc`型协议只对`类`有效，因此只能在`类`中检查协议的一致性。详情查看*[Using Siwft with Cocoa and Objectivei-c](https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/BuildingCocoaApps/index.html#//apple_ref/doc/uid/TP40014216)*。 - class Circle: HasArea { - let pi = 3.1415927 - var radius: Double - var area:≈radius } - init(radius: Double) { self.radius = radius } - } - class Country: HasArea { - var area: Double - init(area: Double) { self.area = area } - } +```swift +class Circle: HasArea { + let pi = 3.1415927 + var radius: Double + var area:≈radius } + init(radius: Double) { self.radius = radius } +} +class Country: HasArea { + var area: Double + init(area: Double) { self.area = area } +} +``` `Circle`和`Country`都遵循了`HasArea`协议，前者把`area`写为计算型属性（computed property），后者则把`area`写为存储型属性（stored property）。 如下所示，`Animal`类没有实现任何协议 - class Animal { - var legs: Int - init(legs: Int) { self.legs = legs } - } +```swift +class Animal { + var legs: Int + init(legs: Int) { self.legs = legs } +} +``` `Circle,Country,Animal`并没有一个相同的基类，所以采用`AnyObject`类型的数组来装载在它们的实例，如下所示： - let objects: AnyObject[] = [ - Circle(radius: 2.0), - Country(area: 243_610), - Animal(legs: 4) - ] +```swift +let objects: AnyObject[] = [ + Circle(radius: 2.0), + Country(area: 243_610), + Animal(legs: 4) +] +``` 如下所示，在迭代时检查`object`数组的元素是否`遵循`了`HasArea`协议： - for object in objects { - if let objectWithArea = object as? HasArea { - println("Area is \(objectWithArea.area)") - } else { - println("Something that doesn't have an area") - } - } - // Area is 12.5663708 - // Area is 243610.0 - // Something that doesn't have an area +```swift +for object in objects { + if let objectWithArea = object as? HasArea { + println("Area is \(objectWithArea.area)") + } else { + println("Something that doesn't have an area") + } +} +// Area is 12.5663708 +// Area is 243610.0 +// Something that doesn't have an area +``` 当数组中的元素遵循`HasArea`协议时，通过`as?`操作符将其`可选绑定(optional binding)`到`objectWithArea`常量上。 @@ -559,36 +618,38 @@ 像`someOptionalMethod?(someArgument)`一样，你可以在可选方法名称后加上`?`来检查该方法是否被实现。`可选方法`和`可选属性`都会返回一个`可选值(optional value)`，当其不可访问时，`?`之后语句不会执行，并返回`nil`。 -> 注意： -> +> 注意： 可选协议只能在含有`@objc`前缀的协议中生效。且`@objc`的协议只能被`类`遵循。 `Counter`类使用`CounterDataSource`类型的外部数据源来提供`增量值(increment amount)`，如下所示： - @objc protocol CounterDataSource { - @optional func incrementForCount(count: Int) -> Int - @optional var fixedIncrement: Int { get } - } +```swift +@objc protocol CounterDataSource { + @optional func incrementForCount(count: Int) -> Int + @optional var fixedIncrement: Int { get } +} +``` `CounterDataSource`含有`incrementForCount`的`可选方法`和`fiexdIncrement`的`可选属性`。 -> 注意： -> +> 注意： `CounterDataSource`中的属性和方法都是可选的，因此可以在类中声明但不实现这些成员，尽管技术上允许这样做，不过最好不要这样写。 `Counter`类含有`CounterDataSource?`类型的可选属性`dataSource`，如下所示： - @objc class Counter { - var count = 0 - var dataSource: CounterDataSource? - func increment() { - if let amount = dataSource?.incrementForCount?(count) { - count += amount - } else if let amount = dataSource?.fixedIncrement? { - count += amount - } - } - } +```swift +@objc class Counter { + var count = 0 + var dataSource: CounterDataSource? + func increment() { + if let amount = dataSource?.incrementForCount?(count) { + count += amount + } else if let amount = dataSource?.fixedIncrement? { + count += amount + } + } +} +``` `count`属性用于存储当前的值，`increment`方法用来为`count`赋值。 @@ -603,47 +664,55 @@ `ThreeSource`实现了`CounterDataSource`协议，如下所示： - class ThreeSource: CounterDataSource { - let fixedIncrement = 3 - } +```swift +class ThreeSource: CounterDataSource { + let fixedIncrement = 3 +} +``` 使用`ThreeSource`作为数据源开实例化一个`Counter`： - var counter = Counter() - counter.dataSource = ThreeSource() - for _ in 1...4 { - counter.increment() - println(counter.count) - } - // 3 - // 6 - // 9 - // 12 +```swift +var counter = Counter() +counter.dataSource = ThreeSource() +for _ in 1...4 { + counter.increment() + println(counter.count) +} +// 3 +// 6 +// 9 +// 12 +``` `TowardsZeroSource`实现了`CounterDataSource`协议中的`incrementForCount`方法，如下所示： - class TowardsZeroSource: CounterDataSource { - func incrementForCount(count: Int) -> Int { - if count == 0 { - return 0 - } else if count < 0 { - return 1 - } else { - return -1 - } - } - } +```swift +class TowardsZeroSource: CounterDataSource { +func incrementForCount(count: Int) -> Int { + if count == 0 { + return 0 + } else if count < 0 { + return 1 + } else { + return -1 + } + } +} +``` 下边是执行的代码： - counter.count = -4 - counter.dataSource = TowardsZeroSource() - for _ in 1...5 { - counter.increment() - println(counter.count) - } - // -3 - // -2 - // -1 - // 0 - // 0 +```swift +counter.count = -4 +counter.dataSource = TowardsZeroSource() +for _ in 1...5 { + counter.increment() + println(counter.count) +} +// -3 +// -2 +// -1 +// 0 +// 0 +```