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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>1.2.</b> Swift 初见
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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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<section class="normal" id="section-gitbook_117">
<blockquote>
<p>翻译：<a href="https://github.com/bruce0505" target="_blank">bruce0505</a><br>校对：<a href="https://github.com/fd5788" target="_blank">fd5788</a></p>
</blockquote>
<h1 id="-deinitialization-">析构过程（Deinitialization）</h1>
<hr>
<p>本页包含内容：</p>
<ul>
<li><a href="#how_deinitialization_works">析构过程原理</a></li>
<li><a href="#deinitializers_in_action">析构函数操作</a></li>
</ul>
<p>在一个类的实例被释放之前，析构函数被立即调用。用关键字<code>deinit</code>来标示析构函数，类似于初始化函数用<code>init</code>来标示。析构函数只适用于类类型。</p>
<p><a name="how_deinitialization_works"></a></p>
<h2 id="-">析构过程原理</h2>
<p>Swift 会自动释放不再需要的实例以释放资源。如<a href="16_Automatic_Reference_Counting.html">自动引用计数</a>那一章描述，Swift 通过<em>自动引用计数</em>（ARC）处理实例的内存管理。通常当你的实例被释放时不需要手动地去清理。但是，当使用自己的资源时，你可能需要进行一些额外的清理。例如，如果创建了一个自定义的类来打开一个文件，并写入一些数据，你可能需要在类实例被释放之前关闭该文件。</p>
<p>在类的定义中，每个类最多只能有一个析构函数。析构函数不带任何参数，在写法上不带括号：</p>
<pre><code class="lang-swift">deinit {
// 执行析构过程
}
</code></pre>
<p>析构函数是在实例释放发生前一步被自动调用。不允许主动调用自己的析构函数。子类继承了父类的析构函数，并且在子类析构函数实现的最后，父类的析构函数被自动调用。即使子类没有提供自己的析构函数，父类的析构函数也总是被调用。</p>
<p>因为直到实例的析构函数被调用时，实例才会被释放，所以析构函数可以访问所有请求实例的属性，并且根据那些属性可以修改它的行为（比如查找一个需要被关闭的文件的名称）。</p>
<p><a name="deinitializers_in_action"></a></p>
<h2 id="-">析构函数操作</h2>
<p>这里是一个析构函数操作的例子。这个例子是一个简单的游戏，定义了两种新类型，<code>Bank</code>和<code>Player</code>。<code>Bank</code>结构体管理一个虚拟货币的流通，在这个流通中<code>Bank</code>永远不可能拥有超过 10,000 的硬币。在这个游戏中有且只能有一个<code>Bank</code>存在，因此<code>Bank</code>由带有静态属性和静态方法的结构体实现，从而存储和管理其当前的状态。</p>
<pre><code class="lang-swift">struct Bank {
static var coinsInBank = 10_000
static func vendCoins(var numberOfCoinsToVend: Int) -> Int {
numberOfCoinsToVend = min(numberOfCoinsToVend, coinsInBank)
coinsInBank -= numberOfCoinsToVend
return numberOfCoinsToVend
}
static func receiveCoins(coins: Int) {
coinsInBank += coins
}
}
</code></pre>
<p><code>Bank</code>根据它的<code>coinsInBank</code>属性来跟踪当前它拥有的硬币数量。银行还提供两个方法——<code>vendCoins</code>和<code>receiveCoins</code>——用来处理硬币的分发和收集。</p>
<p><code>vendCoins</code>方法在 bank 分发硬币之前检查是否有足够的硬币。如果没有足够多的硬币，<code>Bank</code>返回一个比请求时小的数字(如果没有硬币留在 bank 中就返回 0)。<code>vendCoins</code>方法声明<code>numberOfCoinsToVend</code>为一个变量参数，这样就可以在方法体的内部修改数字，而不需要定义一个新的变量。<code>vendCoins</code>方法返回一个整型值，表明了提供的硬币的实际数目。</p>
<p><code>receiveCoins</code>方法只是将 bank 的硬币存储和接收到的硬币数目相加，再保存回 bank。</p>
<p><code>Player</code>类描述了游戏中的一个玩家。每一个 player 在任何时刻都有一定数量的硬币存储在他们的钱包中。这通过 player 的<code>coinsInPurse</code>属性来体现：</p>
<pre><code class="lang-swift">class Player {
var coinsInPurse: Int
init(coins: Int) {
coinsInPurse = Bank.vendCoins(coins)
}
func winCoins(coins: Int) {
coinsInPurse += Bank.vendCoins(coins)
}
deinit {
Bank.receiveCoins(coinsInPurse)
}
}
</code></pre>
<p>每个<code>Player</code>实例都由一个指定数目硬币组成的启动额度初始化，这些硬币在 bank 初始化的过程中得到。如果没有足够的硬币可用，<code>Player</code>实例可能收到比指定数目少的硬币。</p>
<p><code>Player</code>类定义了一个<code>winCoins</code>方法，该方法从银行获取一定数量的硬币，并把它们添加到玩家的钱包。<code>Player</code>类还实现了一个析构函数，这个析构函数在<code>Player</code>实例释放前一步被调用。这里析构函数只是将玩家的所有硬币都返回给银行：</p>
<pre><code class="lang-swift">var playerOne: Player? = Player(coins: 100)
println("A new player has joined the game with \(playerOne!.coinsInPurse) coins")
// 输出 "A new player has joined the game with 100 coins"
println("There are now \(Bank.coinsInBank) coins left in the bank")
// 输出 "There are now 9900 coins left in the bank"
</code></pre>
<p>一个新的<code>Player</code>实例随着一个 100 个硬币（如果有）的请求而被创建。这<code>个Player</code>实例存储在一个名为<code>playerOne</code>的可选<code>Player</code>变量中。这里使用一个可选变量，是因为玩家可以随时离开游戏。设置为可选使得你可以跟踪当前是否有玩家在游戏中。</p>
<p>因为<code>playerOne</code>是可选的，所以由一个感叹号（<code>!</code>）来修饰，每当其<code>winCoins</code>方法被调用时，<code>coinsInPurse</code>属性被访问并打印出它的默认硬币数目。</p>
<pre><code class="lang-swift">playerOne!.winCoins(2_000)
println("PlayerOne won 2000 coins & now has \ (playerOne!.coinsInPurse) coins")
// 输出 "PlayerOne won 2000 coins & now has 2100 coins"
println("The bank now only has \(Bank.coinsInBank) coins left")
// 输出 "The bank now only has 7900 coins left"
</code></pre>
<p>这里，player 已经赢得了 2,000 硬币。player 的钱包现在有 2,100 硬币，bank 只剩余 7,900 硬币。</p>
<pre><code class="lang-swift">playerOne = nil
println("PlayerOne has left the game")
// 输出 "PlayerOne has left the game"
println("The bank now has \(Bank.coinsInBank) coins")
// 输出 "The bank now has 10000 coins"
</code></pre>
<p>玩家现在已经离开了游戏。这表明是要将可选的<code>playerOne</code>变量设置为<code>nil</code>，意思是“没有<code>Player</code>实例”。当这种情况发生的时候，<code>playerOne</code>变量对<code>Player</code>实例的引用被破坏了。没有其它属性或者变量引用<code>Player</code>实例，因此为了清空它占用的内存从而释放它。在这发生前一步，其析构函数被自动调用，其硬币被返回到银行。</p>
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