利用runtime添加属性

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了解OC的都应该知道，在一般情况下，我们是不能向Category中添加属性的，只能添加方法，但有些情况向，我们确实需要向Category中添加属性，而且很多系统的API也有一些在Category添加属性的情况，例如我们属性的UITableView的section和row属性，就是定义在一个名为NSIndexPath的分类里的，如下
这里写图片描述

那这到底是怎么实现的呢？

iOS运行时机制简介

iOS运行时机制，简单来说，就是苹果给开发这提供的一套在运行时动态创建类、添加属性/方法（不止这些，还有一些其他功能）的API，它是一套纯C语言的API，使用相应的API就可以通过Category给一个原本存在的类添加属性。

实例

<code class="hljs objectivec has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">#import <span class="hljs-title" style="box-sizing: border-box;"><Foundation/Foundation.h></span></span><span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">#import <span class="hljs-title" style="box-sizing: border-box;"><objc/runtime.h></span></span><span class="hljs-class" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">@interface</span> <span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">NSObject</span> (<span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">CategoryWithProperty</span>)</span><span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">/** *  要在Category中扩展的属性 */</span><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">@property</span> (<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">nonatomic</span>, <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">strong</span>) <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">NSObject</span> *property;<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">@end</span><span class="hljs-class" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">@implementation</span> <span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">NSObject</span> (<span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">CategoryWithProperty</span>)</span>- (<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">NSObject</span> *)property {    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> objc_getAssociatedObject(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">self</span>, <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">@selector</span>(property));}- (<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span>)setProperty:(<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">NSObject</span> *)value {    objc_setAssociatedObject(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">self</span>, <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">@selector</span>(property), value, OBJC_ASSOCIATION_RETAIN_NONATOMIC);}<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">@end</span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li><li style="box-sizing: border-box; padding: 0px 5px;">23</li></ul>

这样就可以在Category中添加属性了。

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