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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>1.1.</b> 关于 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.</b> Swift 教程
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<a href="../chapter2/01_The_Basics.html">
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<i class="fa fa-check"></i> <b>2.1.</b> 基础部分
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<a href="../chapter2/02_Basic_Operators.html">
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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>2.3.</b> 字符串和字符
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</a>
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<li class="chapter " data-level="2.4" data-path="chapter2/04_Collection_Types.html">
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<a href="../chapter2/04_Collection_Types.html">
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<i class="fa fa-check"></i> <b>2.4.</b> 集合类型
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</a>
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<li class="chapter " data-level="2.5" data-path="chapter2/05_Control_Flow.html">
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<a href="../chapter2/05_Control_Flow.html">
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<i class="fa fa-check"></i> <b>2.5.</b> 控制流
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</a>
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<li class="chapter " data-level="2.6" data-path="chapter2/06_Functions.html">
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<a href="../chapter2/06_Functions.html">
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<i class="fa fa-check"></i> <b>2.6.</b> 函数
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</a>
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<li class="chapter " data-level="2.7" data-path="chapter2/07_Closures.html">
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<a href="../chapter2/07_Closures.html">
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<i class="fa fa-check"></i> <b>2.7.</b> 闭包
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</a>
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<li class="chapter " data-level="2.8" data-path="chapter2/08_Enumerations.html">
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<a href="../chapter2/08_Enumerations.html">
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<i class="fa fa-check"></i> <b>2.8.</b> 枚举
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</a>
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<li class="chapter " data-level="2.9" data-path="chapter2/09_Classes_and_Structures.html">
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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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</a>
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</li>
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<li class="chapter " data-level="2.10" data-path="chapter2/10_Properties.html">
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<a href="../chapter2/10_Properties.html">
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<i class="fa fa-check"></i> <b>2.10.</b> 属性
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</a>
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</li>
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<li class="chapter " data-level="2.11" data-path="chapter2/11_Methods.html">
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<a href="../chapter2/11_Methods.html">
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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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</li>
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<li class="chapter " data-level="2.12" data-path="chapter2/12_Subscripts.html">
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<a href="../chapter2/12_Subscripts.html">
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<i class="fa fa-check"></i> <b>2.12.</b> 附属脚本
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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>
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</li>
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<li class="chapter " data-level="2.14" data-path="chapter2/14_Initialization.html">
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<a href="../chapter2/14_Initialization.html">
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<i class="fa fa-check"></i> <b>2.14.</b> 构造过程
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</a>
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</li>
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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> 析构过程
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</a>
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</li>
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<li class="chapter " data-level="2.16" data-path="chapter2/16_Automatic_Reference_Counting.html">
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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
|
||
</a>
|
||
|
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|
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</li>
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|
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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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</a>
|
||
|
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</li>
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|
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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> 类型嵌套
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||
</a>
|
||
|
||
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||
</li>
|
||
|
||
<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>
|
||
|
||
|
||
</li>
|
||
|
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<li class="chapter " data-level="2.21" data-path="chapter2/21_Protocols.html">
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|
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<a href="../chapter2/21_Protocols.html">
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<i class="fa fa-check"></i> <b>2.21.</b> 协议
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||
</a>
|
||
|
||
|
||
</li>
|
||
|
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<li class="chapter " data-level="2.22" data-path="chapter2/22_Generics.html">
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|
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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
|
||
</a>
|
||
|
||
|
||
</li>
|
||
|
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<li class="chapter " data-level="2.23" data-path="chapter2/23_Advanced_Operators.html">
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|
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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
|
||
</a>
|
||
|
||
|
||
</li>
|
||
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</ul>
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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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<ul class="articles">
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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>
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<li class="chapter " data-level="3.3" data-path="chapter3/03_Types.html">
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<a href="../chapter3/03_Types.html">
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<i class="fa fa-check"></i> <b>3.3.</b> 类型
|
||
</a>
|
||
|
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</li>
|
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|
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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
|
||
</a>
|
||
|
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</li>
|
||
|
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<li class="chapter " data-level="3.5" data-path="chapter3/10_Statements.html">
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<a href="../chapter3/10_Statements.html">
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<i class="fa fa-check"></i> <b>3.5.</b> 语句
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||
</a>
|
||
|
||
|
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</li>
|
||
|
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<li class="chapter " data-level="3.6" data-path="chapter3/05_Declarations.html">
|
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|
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<a href="../chapter3/05_Declarations.html">
|
||
<i class="fa fa-check"></i> <b>3.6.</b> 声明
|
||
</a>
|
||
|
||
|
||
</li>
|
||
|
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<li class="chapter " data-level="3.7" 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>3.7.</b> 特性
|
||
</a>
|
||
|
||
|
||
</li>
|
||
|
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<li class="chapter " data-level="3.8" data-path="chapter3/07_Patterns.html">
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|
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<a href="../chapter3/07_Patterns.html">
|
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<i class="fa fa-check"></i> <b>3.8.</b> 模式
|
||
</a>
|
||
|
||
|
||
</li>
|
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|
||
<li class="chapter " data-level="3.9" data-path="chapter3/08_Generic_Parameters_and_Arguments.html">
|
||
|
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<a href="../chapter3/08_Generic_Parameters_and_Arguments.html">
|
||
<i class="fa fa-check"></i> <b>3.9.</b> 泛型参数
|
||
</a>
|
||
|
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|
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</li>
|
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|
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<li class="chapter " data-level="3.10" data-path="chapter3/09_Summary_of_the_Grammar.html">
|
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|
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<a href="../chapter3/09_Summary_of_the_Grammar.html">
|
||
<i class="fa fa-check"></i> <b>3.10.</b> 语法总结
|
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</a>
|
||
|
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|
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</li>
|
||
|
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|
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</ul>
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|
||
<a href="../chapter1/02_a_swift_tour.html" title="Swift 初见" class="chapter done " data-progress="1.2" style="left: 7.894736842105263%;"></a>
|
||
|
||
<a href="../chapter2/chapter2.html" title="Swift 教程" class="chapter done new-chapter" data-progress="2" style="left: 10.526315789473685%;"></a>
|
||
|
||
<a href="../chapter2/01_The_Basics.html" title="基础部分" class="chapter done " data-progress="2.1" style="left: 13.157894736842104%;"></a>
|
||
|
||
<a href="../chapter2/10_Properties.html" title="属性" class="chapter done " data-progress="2.10" style="left: 15.789473684210526%;"></a>
|
||
|
||
<a href="../chapter2/11_Methods.html" title="方法" class="chapter done " data-progress="2.11" style="left: 18.42105263157895%;"></a>
|
||
|
||
<a href="../chapter2/12_Subscripts.html" title="附属脚本" class="chapter done " data-progress="2.12" style="left: 21.05263157894737%;"></a>
|
||
|
||
<a href="../chapter2/13_Inheritance.html" title="继承" class="chapter done " data-progress="2.13" style="left: 23.68421052631579%;"></a>
|
||
|
||
<a href="../chapter2/14_Initialization.html" title="构造过程" class="chapter done " data-progress="2.14" style="left: 26.31578947368421%;"></a>
|
||
|
||
<a href="../chapter2/15_Deinitialization.html" title="析构过程" class="chapter done " data-progress="2.15" style="left: 28.94736842105263%;"></a>
|
||
|
||
<a href="../chapter2/16_Automatic_Reference_Counting.html" title="自动引用计数" class="chapter done " data-progress="2.16" style="left: 31.57894736842105%;"></a>
|
||
|
||
<a href="../chapter2/17_Optional_Chaining.html" title="可选链" class="chapter done " data-progress="2.17" style="left: 34.21052631578947%;"></a>
|
||
|
||
<a href="../chapter2/18_Type_Casting.html" title="类型检查" class="chapter done " data-progress="2.18" style="left: 36.8421052631579%;"></a>
|
||
|
||
<a href="../chapter2/19_Nested_Types.html" title="类型嵌套" class="chapter done " data-progress="2.19" style="left: 39.473684210526315%;"></a>
|
||
|
||
<a href="../chapter2/02_Basic_Operators.html" title="基本运算符" class="chapter done " data-progress="2.2" style="left: 42.10526315789474%;"></a>
|
||
|
||
<a href="../chapter2/20_Extensions.html" title="扩展" class="chapter done " data-progress="2.20" style="left: 44.73684210526316%;"></a>
|
||
|
||
<a href="../chapter2/21_Protocols.html" title="协议" class="chapter done " data-progress="2.21" style="left: 47.36842105263158%;"></a>
|
||
|
||
<a href="../chapter2/22_Generics.html" title="泛型" class="chapter done " data-progress="2.22" style="left: 50%;"></a>
|
||
|
||
<a href="../chapter2/23_Advanced_Operators.html" title="高级操作符" class="chapter done " data-progress="2.23" style="left: 52.63157894736842%;"></a>
|
||
|
||
<a href="../chapter2/03_Strings_and_Characters.html" title="字符串和字符" class="chapter done " data-progress="2.3" style="left: 55.26315789473684%;"></a>
|
||
|
||
<a href="../chapter2/04_Collection_Types.html" title="集合类型" class="chapter done " data-progress="2.4" style="left: 57.89473684210526%;"></a>
|
||
|
||
<a href="../chapter2/05_Control_Flow.html" title="控制流" class="chapter done " data-progress="2.5" style="left: 60.526315789473685%;"></a>
|
||
|
||
<a href="../chapter2/06_Functions.html" title="函数" class="chapter done " data-progress="2.6" style="left: 63.1578947368421%;"></a>
|
||
|
||
<a href="../chapter2/07_Closures.html" title="闭包" class="chapter done " data-progress="2.7" style="left: 65.78947368421052%;"></a>
|
||
|
||
<a href="../chapter2/08_Enumerations.html" title="枚举" class="chapter done " data-progress="2.8" style="left: 68.42105263157895%;"></a>
|
||
|
||
<a href="../chapter2/09_Classes_and_Structures.html" title="类和结构体" class="chapter done " data-progress="2.9" style="left: 71.05263157894737%;"></a>
|
||
|
||
<a href="../chapter3/chapter3.html" title="语言参考" class="chapter done new-chapter" data-progress="3" style="left: 73.6842105263158%;"></a>
|
||
|
||
<a href="../chapter3/01_About_the_Language_Reference.html" title="关于语言参考" class="chapter done " data-progress="3.1" style="left: 76.3157894736842%;"></a>
|
||
|
||
<a href="../chapter3/09_Summary_of_the_Grammar.html" title="语法总结" class="chapter done " data-progress="3.10" style="left: 78.94736842105263%;"></a>
|
||
|
||
<a href="../chapter3/02_Lexical_Structure.html" title="词法结构" class="chapter done " data-progress="3.2" style="left: 81.57894736842105%;"></a>
|
||
|
||
<a href="../chapter3/03_Types.html" title="类型" class="chapter done " data-progress="3.3" style="left: 84.21052631578948%;"></a>
|
||
|
||
<a href="../chapter3/04_Expressions.html" title="表达式" class="chapter done " data-progress="3.4" style="left: 86.84210526315789%;"></a>
|
||
|
||
<a href="../chapter3/10_Statements.html" title="语句" class="chapter " data-progress="3.5" style="left: 89.47368421052632%;"></a>
|
||
|
||
<a href="../chapter3/05_Declarations.html" title="声明" class="chapter " data-progress="3.6" style="left: 92.10526315789474%;"></a>
|
||
|
||
<a href="../chapter3/06_Attributes.html" title="特性" class="chapter " data-progress="3.7" style="left: 94.73684210526316%;"></a>
|
||
|
||
<a href="../chapter3/07_Patterns.html" title="模式" class="chapter " data-progress="3.8" style="left: 97.36842105263158%;"></a>
|
||
|
||
<a href="../chapter3/08_Generic_Parameters_and_Arguments.html" title="泛型参数" class="chapter " data-progress="3.9" style="left: 100%;"></a>
|
||
|
||
</div>
|
||
</div>
|
||
|
||
<div class="page-inner">
|
||
|
||
<section class="normal" id="section-gitbook_173">
|
||
|
||
<blockquote>
|
||
<p>翻译：sg552</p>
|
||
<p>校对：numbbbbb</p>
|
||
</blockquote>
|
||
<h1 id="-expressions-">表达式（Expressions）</h1>
|
||
<hr>
|
||
<p>本页包含内容：</p>
|
||
<ul>
|
||
<li>[前缀表达式（Prefix Expressions）]</li>
|
||
<li>[二元表达式（Binary Expressions）]</li>
|
||
<li>[赋值表达式（Assignment Operator）]</li>
|
||
<li>[三元条件运算符（Ternary Conditional Operator）]</li>
|
||
<li>[类型转换运算符（Type-Casting Operators）]</li>
|
||
<li>[主要表达式（Primary Expressions）]</li>
|
||
<li>[后缀表达式（Postfix Expressions）]</li>
|
||
</ul>
|
||
<p>Swift 中存在四种表达式： 前缀（prefix）表达式，二元（binary）表达式，主要（primary）表达式和后缀（postfix）表达式。表达式可以返回一个值，以及运行某些逻辑（causes a side effect）。</p>
|
||
<p>前缀表达式和二元表达式就是对某些表达式使用各种运算符（operators）。 主要表达式是最短小的表达式，它提供了获取（变量的）值的一种途径。 后缀表达式则允许你建立复杂的表达式，例如配合函数调用和成员访问。 每种表达式都在下面有详细论述～</p>
|
||
<blockquote>
|
||
<p>表达式的语法</p>
|
||
<p><em>expression</em> → <em>prefix-expression</em><em>binary-expressions(</em>opt)
|
||
<em>expression-list</em> → <em>expression</em>| <em>expression</em>,<em>expression-list</em></p>
|
||
</blockquote>
|
||
<h2 id="-prefix-expressions-">前缀表达式（Prefix Expressions）</h2>
|
||
<p>前缀表达式由 前缀符号和表达式组成。（这个前缀符号只能接收一个参数）</p>
|
||
<p>Swift 标准库支持如下的前缀操作符：</p>
|
||
<ul>
|
||
<li>++ 自增1 （increment）</li>
|
||
<li>-- 自减1 （decrement）</li>
|
||
<li>! 逻辑否 （Logical NOT ）</li>
|
||
<li>~ 按位否 （Bitwise NOT ）</li>
|
||
<li>+ 加（Unary plus）</li>
|
||
<li>- 减（Unary minus）</li>
|
||
</ul>
|
||
<p>对于这些操作符的使用，请参见： Basic Operators and Advanced Operators</p>
|
||
<p>作为对上面标准库运算符的补充，你也可以对 某个函数的参数使用 '&'运算符。 更多信息，请参见： "In-Out parameters".</p>
|
||
<blockquote>
|
||
<p>前缀表达式的语法</p>
|
||
<p><em>prefix-expression</em> → <em>prefix-operator</em> (opt) <em>postfix-expression</em>
|
||
<em>prefix-expression</em> → <em>in-out-expression</em>
|
||
<em>in-out-expression</em> → &<em>identifier</em></p>
|
||
</blockquote>
|
||
<h2 id="-binary-expressions-">二元表达式（Binary Expressions）</h2>
|
||
<p>二元表达式由 "左边参数" + "二元运算符" + "右边参数" 组成, 它有如下的形式：</p>
|
||
<p> <code>left-hand argument</code> <code>operator</code> <code>right-hand argument</code></p>
|
||
<p>Swift 标准库提供了如下的二元运算符：</p>
|
||
<ul>
|
||
<li>求幂相关（无结合，优先级160）<ul>
|
||
<li><< 按位左移（Bitwise left shift）</li>
|
||
<li><blockquote>
|
||
<blockquote>
|
||
<p>按位右移（Bitwise right shift）</p>
|
||
</blockquote>
|
||
</blockquote>
|
||
</li>
|
||
</ul>
|
||
</li>
|
||
<li>乘除法相关（左结合，优先级150）<ul>
|
||
<li>* 乘</li>
|
||
<li>/ 除</li>
|
||
<li>% 求余</li>
|
||
<li>&* 乘法，忽略溢出（ Multiply, ignoring overflow）</li>
|
||
<li>&/ 除法，忽略溢出（Divide, ignoring overflow）</li>
|
||
<li>&% 求余, 忽略溢出（ Remainder, ignoring overflow）</li>
|
||
<li>& 位与（ Bitwise AND）</li>
|
||
</ul>
|
||
</li>
|
||
<li>加减法相关（左结合, 优先级140）<ul>
|
||
<li>+ 加</li>
|
||
<li>- 减</li>
|
||
<li>&+ Add with overflow</li>
|
||
<li>&- Subtract with overflow</li>
|
||
<li>| 按位或（Bitwise OR ）</li>
|
||
<li>^ 按位异或（Bitwise XOR）</li>
|
||
</ul>
|
||
</li>
|
||
<li>Range （无结合,优先级 135）<ul>
|
||
<li>.. 半闭值域 Half-closed range</li>
|
||
<li>... 全闭值域 Closed range</li>
|
||
</ul>
|
||
</li>
|
||
<li>类型转换 （无结合,优先级 132）<ul>
|
||
<li>is 类型检查（ type check）</li>
|
||
<li>as 类型转换（ type cast）</li>
|
||
</ul>
|
||
</li>
|
||
<li>Comparative （无结合,优先级 130）<ul>
|
||
<li>< 小于</li>
|
||
<li><= 小于等于</li>
|
||
<li><blockquote>
|
||
<p>大于</p>
|
||
</blockquote>
|
||
</li>
|
||
<li><blockquote>
|
||
<p>= 大于等于</p>
|
||
</blockquote>
|
||
</li>
|
||
<li>== 等于</li>
|
||
<li>!= 不等</li>
|
||
<li>=== 恒等于</li>
|
||
<li>!== 不恒等</li>
|
||
<li>~= 模式匹配（ Pattern match）</li>
|
||
</ul>
|
||
</li>
|
||
<li>合取（ Conjunctive） （左结合,优先级 120）<ul>
|
||
<li>&& 逻辑与（Logical AND）</li>
|
||
</ul>
|
||
</li>
|
||
<li>析取（Disjunctive） （左结合,优先级 110）<ul>
|
||
<li>|| 逻辑或（ Logical OR）</li>
|
||
</ul>
|
||
</li>
|
||
<li>三元条件（Ternary Conditional ）（右结合,优先级 100）<ul>
|
||
<li>?: 三元条件 Ternary conditional</li>
|
||
</ul>
|
||
</li>
|
||
<li>赋值 （Assignment） （右结合, 优先级 90）<ul>
|
||
<li>= 赋值（Assign）</li>
|
||
<li>*= Multiply and assign</li>
|
||
<li>/= Divide and assign</li>
|
||
<li>%= Remainder and assign</li>
|
||
<li>+= Add and assign</li>
|
||
<li>-= Subtract and assign</li>
|
||
<li><<= Left bit shift and assign</li>
|
||
<li><blockquote>
|
||
<blockquote>
|
||
<p>= Right bit shift and assign</p>
|
||
</blockquote>
|
||
</blockquote>
|
||
</li>
|
||
<li>&= Bitwise AND and assign</li>
|
||
<li>^= Bitwise XOR and assign</li>
|
||
<li>|= Bitwise OR and assign</li>
|
||
<li>&&= Logical AND and assign</li>
|
||
<li>||= Logical OR and assign</li>
|
||
</ul>
|
||
</li>
|
||
</ul>
|
||
<p>关于这些运算符（operators）的更多信息，请参见：Basic Operators and Advanced Operators.</p>
|
||
<blockquote>
|
||
<blockquote>
|
||
<p>注意</p>
|
||
<p>在解析时, 一个二元表达式表示为一个一级数组（a flat list）, 这个数组（List）根据运算符的先后顺序，被转换成了一个tree. 例如： 2 + 3 <em> 5 首先被认为是： 2, + , <code>3</code>, </em>, 5. 随后它被转换成 tree （2 + （3 * 5））</p>
|
||
</blockquote>
|
||
<p>二元表达式的语法</p>
|
||
<p><em>binary-expression</em> → <em>binary-operator</em><em>prefix-expression</em>
|
||
<em>binary-expression</em> → <em>assignment-operator</em>prefix-expression<em>
|
||
</em>binary-expression<em> → </em>conditional-operator<em>prefix-expression</em>
|
||
<em>binary-expression</em> → <em>type-casting-operator</em>
|
||
<em>binary-expression</em>s → <em>binary-expression</em><em>binary-expressions</em>(opt)</p>
|
||
</blockquote>
|
||
<h2 id="-assignment-operator-">赋值表达式（Assignment Operator）</h2>
|
||
<p>The assigment operator sets a new value for a given expression. It has the following form:
|
||
赋值表达式会对某个给定的表达式赋值。 它有如下的形式；</p>
|
||
<p><code>expression</code> = <code>value</code></p>
|
||
<p>就是把右边的 <em>value</em> 赋值给左边的 <em>expression</em>. 如果左边的<em>expression</em> 需要接收多个参数（是一个tuple ），那么右边必须也是一个具有同样数量参数的tuple. （允许嵌套的tuple）</p>
|
||
<pre><code class="lang-swift">(a, _, (b, c)) = ("test", 9.45, (12, 3))
|
||
// a is "test", b is 12, c is 3, and 9.45 is ignored
|
||
</code></pre>
|
||
<p>赋值运算符不返回任何值。</p>
|
||
<blockquote>
|
||
<p>赋值表达式的语法</p>
|
||
<p><em>assignment-operator</em> → =</p>
|
||
</blockquote>
|
||
<h2 id="-ternary-conditional-operator-">三元条件运算符（Ternary Conditional Operator）</h2>
|
||
<p>三元条件运算符 是根据条件来获取值。 形式如下：</p>
|
||
<pre><code>`condition` ? `expression used if true` : `expression used if false`
|
||
</code></pre><p>如果 <code>condition</code> 是true, 那么返回 第一个表达式的值（此时不会调用第二个表达式）， 否则返回第二个表达式的值（此时不会调用第一个表达式）。</p>
|
||
<p>想看三元条件运算符的例子，请参见： Ternary Conditional Operator.</p>
|
||
<blockquote>
|
||
<p>三元条件表达式</p>
|
||
<p><code>conditional-operator</code> → ?<code>expression</code>:</p>
|
||
</blockquote>
|
||
<h2 id="-type-casting-operators-">类型转换运算符（Type-Casting Operators）</h2>
|
||
<p>有两种类型转换操作符： as 和 is. 它们有如下的形式：</p>
|
||
<pre><code>`expression` as `type`
|
||
`expression` as? `type`
|
||
`expression` is `type`
|
||
</code></pre><p>as 运算符会把<code>目标表达式</code>转换成指定的<code>类型</code>（specified type），过程如下：</p>
|
||
<ul>
|
||
<li><p>如果类型转换成功， 那么目标表达式就会返回指定类型的实例（instance）. 例如：把子类（subclass）变成父类（superclass）时.</p>
|
||
</li>
|
||
<li><p>如果转换失败，则会抛出编译错误（ compile-time error）。</p>
|
||
</li>
|
||
<li><p>如果上述两个情况都不是（也就是说，编译器在编译时期无法确定转换能否成功，） 那么目标表达式就会变成指定的类型的optional. （is an optional of the specified type ） 然后在运行时，如果转换成功， 目标表达式就会作为 optional的一部分来返回， 否则，目标表达式返回nil. 对应的例子是： 把一个 superclass 转换成一个 subclass.</p>
|
||
</li>
|
||
</ul>
|
||
<pre><code class="lang-swift">class SomeSuperType {}
|
||
class SomeType: SomeSuperType {}
|
||
class SomeChildType: SomeType {}
|
||
let s = SomeType()
|
||
|
||
let x = s as SomeSuperType // known to succeed; type is SomeSuperType
|
||
let y = s as Int // known to fail; compile-time error
|
||
let z = s as SomeChildType // might fail at runtime; type is SomeChildType?
|
||
</code></pre>
|
||
<p>使用'as'做类型转换跟正常的类型声明，对于编译器来说是一样的。例如：</p>
|
||
<pre><code class="lang-swift">let y1 = x as SomeType // Type information from 'as'
|
||
let y2: SomeType = x // Type information from an annotation
|
||
</code></pre>
|
||
<p>'is' 运算符在“运行时（runtime）”会做检查。 成功会返回true, 否则 false</p>
|
||
<p>The check must not be known to be true or false at compile time. The following are invalid:
|
||
上述检查在“编译时（compile time）”不能使用。 例如下面的使用是错误的：</p>
|
||
<pre><code class="lang-swift">"hello" is String
|
||
"hello" is Int
|
||
</code></pre>
|
||
<p>关于类型转换的更多内容和例子，请参见： Type Casting.</p>
|
||
<blockquote>
|
||
<p>类型转换的语法</p>
|
||
<p><em>type-casting-operator</em> → is<em>type</em>| as?(opt)<em>type</em></p>
|
||
</blockquote>
|
||
<h2 id="-primary-expressions-">主要表达式（Primary Expressions）</h2>
|
||
<p><code>主要表达式</code>是最基本的表达式。 它们可以跟 前缀表达式，二元表达式，后缀表达式以及其他主要表达式组合使用。</p>
|
||
<blockquote>
|
||
<p>主要表达式的语法</p>
|
||
<p><em>primary-expression</em> → <em>identifier</em><em>generic-argument-clause</em>(opt)
|
||
<em>primary-expression</em> → <em>literal-expression</em>
|
||
<em>primary-expression</em> → <em>self-expression</em>
|
||
<em>primary-expression</em> → <em>superclass-expression</em>
|
||
<em>primary-expression</em> → <em>closure-expression</em>
|
||
<em>primary-expression</em> → <em>parenthesized-expression</em>
|
||
<em>primary-expression</em> → <em>implicit-member-expression</em>
|
||
<em>primary-expression</em> → <em>wildcard-expression</em></p>
|
||
</blockquote>
|
||
<h3 id="-literal-expression-">字符型表达式（Literal Expression）</h3>
|
||
<p>由这些内容组成：普通的字符（string, number） , 一个字符的字典或者数组，或者下面列表中的特殊字符。</p>
|
||
<table>
|
||
<thead>
|
||
<tr>
|
||
<th>字符（Literal）</th>
|
||
<th>类型（Type）</th>
|
||
<th>值（Value）</th>
|
||
</tr>
|
||
</thead>
|
||
<tbody>
|
||
<tr>
|
||
<td>_<em>FILE_</em></td>
|
||
<td>String</td>
|
||
<td>所在的文件名</td>
|
||
</tr>
|
||
<tr>
|
||
<td>_<em>LINE_</em></td>
|
||
<td>Int</td>
|
||
<td>所在的行数</td>
|
||
</tr>
|
||
<tr>
|
||
<td>_<em>COLUMN_</em></td>
|
||
<td>Int</td>
|
||
<td>所在的列数</td>
|
||
</tr>
|
||
<tr>
|
||
<td>_<em>FUNCTION_</em></td>
|
||
<td>String</td>
|
||
<td>所在的function 的名字</td>
|
||
</tr>
|
||
</tbody>
|
||
</table>
|
||
<p>在某个函数（function）中，<code>__FUNCTION__</code> 会返回当前函数的名字。 在某个方法（method）中，它会返回当前方法的名字。 在某个property 的getter/setter中会返回这个属性的名字。 在init/subscript中 只有的特殊成员（member）中会返回这个keyword的名字，在某个文件的顶端（the top level of a file），它返回的是当前module的名字。</p>
|
||
<p>一个array literal，是一个有序的值的集合。 它的形式是：</p>
|
||
<pre><code>[`value 1`, `value 2`, `...`]
|
||
</code></pre><p>数组中的最后一个表达式可以紧跟一个逗号（','）. []表示空数组 。 array literal的type是 T[], 这个T就是数组中元素的type. 如果该数组中有多种type, T则是跟这些type的公共supertype最接近的type.（closest common supertype）</p>
|
||
<p>一个<code>dictionary literal</code> 是一个包含无序的键值对（key-value pairs）的集合，它的形式是:</p>
|
||
<pre><code>[`key 1`: `value 1`, `key 2`: `value 2`, `...`]
|
||
</code></pre><p>dictionary 的最后一个表达式可以是一个逗号（','）. [:] 表示一个空的dictionary. 它的type是 Dictionary<KeyType, ValueType> （这里KeyType表示 key的type, ValueType表示 value的type） 如果这个dictionary 中包含多种 types, 那么KeyType, Value 则对应着它们的公共supertype最接近的type（ closest common supertype）.</p>
|
||
<blockquote>
|
||
<p>字符型表达式的语法</p>
|
||
<p><em>literal-expression</em> → <em>literal</em>
|
||
<em>literal-expression</em> → <em>array-literal</em>| <em>dictionary-literal</em>
|
||
<em>literal-expression</em> → <em>_<em>FILE_</em></em>| <em>_<em>LINE_</em></em>| <em>_<em>COLUMN_</em></em>| <em>_<em>FUNCTION_</em></em>
|
||
<em>array-literal</em> → [<em>array-literal-items</em>opt]
|
||
<em>array-literal-items</em> → <em>array-literal-item</em>,(opt) | <em>array-literal-item</em>,<em>array-literal-items</em>
|
||
<em>array-literal-item</em> → <em>expression</em>
|
||
<em>dictionary-literal</em> → [<em>dictionary-literal-items</em>] [:]
|
||
<em>dictionary-literal-items</em> → <em>dictionary-literal-item</em>,(opt)| <em>dictionary-literal-item</em>,<em>dictionary-literal-items</em>
|
||
<em>dictionary-literal-item</em> → <em>expression</em>:<em>expression</em></p>
|
||
</blockquote>
|
||
<h3 id="self-self-expression-">self表达式（Self Expression）</h3>
|
||
<p>self表达式是对 当前type 或者当前instance的引用。它的形式如下：</p>
|
||
<blockquote>
|
||
<p>self
|
||
self.<code>member name</code>
|
||
self[<code>subscript index</code>]
|
||
self（<code>initializer arguments</code>）
|
||
self.init（<code>initializer arguments</code>）</p>
|
||
</blockquote>
|
||
<p>如果在 initializer, subscript, instance method中，self等同于当前type的instance. 在一个静态方法（static method）, 类方法（class method）中， self等同于当前的type.</p>
|
||
<p>当访问 member（成员变量时）， self 用来区分重名变量（例如函数的参数）. 例如，
|
||
（下面的 self.greeting 指的是 var greeting: String, 而不是 init（greeting: String） ）</p>
|
||
<pre><code class="lang-swift">class SomeClass {
|
||
var greeting: String
|
||
init（greeting: String） {
|
||
self.greeting = greeting
|
||
}
|
||
}
|
||
</code></pre>
|
||
<p>在mutating 方法中， 你可以使用self 对 该instance进行赋值。</p>
|
||
<pre><code class="lang-swift">struct Point {
|
||
var x = 0.0, y = 0.0
|
||
mutating func moveByX（deltaX: Double, y deltaY: Double） {
|
||
self = Point（x: x + deltaX, y: y + deltaY）
|
||
}
|
||
}
|
||
</code></pre>
|
||
<blockquote>
|
||
<p>self表达式的语法</p>
|
||
<p><em>self-expression</em> → self
|
||
<em>self-expression</em> → self.<em>identifier</em>
|
||
<em>self-expression</em> → self[<em>expression</em>]
|
||
<em>self-expression</em> → self.init</p>
|
||
</blockquote>
|
||
<h3 id="-superclass-expression-">超类表达式（Superclass Expression）</h3>
|
||
<p>超类表达式可以使我们在某个class中访问它的超类. 它有如下形式：</p>
|
||
<pre><code>super.`member name`
|
||
super[`subscript index`]
|
||
super.init（`initializer arguments`）
|
||
</code></pre><p>形式1 用来访问超类的某个成员（member）. 形式2 用来访问该超类的 subscript 实现。 形式3 用来访问该超类的 initializer.</p>
|
||
<p>子类（subclass）可以通过超类（superclass）表达式在它们的 member, subscripting 和 initializers 中来利用它们超类中的某些实现（既有的方法或者逻辑）。</p>
|
||
<blockquote>
|
||
<p>GRAMMAR OF A SUPERCLASS EXPRESSION</p>
|
||
<p><em>superclass-expression</em> → <em>superclass-method-expression</em> | <em>superclass-subscript-expression</em>| <em>superclass-initializer-expression</em>
|
||
<em>superclass-method-expression</em> → super.<em>identifier</em>
|
||
<em>superclass-subscript-expression</em> → super[<em>expression</em>]
|
||
<em>superclass-initializer-expression</em> → super.init</p>
|
||
</blockquote>
|
||
<h3 id="-closure-expression-">闭包表达式（Closure Expression）</h3>
|
||
<p>闭包（closure） 表达式可以建立一个闭包（在其他语言中也叫 lambda, 或者 匿名函数（anonymous function））. 跟函数（function）的声明一样， 闭包（closure）包含了可执行的代码（跟方法主体（statement）类似） 以及接收（capture）的参数。 它的形式如下：</p>
|
||
<pre><code class="lang-swift"> { （parameters） -> return type in
|
||
statements
|
||
}
|
||
</code></pre>
|
||
<p>闭包的参数声明形式跟方法中的声明一样, 请参见：Function Declaration.</p>
|
||
<p>闭包还有几种特殊的形式, 让使用更加简洁：</p>
|
||
<ul>
|
||
<li>闭包可以省略 它的参数的type 和返回值的type. 如果省略了参数和参数类型，就也要省略 'in'关键字。 如果被省略的type 无法被编译器获知（inferred） ，那么就会抛出编译错误。</li>
|
||
<li>闭包可以省略参数，转而在方法体（statement）中使用 $0, $1, $2 来引用出现的第一个，第二个，第三个参数。</li>
|
||
<li>如果闭包中只包含了一个表达式，那么该表达式就会自动成为该闭包的返回值。 在执行 'type inference '时，该表达式也会返回。</li>
|
||
</ul>
|
||
<p>下面几个 闭包表达式是 等价的：</p>
|
||
<pre><code class="lang-swift">myFunction {
|
||
（x: Int, y: Int） -> Int in
|
||
return x + y
|
||
}
|
||
|
||
myFunction {
|
||
（x, y） in
|
||
return x + y
|
||
}
|
||
|
||
myFunction { return $0 + $1 }
|
||
|
||
myFunction { $0 + $1 }
|
||
</code></pre>
|
||
<p>关于 向闭包中传递参数的内容，参见： Function Call Expression.</p>
|
||
<p>闭包表达式可以通过一个参数列表（capture list） 来显式指定它需要的参数。 参数列表 由中括号 [] 括起来，里面的参数由逗号','分隔。一旦使用了参数列表，就必须使用'in'关键字（在任何情况下都得这样做，包括忽略参数的名字，type, 返回值时等等）。</p>
|
||
<p>在闭包的参数列表（ capture list）中， 参数可以声明为 'weak' 或者 'unowned' .</p>
|
||
<pre><code class="lang-swift">myFunction { print（self.title） } // strong capture
|
||
myFunction { [weak self] in print（self!.title） } // weak capture
|
||
myFunction { [unowned self] in print（self.title） } // unowned capture
|
||
</code></pre>
|
||
<p>在参数列表中，也可以使用任意表达式来赋值. 该表达式会在 闭包被执行时赋值，然后按照不同的力度来获取（这句话请慎重理解）。（captured with the specified strength. ） 例如：</p>
|
||
<pre><code class="lang-swift">// Weak capture of "self.parent" as "parent"
|
||
myFunction { [weak parent = self.parent] in print（parent!.title） }
|
||
</code></pre>
|
||
<p>关于闭包表达式的更多信息和例子，请参见： Closure Expressions.</p>
|
||
<blockquote>
|
||
<p>闭包表达式的语法</p>
|
||
<p><em>closure-expression</em> → {<em>closure-signature</em>opt<em>statements</em>}
|
||
<em>closure-signature</em> → <em>parameter-clause</em><em>function-result</em>(opt)in
|
||
<em>closure-signature</em> → <em>identifier-list</em><em>function-result</em>(opt)in
|
||
<em>closure-signature</em> → <em>capture-list</em><em>parameter-clause</em><em>function-result</em>(opt)in
|
||
<em>closure-signature</em> → <em>capture-list</em><em>identifier-list</em><em>function-result</em>(opt)in
|
||
<em>closure-signature</em> → <em>capture-list</em>in
|
||
<em>capture-list</em> → [<em>capture-specifier</em><em>expression</em>]
|
||
<em>capture-specifier</em> → weak| unowned| unowned（safe）| unowned（unsafe）</p>
|
||
</blockquote>
|
||
<h3 id="-implicit-member-expression-">隐式成员表达式（Implicit Member Expression）</h3>
|
||
<p>在可以判断出类型（type）的上下文（context）中，隐式成员表达式是访问某个type的member（ 例如 class method, enumeration case） 的简洁方法。 它的形式是：</p>
|
||
<p>.<code>member name</code></p>
|
||
<p>例子：</p>
|
||
<pre><code class="lang-swift">var x = MyEnumeration.SomeValue
|
||
x = .AnotherValue
|
||
</code></pre>
|
||
<blockquote>
|
||
<p> 隐式成员表达式的语法</p>
|
||
<p> <em>implicit-member-expression</em> → .<em>identifier</em></p>
|
||
</blockquote>
|
||
<h3 id="-parenthesized-expression-">圆括号表达式（Parenthesized Expression）</h3>
|
||
<p>圆括号表达式由多个子表达式和逗号','组成。 每个子表达式前面可以有 identifier x: 这样的可选前缀。形式如下：</p>
|
||
<p>（<code>identifier 1</code>: <code>expression 1</code>, <code>identifier 2</code>: <code>expression 2</code>, <code>...</code>）</p>
|
||
<p>圆括号表达式用来建立tuples ， 然后把它做为参数传递给 function. 如果某个圆括号表达式中只有一个 子表达式，那么它的type就是 子表达式的type。例如： （1）的 type是Int, 而不是（Int）</p>
|
||
<blockquote>
|
||
<p>圆括号表达式的语法</p>
|
||
<p><em>parenthesized-expression</em> → （<em>expression-element-list</em> (opt)）
|
||
<em>expression-element-list</em> → <em>expression-element</em>| <em>expression-element</em>,<em>expression-element-list</em>
|
||
<em>expression-element</em> → <em>expression</em>| <em>identifier</em>:<em>expression</em></p>
|
||
</blockquote>
|
||
<h3 id="-wildcard-expression-">通配符表达式（Wildcard Expression）</h3>
|
||
<p>通配符表达式用来忽略传递进来的某个参数。例如：下面的代码中，10被传递给x, 20被忽略（译注：好奇葩的语法。。。）</p>
|
||
<pre><code class="lang-swift">（x, _） = （10, 20）
|
||
// x is 10, 20 is ignored
|
||
</code></pre>
|
||
<blockquote>
|
||
<p>通配符表达式的语法</p>
|
||
<p><em>wildcard-expression</em> → _</p>
|
||
</blockquote>
|
||
<h2 id="-postfix-expressions-">后缀表达式（Postfix Expressions）</h2>
|
||
<p>后缀表达式就是在某个表达式的后面加上 操作符。 严格的讲，每个主要表达式（primary expression）都是一个后缀表达式</p>
|
||
<p>Swift 标准库提供了下列后缀表达式：</p>
|
||
<ul>
|
||
<li>++ Increment</li>
|
||
<li>-- Decrement</li>
|
||
</ul>
|
||
<p>对于这些操作符的使用，请参见： Basic Operators and Advanced Operators</p>
|
||
<blockquote>
|
||
<p>后缀表达式的语法</p>
|
||
<p><em>postfix-expression</em> → <em>primary-expression</em>
|
||
<em>postfix-expression</em> → <em>postfix-expression</em><em>postfix-operator</em>
|
||
<em>postfix-expression</em> → <em>function-call-expression</em>
|
||
<em>postfix-expression</em> → <em>initializer-expression</em>
|
||
<em>postfix-expression</em> → <em>explicit-member-expression</em>
|
||
<em>postfix-expression</em> → <em>postfix-self-expression</em>
|
||
<em>postfix-expression</em> → <em>dynamic-type-expression</em>
|
||
<em>postfix-expression</em> → <em>subscript-expression</em>
|
||
<em>postfix-expression</em> → <em>forced-value-expression</em>
|
||
<em>postfix-expression</em> → <em>optional-chaining-expression</em></p>
|
||
</blockquote>
|
||
<h3 id="-function-call-expression-">函数调用表达式（Function Call Expression）</h3>
|
||
<p>函数调用表达式由函数名和参数列表组成。它的形式如下：</p>
|
||
<p><code>function name</code>（<code>argument value 1</code>, <code>argument value 2</code>）</p>
|
||
<p>The function name can be any expression whose value is of a function type.
|
||
（不用翻译了, 太罗嗦）</p>
|
||
<p>如果该function 的声明中指定了参数的名字，那么在调用的时候也必须得写出来. 例如：</p>
|
||
<p><code>function name</code>（<code>argument name 1</code>: <code>argument value 1</code>, <code>argument name 2</code>: <code>argument value 2</code>）</p>
|
||
<p>可以在 函数调用表达式的尾部（最后一个参数之后）加上 一个闭包（closure） ， 该闭包会被目标函数理解并执行。它具有如下两种写法：</p>
|
||
<pre><code class="lang-swift">// someFunction takes an integer and a closure as its arguments
|
||
someFunction（x, {$0 == 13}）
|
||
someFunction（x） {$0 == 13}
|
||
</code></pre>
|
||
<p>如果闭包是该函数的唯一参数，那么圆括号可以省略。</p>
|
||
<pre><code class="lang-swift">// someFunction takes a closure as its only argument
|
||
myData.someMethod（） {$0 == 13}
|
||
myData.someMethod {$0 == 13}
|
||
</code></pre>
|
||
<blockquote>
|
||
<p>GRAMMAR OF A FUNCTION CALL EXPRESSION</p>
|
||
<p><em>function-call-expression</em> → <em>postfix-expression</em><em>parenthesized-expression</em>
|
||
<em>function-call-expression</em> → <em>postfix-expression</em><em>parenthesized-expression</em>(opt)<em>trailing-closure</em>
|
||
<em>trailing-closure</em> → <em>closure-expression</em></p>
|
||
</blockquote>
|
||
<h3 id="-initializer-expression-">初始化函数表达式（Initializer Expression）</h3>
|
||
<p>Initializer表达式用来给某个Type初始化。 它的形式如下：</p>
|
||
<p><code>expression</code>.init（<code>initializer arguments</code>）</p>
|
||
<p>（Initializer表达式用来给某个Type初始化。） 跟函数（function）不同， initializer 不能返回值。</p>
|
||
<pre><code class="lang-swift">var x = SomeClass.someClassFunction // ok
|
||
var y = SomeClass.init // error
|
||
```swift
|
||
|
||
可以通过 initializer 表达式来委托调用（delegate to ）到superclass的initializers.
|
||
|
||
```swift
|
||
class SomeSubClass: SomeSuperClass {
|
||
init（） {
|
||
// subclass initialization goes here
|
||
super.init（）
|
||
}
|
||
}
|
||
</code></pre>
|
||
<blockquote>
|
||
<p>initializer表达式的语法</p>
|
||
<p><em>initializer-expression</em> → <em>postfix-expression</em>.init</p>
|
||
</blockquote>
|
||
<h3 id="-explicit-member-expression-">显式成员表达式（Explicit Member Expression）</h3>
|
||
<p>显示成员表达式允许我们访问type, tuple, module的成员变量。它的形式如下：</p>
|
||
<p><code>expression</code>.<code>member name</code></p>
|
||
<p>该member 就是某个type在声明时候所定义（declaration or extension） 的变量, 例如：</p>
|
||
<pre><code class="lang-swift">class SomeClass {
|
||
var someProperty = 42
|
||
}
|
||
let c = SomeClass（）
|
||
let y = c.someProperty // Member access
|
||
</code></pre>
|
||
<p>对于tuple, 要根据它们出现的顺序（0, 1, 2...）来使用:</p>
|
||
<pre><code class="lang-swift">var t = （10, 20, 30）
|
||
t.0 = t.1
|
||
// Now t is （20, 20, 30）
|
||
</code></pre>
|
||
<p>The members of a module access the top-level declarations of that module.
|
||
（不确定：对于某个module的member的调用，只能调用在top-level声明中的member.）</p>
|
||
<blockquote>
|
||
<p>显示成员表达式的语法</p>
|
||
<p><em>explicit-member-expression</em> → <em>postfix-expression</em>.<em>decimal-digit</em>
|
||
<em>explicit-member-expression</em> → <em>postfix-expression</em>.<em>identifier</em><em>generic-argument-clause</em>(opt)</p>
|
||
</blockquote>
|
||
<h3 id="-self-postfix-self-expression-">后缀self表达式（Postfix Self Expression）</h3>
|
||
<p>后缀表达式由 某个表达式 + '.self' 组成. 形式如下：</p>
|
||
<p><code>expression</code>.self
|
||
<code>type</code>.self</p>
|
||
<p>形式1 表示会返回 expression 的值。例如： x.self 返回 x</p>
|
||
<p>形式2：返回对应的type。我们可以用它来动态的获取某个instance的type。</p>
|
||
<blockquote>
|
||
<p>后缀self表达式的语法</p>
|
||
<p><em>postfix-self-expression</em> → <em>postfix-expression</em>.self</p>
|
||
</blockquote>
|
||
<h3 id="dynamic-dynamic-type-expression-">dynamic表达式（Dynamic Type Expression）</h3>
|
||
<p>（因为dynamicType是一个独有的方法，所以这里保留了英文单词，未作翻译, --- 类似与self expression）</p>
|
||
<p>dynamicType 表达式由 某个表达式 + '.dynamicType' 组成。</p>
|
||
<p><code>expression</code>.dynamicType</p>
|
||
<p>上面的形式中， expression 不能是某type的名字（当然了，如果我都知道它的名字了还需要动态来获取它吗）。动态类型表达式会返回"运行时"某个instance的type, 具体请看下面的列子：</p>
|
||
<pre><code class="lang-swift">class SomeBaseClass {
|
||
class func printClassName（） {
|
||
println（"SomeBaseClass"）
|
||
}
|
||
}
|
||
class SomeSubClass: SomeBaseClass {
|
||
override class func printClassName（） {
|
||
println（"SomeSubClass"）
|
||
}
|
||
}
|
||
let someInstance: SomeBaseClass = SomeSubClass（）
|
||
|
||
// someInstance is of type SomeBaseClass at compile time, but
|
||
// someInstance is of type SomeSubClass at runtime
|
||
someInstance.dynamicType.printClassName（）
|
||
// prints "SomeSubClass"
|
||
</code></pre>
|
||
<blockquote>
|
||
<p>dynamic type 表达式</p>
|
||
<p><em>dynamic-type-expression</em> → <em>postfix-expression</em>.dynamicType</p>
|
||
</blockquote>
|
||
<h3 id="-subscript-expression-">附属脚本表达式（Subscript Expression）</h3>
|
||
<p>附属脚本表达式提供了通过附属脚本访问getter/setter 的方法。它的形式是：</p>
|
||
<p><code>expression</code>[<code>index expressions</code>]</p>
|
||
<p>可以通过附属脚本表达式通过getter获取某个值，或者通过setter赋予某个值.</p>
|
||
<p>关于subscript的声明，请参见： Protocol Subscript Declaration.</p>
|
||
<blockquote>
|
||
<p>附属脚本表达式的语法</p>
|
||
<p><em>subscript-expression</em> → <em>postfix-expression</em>[<em>expression-list</em>]</p>
|
||
</blockquote>
|
||
<h3 id="-forced-value-expression-">强制取值表达式（Forced-Value Expression）</h3>
|
||
<p>强制取值表达式用来获取某个目标表达式的值（该目标表达式的值必须不是nil ）。它的形式如下：</p>
|
||
<p><code>expression</code>!</p>
|
||
<p>如果该表达式的值不是nil, 则返回对应的值。 否则，抛出运行时错误（runtime error）。</p>
|
||
<blockquote>
|
||
<p>强制取值表达式的语法</p>
|
||
<p><em>forced-value-expression</em> → <em>postfix-expression</em>!</p>
|
||
</blockquote>
|
||
<h3 id="-optional-chaining-expression-">可选链表达式（Optional-Chaining Expression）</h3>
|
||
<p>可选链表达式由目标表达式 + '?' 组成，形式如下：</p>
|
||
<p><code>expression</code>?</p>
|
||
<p>后缀'?' 返回目标表达式的值，把它做为可选的参数传递给后续的表达式</p>
|
||
<p>如果某个后缀表达式包含了可选链表达式，那么它的执行过程就比较特殊： 首先先判断该可选链表达式的值，如果是 nil, 整个后缀表达式都返回 nil, 如果该可选链的值不是nil, 则正常返回该后缀表达式的值（依次执行它的各个子表达式）。在这两种情况下，该后缀表达式仍然是一个optional type（In either case, the value of the postfix expression is still of an optional type）</p>
|
||
<p>如果某个"后缀表达式"的"子表达式"中包含了"可选链表达式"，那么只有最外层的表达式返回的才是一个optional type. 例如，在下面的例子中， 如果c 不是nil, 那么 c?.property.performAction（） 这句代码在执行时，就会先获得c 的property方法，然后调用 performAction（）方法。 然后对于 "c?.property.performAction（）" 这个整体，它的返回值是一个optional type.</p>
|
||
<pre><code class="lang-swift">var c: SomeClass?
|
||
var result: Bool? = c?.property.performAction（）
|
||
</code></pre>
|
||
<p>如果不使用可选链表达式，那么 上面例子的代码跟下面例子等价：</p>
|
||
<pre><code class="lang-swift">if let unwrappedC = c {
|
||
result = unwrappedC.property.performAction（）
|
||
}
|
||
</code></pre>
|
||
<blockquote>
|
||
<p>可选链表达式的语法</p>
|
||
<p><em>optional-chaining-expression</em> → <em>postfix-expression</em>?</p>
|
||
</blockquote>
|
||
|
||
|
||
</section>
|
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|
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