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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>2.1.</b> 基础部分
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<i class="fa fa-check"></i> <b>2.2.</b> 基本操作符
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<i class="fa fa-check"></i> <b>2.3.</b> 字符串和字符
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<i class="fa fa-check"></i> <b>2.4.</b> 集合类型
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<i class="fa fa-check"></i> <b>2.5.</b> 控制流
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<i class="fa fa-check"></i> <b>2.6.</b> 函数
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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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<a href="../chapter2/09_Classes_and_Structures.html">
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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>2.11.</b> 方法
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</a>
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<a href="../chapter2/12_Subscripts.html">
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<i class="fa fa-check"></i> <b>2.12.</b> 下标
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</a>
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</li>
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<li class="chapter " data-level="2.13" data-path="chapter2/13_Inheritance.html">
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<a href="../chapter2/13_Inheritance.html">
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<i class="fa fa-check"></i> <b>2.13.</b> 继承
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<a href="../chapter2/14_Initialization.html">
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<i class="fa fa-check"></i> <b>2.14.</b> 构造函数
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<li class="chapter " data-level="2.15" data-path="chapter2/15_Deinitialization.html">
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<a href="../chapter2/15_Deinitialization.html">
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<i class="fa fa-check"></i> <b>2.15.</b> 析构函数
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</a>
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<a href="../chapter2/16_Automatic_Reference_Counting.html">
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<i class="fa fa-check"></i> <b>2.16.</b> 自动引用计数
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</a>
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</li>
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<li class="chapter " data-level="2.17" data-path="chapter2/17_Optional_Chaining.html">
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<a href="../chapter2/17_Optional_Chaining.html">
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<i class="fa fa-check"></i> <b>2.17.</b> 可选链
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</a>
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<li class="chapter " data-level="2.18" data-path="chapter2/18_Type_Casting.html">
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<a href="../chapter2/18_Type_Casting.html">
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<i class="fa fa-check"></i> <b>2.18.</b> 类型检查
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<li class="chapter " data-level="2.19" data-path="chapter2/19_Nested_Types.html">
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<a href="../chapter2/19_Nested_Types.html">
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<i class="fa fa-check"></i> <b>2.19.</b> 嵌套类型
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</a>
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<li class="chapter " data-level="2.20" data-path="chapter2/20_Extensions.html">
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<a href="../chapter2/20_Extensions.html">
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<i class="fa fa-check"></i> <b>2.20.</b> 扩展
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</a>
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<li class="chapter " data-level="2.21" data-path="chapter2/21_Protocols.html">
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<a href="../chapter2/21_Protocols.html">
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<i class="fa fa-check"></i> <b>2.21.</b> 接口
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</a>
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</li>
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<li class="chapter " data-level="2.22" data-path="chapter2/22_Generics.html">
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<a href="../chapter2/22_Generics.html">
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<i class="fa fa-check"></i> <b>2.22.</b> 泛型
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</a>
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</li>
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<li class="chapter " data-level="2.23" data-path="chapter2/23_Advanced_Operators.html">
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<a href="../chapter2/23_Advanced_Operators.html">
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<i class="fa fa-check"></i> <b>2.23.</b> 高级操作符
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</a>
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</li>
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</li>
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<li class="chapter " data-level="3" data-path="chapter3/chapter3.html">
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<a href="../chapter3/chapter3.html">
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<i class="fa fa-check"></i> <b>3.</b> 语言参考
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</a>
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<li class="chapter " data-level="3.1" data-path="chapter3/01_About_the_Language_Reference.html">
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<a href="../chapter3/01_About_the_Language_Reference.html">
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<i class="fa fa-check"></i> <b>3.1.</b> 关于语言参考
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</a>
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</li>
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<li class="chapter " data-level="3.2" data-path="chapter3/02_Lexical_Structure.html">
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<a href="../chapter3/02_Lexical_Structure.html">
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<i class="fa fa-check"></i> <b>3.2.</b> 词法结构
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</a>
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<li class="chapter " data-level="3.3" data-path="chapter3/03_Types.html">
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<i class="fa fa-check"></i> <b>3.3.</b> 类型
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</a>
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<li class="chapter " data-level="3.4" data-path="chapter3/04_Expressions.html">
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<a href="../chapter3/04_Expressions.html">
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<i class="fa fa-check"></i> <b>3.4.</b> 表达式
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</a>
|
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|
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<li class="chapter " data-level="3.5" data-path="chapter3/05_Declarations.html">
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<a href="../chapter3/05_Declarations.html">
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<i class="fa fa-check"></i> <b>3.5.</b> 声明
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</a>
|
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|
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|
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|
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|
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<li class="chapter " data-level="3.6" data-path="chapter3/06_Attributes.html">
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<a href="../chapter3/06_Attributes.html">
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<i class="fa fa-check"></i> <b>3.6.</b> 属性
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</a>
|
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|
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|
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</li>
|
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|
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<li class="chapter " data-level="3.7" data-path="chapter3/07_Patterns.html">
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<a href="../chapter3/07_Patterns.html">
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<i class="fa fa-check"></i> <b>3.7.</b> 模式
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</a>
|
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</li>
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|
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<li class="chapter " data-level="3.8" data-path="chapter3/08_Generic_Parameters_and_Arguments.html">
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<a href="../chapter3/08_Generic_Parameters_and_Arguments.html">
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<i class="fa fa-check"></i> <b>3.8.</b> 泛型参数
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</a>
|
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|
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</li>
|
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|
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<li class="chapter " data-level="3.9" data-path="chapter3/09_Summary_of_the_Grammar.html">
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<a href="../chapter3/09_Summary_of_the_Grammar.html">
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<i class="fa fa-check"></i> <b>3.9.</b> 语法总结
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</a>
|
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|
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</li>
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<a href="../chapter2/10_Properties.html" title="属性" class="chapter done " data-progress="2.10" style="left: 16.216216216216218%;"></a>
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<a href="../chapter2/11_Methods.html" title="方法" class="chapter done " data-progress="2.11" style="left: 18.91891891891892%;"></a>
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<a href="../chapter2/12_Subscripts.html" title="下标" class="chapter done " data-progress="2.12" style="left: 21.62162162162162%;"></a>
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<a href="../chapter2/13_Inheritance.html" title="继承" class="chapter done " data-progress="2.13" style="left: 24.324324324324323%;"></a>
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<a href="../chapter2/19_Nested_Types.html" title="嵌套类型" class="chapter " data-progress="2.19" style="left: 40.54054054054054%;"></a>
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<a href="../chapter2/03_Strings_and_Characters.html" title="字符串和字符" class="chapter " data-progress="2.3" style="left: 56.75675675675676%;"></a>
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<a href="../chapter2/04_Collection_Types.html" title="集合类型" class="chapter " data-progress="2.4" style="left: 59.45945945945946%;"></a>
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<a href="../chapter2/05_Control_Flow.html" title="控制流" class="chapter " data-progress="2.5" style="left: 62.16216216216216%;"></a>
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<a href="../chapter2/06_Functions.html" title="函数" class="chapter " data-progress="2.6" style="left: 64.86486486486487%;"></a>
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<a href="../chapter3/02_Lexical_Structure.html" title="词法结构" class="chapter " data-progress="3.2" style="left: 81.08108108108108%;"></a>
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<a href="../chapter3/03_Types.html" title="类型" class="chapter " data-progress="3.3" style="left: 83.78378378378379%;"></a>
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<a href="../chapter3/04_Expressions.html" title="表达式" class="chapter " data-progress="3.4" style="left: 86.48648648648648%;"></a>
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<a href="../chapter3/05_Declarations.html" title="声明" class="chapter " data-progress="3.5" style="left: 89.1891891891892%;"></a>
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<a href="../chapter3/06_Attributes.html" title="属性" class="chapter " data-progress="3.6" style="left: 91.89189189189189%;"></a>
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<a href="../chapter3/07_Patterns.html" title="模式" class="chapter " data-progress="3.7" style="left: 94.5945945945946%;"></a>
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<a href="../chapter3/08_Generic_Parameters_and_Arguments.html" title="泛型参数" class="chapter " data-progress="3.8" style="left: 97.29729729729729%;"></a>
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<a href="../chapter3/09_Summary_of_the_Grammar.html" title="语法总结" class="chapter " data-progress="3.9" style="left: 100%;"></a>
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</div>
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</div>
|
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|
||
<div class="page-inner">
|
||
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||
<section class="normal" id="section-gitbook_23">
|
||
|
||
<h1 id="-">自动引用计数</h1>
|
||
<p>本页包含内容：</p>
|
||
<ul>
|
||
<li>自动引用计数的工作机制</li>
|
||
<li>自动引用计数实践</li>
|
||
<li>类实例间的强引用环</li>
|
||
<li>类实例间的强引用环分解</li>
|
||
<li>闭包的强引用环</li>
|
||
<li>闭包的强引用环分解</li>
|
||
</ul>
|
||
<p>Swift使用自动引用计数(ARC)这一机制来跟踪和管理你的应用程序的内存。通常情况下，Swift的内存管理机制会一直起着作用，你无须自己来考虑内存的管理。ARC会在类的实例不再被使用时，自动释放其占用的内存。</p>
|
||
<p>然而，在少数情况下，ARC为了能帮助你管理内存，需要更多的关于你的代码之间关系的信息。本章描述了这些情况，并且为你示范怎样启用ARC来管理你的应用程序的内存。</p>
|
||
<blockquote>
|
||
<p>注意: 引用计数仅仅应用于类的实例。结构体和枚举类型是值类型，不是引用类型，也不是通过引用的方式存储和传递。</p>
|
||
</blockquote>
|
||
<h2 id="-">自动引用计数的工作机制</h2>
|
||
<p>当你每次创建一个类的新的实例的时候，ARC会分配一大块内存用来储存实例的信息。内存中会包含实例的类型信息，以及这个实例所有相关属性的值。此外，当实例不再被使用时，ARC释放实例所占用的内存，并让释放的内存能挪作他用。这确保了不再被使用的实例，不会一直占用内存空间。</p>
|
||
<p>然而，当ARC收回和释放了正在被使用中的实例，该实例的属性和方法将不能再被访问和调用。实际上，如果你试图访问这个实例，你的应用程序很可能会崩溃。</p>
|
||
<p>为了确保使用中的实例不会被回收，ARC会跟踪和计算每一个类实例正在被多少属性，常量和变量所引用。哪怕实例的引用数为一，ARC都不会回收这个实例。</p>
|
||
<p>为了使之成为可能，无论你将类实例赋值给属性，常量或者是变量，属性，常量或者变量，都会对此实例创建强引用。之所以称之为强引用，是因为它会将实例牢牢的保持住，只要强引用还在，实例是不允许被释放的。</p>
|
||
<h2 id="-">自动引用计数实践</h2>
|
||
|
||
|
||
</section>
|
||
|
||
</div>
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</div>
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