codeigniter是如何实现钩子机制的？

codeigniter是如何实现钩子机制的？，有需要的朋友可以参考下。

记得上一次去到喜啦面试，面试官问我一个问题：codeigniter是如何实现钩子机制的？
当时答不上来，后来回来之后查了一些资料才明白，所以在这里记录一下：
codeigniter的钩子是这样实现的：首先在框架的核心文件system/core/CodeIniter.php文件的 122行，载入Hooks类，接着在该文件中定义了几个挂载点，比如pre_system（129行）、post_controller_constructor（295行）等，并在这些挂载点上面执行hooks类的_call_hook() 方法。

另附codeigniter的hooks类的源代码：

1. /**
2. * CodeIgniter
3. *
4. * An open source application development framework for PHP 5.1.6 or newer
5. *
6. * @package CodeIgniter
7. * @author EllisLab Dev Team
8. * @copyright Copyright (c) 2008 - 2014, EllisLab, Inc.
9. * @copyright Copyright (c) 2014 - 2015, British Columbia Institute of Technology (http://bcit.ca/)
10. * @license http://codeigniter.com/user_guide/license.html
11. * [url=home.php?mod=space&uid=17823]@LINK[/url] http://codeigniter.com
12. * @since Version 1.0
13. * @filesource
14. */
16. // ------------------------------------------------------------------------
18. /**
19. * CodeIgniter Hooks Class
20. *
21. * Provides a mechanism to extend the base system without hacking.
22. *
23. * @package CodeIgniter
24. * @subpackage Libraries
25. * @category Libraries
26. * @author EllisLab Dev Team
27. * @link http://codeigniter.com/user_guide/libraries/encryption.html
28. */
29. class CI_Hooks {
31. /**
32. * Determines wether hooks are enabled
33. *
34. * @var bool
35. */
36. var $enabled = FALSE;
37. /**
38. * List of all hooks set in config/hooks.php
39. *
40. * @var array
41. */
42. var $hooks = array();
43. /**
44. * Determines wether hook is in progress, used to prevent infinte loops
45. *
46. * @var bool
47. */
48. var $in_progress = FALSE;
50. /**
51. * Constructor
52. *
53. */
54. function __construct()
55. {
56. $this->_initialize();
57. log_message('debug', "Hooks Class Initialized");
58. }
60. // --------------------------------------------------------------------
62. /**
63. * Initialize the Hooks Preferences
64. *
65. * @access private
66. * @return void
67. */
68. function _initialize()
69. {
70. $CFG =& load_class('Config', 'core');
72. // If hooks are not enabled in the config file
73. // there is nothing else to do
75. if ($CFG->item('enable_hooks') == FALSE)
76. {
77. return;
78. }
80. // Grab the "hooks" definition file.
81. // If there are no hooks, we're done.
83. if (defined('ENVIRONMENT') AND is_file(APPPATH.'config/'.ENVIRONMENT.'/hooks.php'))
84. {
85. include(APPPATH.'config/'.ENVIRONMENT.'/hooks.php');
86. }
87. elseif (is_file(APPPATH.'config/hooks.php'))
88. {
89. include(APPPATH.'config/hooks.php');
90. }
93. if ( ! isset($hook) OR ! is_array($hook))
94. {
95. return;
96. }
98. $this->hooks =& $hook;
99. $this->enabled = TRUE;
100. }
102. // --------------------------------------------------------------------
104. /**
105. * Call Hook
106. *
107. * Calls a particular hook
108. *
109. * @access private
110. * @param string the hook name
111. * @return mixed
112. */
113. function _call_hook($which = '')
114. {
115. if ( ! $this->enabled OR ! isset($this->hooks[$which]))
116. {
117. return FALSE;
118. }
120. if (isset($this->hooks[$which][0]) AND is_array($this->hooks[$which][0]))
121. {
122. foreach ($this->hooks[$which] as $val)
123. {
124. $this->_run_hook($val);
125. }
126. }
127. else
128. {
129. $this->_run_hook($this->hooks[$which]);
130. }
132. return TRUE;
133. }
135. // --------------------------------------------------------------------
137. /**
138. * Run Hook
139. *
140. * Runs a particular hook
141. *
142. * @access private
143. * @param array the hook details
144. * @return bool
145. */
146. function _run_hook($data)
147. {
148. if ( ! is_array($data))
149. {
150. return FALSE;
151. }
153. // -----------------------------------
154. // Safety - Prevents run-away loops
155. // -----------------------------------
157. // If the script being called happens to have the same
158. // hook call within it a loop can happen
160. if ($this->in_progress == TRUE)
161. {
162. return;
163. }
165. // -----------------------------------
166. // Set file path
167. // -----------------------------------
169. if ( ! isset($data['filepath']) OR ! isset($data['filename']))
170. {
171. return FALSE;
172. }
174. $filepath = APPPATH.$data['filepath'].'/'.$data['filename'];
176. if ( ! file_exists($filepath))
177. {
178. return FALSE;
179. }
181. // -----------------------------------
182. // Set class/function name
183. // -----------------------------------
185. $class = FALSE;
186. $function = FALSE;
187. $params = '';
189. if (isset($data['class']) AND $data['class'] != '')
190. {
191. $class = $data['class'];
192. }
194. if (isset($data['function']))
195. {
196. $function = $data['function'];
197. }
199. if (isset($data['params']))
200. {
201. $params = $data['params'];
202. }
204. if ($class === FALSE AND $function === FALSE)
205. {
206. return FALSE;
207. }
209. // -----------------------------------
210. // Set the in_progress flag
211. // -----------------------------------
213. $this->in_progress = TRUE;
215. // -----------------------------------
216. // Call the requested class and/or function
217. // -----------------------------------
219. if ($class !== FALSE)
220. {
221. if ( ! class_exists($class))
222. {
223. require($filepath);
224. }
226. $HOOK = new $class;
227. $HOOK->$function($params);
228. }
229. else
230. {
231. if ( ! function_exists($function))
232. {
233. require($filepath);
234. }
236. $function($params);
237. }
239. $this->in_progress = FALSE;
240. return TRUE;
241. }
243. }
245. // END CI_Hooks class
247. /* End of file Hooks.php */
248. /* Location: ./system/core/Hooks.php */

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可以看出codeigniter实现钩子机制的方式不够优雅，其实完全可以使用观察者模式来实现钩子机制，将挂载点当做监听的事件。

如何实现, codeigniter