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<a href="../" >The Swift Programming Language 中文版</a>
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<i class="fa fa-check"></i> <b>1.</b> 欢迎使用 Swift
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<i class="fa fa-check"></i> <b>2.2.</b> 基本运算符
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<i class="fa fa-check"></i> <b>2.7.</b> 闭包
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<i class="fa fa-check"></i> <b>2.8.</b> 枚举
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<i class="fa fa-check"></i> <b>2.9.</b> 类和结构体
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<i class="fa fa-check"></i> <b>2.10.</b> 属性
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<i class="fa fa-check"></i> <b>2.16.</b> 自动引用计数
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<i class="fa fa-check"></i> <b>2.17.</b> 可选链
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<i class="fa fa-check"></i> <b>2.18.</b> 类型检查
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<i class="fa fa-check"></i> <b>2.19.</b> 嵌套类型
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<i class="fa fa-check"></i> <b>2.20.</b> 扩展
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<i class="fa fa-check"></i> <b>2.21.</b> 协议
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<i class="fa fa-check"></i> <b>2.22.</b> 泛型
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<i class="fa fa-check"></i> <b>2.23.</b> 高级操作符
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</div>
</div>
<div class="page-inner">
<section class="normal" id="section-gitbook_129">
<blockquote>
<p>翻译：<a href="https://github.com/geek5nan" target="_blank">geek5nan</a><br>校对：<a href="https://github.com/dabing1022" target="_blank">dabing1022</a></p>
</blockquote>
<h1 id="-">协议</h1>
<hr>
<p>本页包含内容：</p>
<ul>
<li><a href="#protocol_syntax">协议的语法（Protocol Syntax）</a></li>
<li><a href="#property_requirements">属性要求（Property Requirements）</a></li>
<li><a href="#method_requirements">方法要求（Method Requirements）</a></li>
<li><a href="#mutating_method_requirements">突变方法要求（Mutating Method Requirements）</a></li>
<li><a href="#protocols_as_types">协议类型（Protocols as Types）</a></li>
<li><a href="#delegation">委托(代理)模式（Delegation）</a></li>
<li><a href="#adding_protocol_conformance_with_an_extension">在扩展中添加协议成员（Adding Protocol Conformance with an Extension）</a></li>
<li><a href="#declaring_protocol_adoption_with_an_extension">通过扩展补充协议声明（Declaring Protocol Adoption with an Extension）</a></li>
<li><a href="#collections_of_protocol_types">集合中的协议类型（Collections of Protocol Types）</a></li>
<li><a href="#protocol_inheritance">协议的继承（Protocol Inheritance）</a></li>
<li><a href="#protocol_composition">协议合成（Protocol Composition）</a></li>
<li><a href="#checking_for_protocol_conformance">检验协议的一致性（Checking for Protocol Conformance）</a></li>
<li><a href="#optional_protocol_requirements">可选协议要求（Optional Protocol Requirements）</a></li>
</ul>
<p><code>Protocol(协议)</code>用于<strong>统一</strong>方法和属性的名称，而不实现任何功能。<code>协议</code>能够被类，枚举，结构体实现，满足协议要求的类，枚举，结构体被称为协议的<code>遵循者</code>。</p>
<p><code>遵循者</code>需要提供<code>协议</code>指定的成员，如属性，方法，操作符，下标等。</p>
<p><a name="protocol_syntax"></a></p>
<h2 id="-">协议的语法</h2>
<p><code>协议</code>的定义与类，结构体，枚举的定义非常相似，如下所示：</p>
<pre><code class="lang-swift">protocol SomeProtocol {
// 协议内容
}
</code></pre>
<p>在类，结构体，枚举的名称后加上<code>协议名称</code>，中间以冒号<code>:</code>分隔即可实现协议；实现多个协议时，各协议之间用逗号<code>,</code>分隔，如下所示：</p>
<pre><code class="lang-swift">struct SomeStructure: FirstProtocol, AnotherProtocol {
// 结构体内容
}
</code></pre>
<p>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：</p>
<pre><code class="lang-swift">class SomeClass: SomeSuperClass, FirstProtocol, AnotherProtocol {
// 类的内容
}
</code></pre>
<p><a name="property_requirements"></a></p>
<h2 id="-">属性要求</h2>
<p><code>协议</code>能够要求其<code>遵循者</code>必须含有一些<strong>特定名称和类型</strong>的<code>实例属性(instance property)</code>或<code>类属性 (type property)</code>，也能够要求属性具有<code>(设置权限)settable</code> 和<code>(访问权限)gettable</code>，但它不要求<code>属性</code>是<code>存储型属性(stored property)</code>还是<code>计算型属性(calculate property)</code>。</p>
<p>如果协议要求属性具有设置权限和访问权限，那常量存储型属性或者只读计算型属性都无法满足此要求。如果协议只要求属性具有访问权限，那任何类型的属性都可以满足此要求，无论这些属性是否具有设置权限。</p>
<p>通常前置<code>var</code>关键字将属性声明为变量。在属性声明后写上<code>{ get set }</code>表示属性为可读写的。<code>{ get }</code>用来表示属性为可读的。即使你为可读的属性实现了<code>setter</code>方法，它也不会出错。</p>
<pre><code class="lang-swift">protocol SomeProtocol {
var musBeSettable : Int { get set }
var doesNotNeedToBeSettable: Int { get }
}
</code></pre>
<p>用类来实现协议时，使用<code>class</code>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<code>static</code>关键字来表示：</p>
<pre><code class="lang-swift">protocol AnotherProtocol {
class var someTypeProperty: Int { get set }
}
protocol FullyNamed {
var fullName: String { get }
}
</code></pre>
<p><code>FullyNamed</code>协议含有<code>fullName</code>属性。因此其<code>遵循者</code>必须含有一个名为<code>fullName</code>，类型为<code>String</code>的可读属性。</p>
<pre><code class="lang-swift">struct Person: FullyNamed{
var fullName: String
}
let john = Person(fullName: "John Appleseed")
//john.fullName 为 "John Appleseed"
</code></pre>
<p><code>Person</code>结构体含有一个名为<code>fullName</code>的<code>存储型属性</code>，完整的<code>遵循</code>了协议。(<em>若协议未被完整遵循，编译时则会报错</em>)。</p>
<p>如下所示，<code>Startship</code>类<code>遵循</code>了<code>FullyNamed</code>协议：</p>
<pre><code class="lang-swift">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
}
}
var ncc1701 = Starship(name: "Enterprise", prefix: "USS")
// ncc1701.fullName == "USS Enterprise"
</code></pre>
<p><code>Starship</code>类将<code>fullName</code>实现为可读的<code>计算型属性</code>。它的每一个实例都有一个名为<code>name</code>的必备属性和一个名为<code>prefix</code>的可选属性。 当<code>prefix</code>存在时，将<code>prefix</code>插入到<code>name</code>之前来为<code>Starship</code>构建<code>fullName</code>。</p>
<p><a name="method_requirements"></a></p>
<h2 id="-">方法要求</h2>
<p><code>协议</code>能够要求其<code>遵循者</code>必备某些特定的<code>实例方法</code>和<code>类方法</code>。协议方法的声明与普通方法声明相似，但它不需要<code>方法</code>内容。</p>
<blockquote>
<p>注意：
协议方法支持<code>变长参数(variadic parameter)</code>，不支持<code>默认参数(default parameter)</code>。</p>
</blockquote>
<p>前置<code>class</code>关键字表示协议中的成员为<code>类成员</code>；当协议用于被<code>枚举</code>或<code>结构体</code>遵循时，则使用<code>static</code>关键字。如下所示：</p>
<pre><code class="lang-swift">protocol SomeProtocol {
class func someTypeMethod()
}
protocol RandomNumberGenerator {
func random() -> Double
}
</code></pre>
<p><code>RandomNumberGenerator</code>协议要求其<code>遵循者</code>必须拥有一个名为<code>random</code>， 返回值类型为<code>Double</code>的实例方法。(我们假设随机数在[0，1]区间内)。</p>
<p><code>LinearCongruentialGenerator</code>类<code>遵循</code>了<code>RandomNumberGenerator</code>协议，并提供了一个叫做<em>线性同余生成器(linear congruential generator)</em>的伪随机数算法。</p>
<pre><code class="lang-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"
</code></pre>
<p><a name="mutating_method_requirements"></a></p>
<h2 id="-">突变方法要求</h2>
<p>能在<code>方法</code>或<code>函数</code>内部改变实例类型的方法称为<code>突变方法</code>。在<code>值类型(Value Type)</code>(<em>译者注：特指结构体和枚举</em>)中的的<code>函数</code>前缀加上<code>mutating</code>关键字来表示该函数允许改变该实例和其属性的类型。 这一变换过程在<a href="11_Methods.html#instance_methods">实例方法(Instance Methods)</a>章节中有详细描述。</p>
<p>(<em>译者注：类中的成员为<code>引用类型(Reference Type)</code>，可以方便的修改实例及其属性的值而无需改变类型；而<code>结构体</code>和<code>枚举</code>中的成员均为<code>值类型(Value Type)</code>，修改变量的值就相当于修改变量的类型，而<code>Swift</code>默认不允许修改类型，因此需要前置<code>mutating</code>关键字用来表示该<code>函数</code>中能够修改类型</em>)</p>
<blockquote>
<p>注意：
用<code>class</code>实现协议中的<code>mutating</code>方法时，不用写<code>mutating</code>关键字；用<code>结构体</code>，<code>枚举</code>实现协议中的<code>mutating</code>方法时，必须写<code>mutating</code>关键字。</p>
</blockquote>
<p>如下所示，<code>Togglable</code>协议含有<code>toggle</code>函数。根据函数名称推测，<code>toggle</code>可能用于<strong>切换或恢复</strong>某个属性的状态。<code>mutating</code>关键字表示它为<code>突变方法</code>：</p>
<pre><code class="lang-swift">protocol Togglable {
mutating func toggle()
}
</code></pre>
<p>当使用<code>枚举</code>或<code>结构体</code>来实现<code>Togglabl</code>协议时，必须在<code>toggle</code>方法前加上<code>mutating</code>关键字。</p>
<p>如下所示，<code>OnOffSwitch</code>枚举<code>遵循</code>了<code>Togglable</code>协议，<code>On</code>，<code>Off</code>两个成员用于表示当前状态</p>
<pre><code class="lang-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
</code></pre>
<p><a name="protocols_as_types"></a></p>
<h2 id="-">协议类型</h2>
<p><code>协议</code>本身不实现任何功能，但你可以将它当做<code>类型</code>来使用。</p>
<p>使用场景：</p>
<ul>
<li>作为函数，方法或构造器中的参数类型，返回值类型</li>
<li>作为常量，变量，属性的类型</li>
<li>作为数组，字典或其他容器中的元素类型</li>
</ul>
<blockquote>
<p>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</p>
</blockquote>
<pre><code class="lang-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
}
}
</code></pre>
<p>这里定义了一个名为 <code>Dice</code>的类，用来代表桌游中的N个面的骰子。</p>
<p><code>Dice</code>含有<code>sides</code>和<code>generator</code>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<code>RandomNumberGenerator</code>的协议类型。所以它能够被赋值为任意<code>遵循</code>该协议的类型。</p>
<p>此外，使用<code>构造器(init)</code>来代替之前版本中的<code>setup</code>操作。构造器中含有一个名为<code>generator</code>，类型为<code>RandomNumberGenerator</code>的形参，使得它可以接收任意遵循<code>RandomNumberGenerator</code>协议的类型。</p>
<p><code>roll</code>方法用来模拟骰子的面值。它先使用<code>generator</code>的<code>random</code>方法来创建一个[0-1]区间内的随机数种子，然后加工这个随机数种子生成骰子的面值。</p>
<p>如下所示，<code>LinearCongruentialGenerator</code>的实例作为随机数生成器传入<code>Dice</code>的<code>构造器</code></p>
<pre><code class="lang-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
</code></pre>
<p><a name="delegation"></a></p>
<h2 id="-">委托(代理)模式</h2>
<p>委托是一种设计模式，它允许类或结构体将一些需要它们负责的功能<code>交由(委托)</code>给其他的类型。</p>
<p>委托模式的实现很简单： 定义<code>协议</code>来<code>封装</code>那些需要被委托的<code>函数和方法</code>， 使其<code>遵循者</code>拥有这些被委托的<code>函数和方法</code>。</p>
<p>委托模式可以用来响应特定的动作或接收外部数据源提供的数据，而无需要知道外部数据源的类型。</p>
<p>下文是两个基于骰子游戏的协议：</p>
<pre><code class="lang-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)
}
</code></pre>
<p><code>DiceGame</code>协议可以在任意含有骰子的游戏中实现，<code>DiceGameDelegate</code>协议可以用来追踪<code>DiceGame</code>的游戏过程。</p>
<p>如下所示，<code>SnakesAndLadders</code>是<code>Snakes and Ladders</code>(译者注：<a href="05_Control_Flow.html">控制流</a>章节有该游戏的详细介绍)游戏的新版本。新版本使用<code>Dice</code>作为骰子，并且实现了<code>DiceGame</code>和<code>DiceGameDelegate</code>协议</p>
<pre><code class="lang-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)
}
}
</code></pre>
<p>游戏的<code>初始化设置(setup)</code>被<code>SnakesAndLadders</code>类的<code>构造器(initializer)</code>实现。所有的游戏逻辑被转移到了<code>play</code>方法中。</p>
<blockquote>
<p>注意：
因为<code>delegate</code>并不是该游戏的必备条件，<code>delegate</code>被定义为遵循<code>DiceGameDelegate</code>协议的可选属性</p>
</blockquote>
<p><code>DicegameDelegate</code>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<code>play()</code>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。</p>
<p>因为<code>delegate</code>是一个遵循<code>DiceGameDelegate</code>的可选属性，因此在<code>play()</code>方法中使用了<code>可选链</code>来调用委托方法。 若<code>delegate</code>属性为<code>nil</code>， 则委托调用<em>优雅地</em>失效。若<code>delegate</code>不为<code>nil</code>，则委托方法被调用</p>
<p>如下所示，<code>DiceGameTracker</code>遵循了<code>DiceGameDelegate</code>协议</p>
<pre><code class="lang-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")
}
}
</code></pre>
<p><code>DiceGameTracker</code>实现了<code>DiceGameDelegate</code>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，<code>numberOfTurns</code>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。</p>
<p><code>gameDidStart</code>方法从<code>game</code>参数获取游戏信息并输出。<code>game</code>在方法中被当做<code>DiceGame</code>类型而不是<code>SnakeAndLadders</code>类型，所以方法中只能访问<code>DiceGame</code>协议中的成员。</p>
<p><code>DiceGameTracker</code>的运行情况，如下所示：</p>
<pre><code class="lang-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
</code></pre>
<p><a name="adding_protocol_conformance_with_an_extension"></a></p>
<h2 id="-">在扩展中添加协议成员</h2>
<p>即便无法修改源代码，依然可以通过<code>扩展(Extension)</code>来扩充已存在类型(<em>译者注： 类，结构体，枚举等</em>)。<code>扩展</code>可以为已存在的类型添加<code>属性</code>，<code>方法</code>，<code>下标</code>，<code>协议</code>等成员。详情请在<a href="20_Extensions.html">扩展</a>章节中查看。</p>
<blockquote>
<p>注意：
通过<code>扩展</code>为已存在的类型<code>遵循</code>协议时，该类型的所有实例也会随之添加协议中的方法</p>
</blockquote>
<p><code>TextRepresentable</code>协议含有一个<code>asText</code>，如下所示：</p>
<pre><code class="lang-swift">protocol TextRepresentable {
func asText() -> String
}
</code></pre>
<p>通过<code>扩展</code>为上一节中提到的<code>Dice</code>类遵循<code>TextRepresentable</code>协议</p>
<pre><code class="lang-swift">extension Dice: TextRepresentable {
cun asText() -> String {
return "A \(sides)-sided dice"
}
}
</code></pre>
<p>从现在起，<code>Dice</code>类型的实例可被当作<code>TextRepresentable</code>类型：</p>
<pre><code class="lang-swift">let d12 = Dice(sides: 12,generator: LinearCongruentialGenerator())
println(d12.asText())
// 输出 "A 12-sided dice"
</code></pre>
<p><code>SnakesAndLadders</code>类也可以通过<code>扩展</code>的方式来遵循协议：</p>
<pre><code class="lang-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"
</code></pre>
<p><a name="declaring_protocol_adoption_with_an_extension"></a></p>
<h2 id="-">通过扩展补充协议声明</h2>
<p>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<code>扩展</code>来补充协议声明：</p>
<pre><code class="lang-swift">struct Hamster {
var name: String
func asText() -> String {
return "A hamster named \(name)"
}
}
extension Hamster: TextRepresentabl {}
</code></pre>
<p>从现在起，<code>Hamster</code>的实例可以作为<code>TextRepresentable</code>类型使用</p>
<pre><code class="lang-swift">let simonTheHamster = Hamster(name: "Simon")
let somethingTextRepresentable: TextRepresentabl = simonTheHamester
println(somethingTextRepresentable.asText())
// 输出 "A hamster named Simon"
</code></pre>
<blockquote>
<p>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</p>
</blockquote>
<p><a name="collections_of_protocol_types"></a></p>
<h2 id="-">集合中的协议类型</h2>
<p>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：</p>
<pre><code class="lang-swift">let things: TextRepresentable[] = [game,d12,simoTheHamster]
</code></pre>
<p>如下所示，<code>things</code>数组可以被直接遍历，并调用其中元素的<code>asText()</code>函数：</p>
<pre><code class="lang-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
</code></pre>
<p><code>thing</code>被当做是<code>TextRepresentable</code>类型而不是<code>Dice</code>，<code>DiceGame</code>，<code>Hamster</code>等类型。因此能且仅能调用<code>asText</code>方法</p>
<p><a name="protocol_inheritance"></a></p>
<h2 id="-">协议的继承</h2>
<p>协议能够<em>继承</em>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<code>,</code>分隔</p>
<pre><code class="lang-swift">protocol InheritingProtocol: SomeProtocol, AnotherProtocol {
// 协议定义
}
</code></pre>
<p>如下所示，<code>PrettyTextRepresentable</code>协议继承了<code>TextRepresentable</code>协议</p>
<pre><code class="lang-swift">protocol PrettyTextRepresentable: TextRepresentable {
func asPrettyText() -> String
}
</code></pre>
<p><code>遵循``PrettyTextRepresentable</code>协议的同时，也需要<code>遵循</code>TextRepresentable`协议。</p>
<p>如下所示，用<code>扩展</code>为<code>SnakesAndLadders</code>遵循<code>PrettyTextRepresentable</code>协议：</p>
<pre><code class="lang-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
}
}
</code></pre>
<p>在<code>for in</code>中迭代出了<code>board</code>数组中的每一个元素：</p>
<ul>
<li>当从数组中迭代出的元素的值大于0时，用<code>▲</code>表示</li>
<li>当从数组中迭代出的元素的值小于0时，用<code>▼</code>表示</li>
<li>当从数组中迭代出的元素的值等于0时，用<code>○</code>表示</li>
</ul>
<p>任意<code>SankesAndLadders</code>的实例都可以使用<code>asPrettyText()</code>方法。</p>
<pre><code class="lang-swift">println(game.asPrettyText())
// A game of Snakes and Ladders with 25 squares:
// ○ ○ ▲ ○ ○ ▲ ○ ○ ▲ ▲ ○ ○ ○ ▼ ○ ○ ○ ○ ▼ ○ ○ ▼ ○ ▼ ○
</code></pre>
<p><a name="protocol_composition"></a></p>
<h2 id="-">协议合成</h2>
<p>一个协议可由多个协议采用<code>protocol<SomeProtocol, AnotherProtocol></code>这样的格式进行组合，称为<code>协议合成(protocol composition)</code>。</p>
<p>举个例子：</p>
<pre><code class="lang-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<Named, Aged>) {
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!
</code></pre>
<p><code>Named</code>协议包含<code>String</code>类型的<code>name</code>属性；<code>Aged</code>协议包含<code>Int</code>类型的<code>age</code>属性。<code>Person</code>结构体<code>遵循</code>了这两个协议。</p>
<p><code>wishHappyBirthday</code>函数的形参<code>celebrator</code>的类型为<code>protocol<Named,Aged></code>。可以传入任意<code>遵循</code>这两个协议的类型的实例</p>
<blockquote>
<p>注意：
<code>协议合成</code>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。</p>
</blockquote>
<p><a name="checking_for_protocol_conformance"></a></p>
<h2 id="-">检验协议的一致性</h2>
<p>使用<code>is</code>检验协议一致性，使用<code>as</code>将协议类型<code>向下转换(downcast)</code>为的其他协议类型。检验与转换的语法和之前相同(<em>详情查看<a href="18_Type_Casting.html">类型检查</a></em>)：</p>
<ul>
<li><code>is</code>操作符用来检查实例是否<code>遵循</code>了某个<code>协议</code>。</li>
<li><code>as?</code>返回一个可选值，当实例<code>遵循</code>协议时，返回该协议类型；否则返回<code>nil</code></li>
<li><code>as</code>用以强制向下转换型。</li>