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：

/**
 * CodeIgniter
 *
 * An open source application development framework for PHP 5.1.6 or newer
 *
 * @package     CodeIgniter
 * @author      EllisLab Dev Team
 * @copyright       Copyright (c) 2008 - 2014, EllisLab, Inc.
 * @copyright       Copyright (c) 2014 - 2015, British Columbia Institute of Technology (http://bcit.ca/)
 * @license     http://codeigniter.com/user_guide/license.html
 * [url=home.php?mod=space&uid=17823]@LINK[/url]        http://codeigniter.com
 * @since       Version 1.0
 * @filesource
 */

// ------------------------------------------------------------------------

/**
 * CodeIgniter Hooks Class
 *
 * Provides a mechanism to extend the base system without hacking.
 *
 * @package     CodeIgniter
 * @subpackage  Libraries
 * @category    Libraries
 * @author      EllisLab Dev Team
 * @link        http://codeigniter.com/user_guide/libraries/encryption.html
 */
class CI_Hooks {

    /**
     * Determines wether hooks are enabled
     *
     * @var bool
     */
    var $enabled       = FALSE;
    /**
     * List of all hooks set in config/hooks.php
     *
     * @var array
     */
    var $hooks         = array();
    /**
     * Determines wether hook is in progress, used to prevent infinte loops
     *
     * @var bool
     */
    var $in_progress   = FALSE;

    /**
     * Constructor
     *
     */
    function __construct()
    {
        $this->_initialize();
        log_message('debug', "Hooks Class Initialized");
    }

    // --------------------------------------------------------------------

    /**
     * Initialize the Hooks Preferences
     *
     * @access  private
     * @return  void
     */
    function _initialize()
    {
        $CFG =& load_class('Config', 'core');

        // If hooks are not enabled in the config file
        // there is nothing else to do

        if ($CFG->item('enable_hooks') == FALSE)
        {
            return;
        }

        // Grab the "hooks" definition file.
        // If there are no hooks, we're done.

        if (defined('ENVIRONMENT') AND is_file(APPPATH.'config/'.ENVIRONMENT.'/hooks.php'))
        {
            include(APPPATH.'config/'.ENVIRONMENT.'/hooks.php');
        }
        elseif (is_file(APPPATH.'config/hooks.php'))
        {
            include(APPPATH.'config/hooks.php');
        }


        if ( ! isset($hook) OR ! is_array($hook))
        {
            return;
        }

        $this->hooks =& $hook;
        $this->enabled = TRUE;
    }

    // --------------------------------------------------------------------

    /**
     * Call Hook
     *
     * Calls a particular hook
     *
     * @access  private
     * @param   string  the hook name
     * @return  mixed
     */
    function _call_hook($which = '')
    {
        if ( ! $this->enabled OR ! isset($this->hooks[$which]))
        {
            return FALSE;
        }

        if (isset($this->hooks[$which][0]) AND is_array($this->hooks[$which][0]))
        {
            foreach ($this->hooks[$which] as $val)
            {
                $this->_run_hook($val);
            }
        }
        else
        {
            $this->_run_hook($this->hooks[$which]);
        }

        return TRUE;
    }

    // --------------------------------------------------------------------

    /**
     * Run Hook
     *
     * Runs a particular hook
     *
     * @access  private
     * @param   array   the hook details
     * @return  bool
     */
    function _run_hook($data)
    {
        if ( ! is_array($data))
        {
            return FALSE;
        }

        // -----------------------------------
        // Safety - Prevents run-away loops
        // -----------------------------------

        // If the script being called happens to have the same
        // hook call within it a loop can happen

        if ($this->in_progress == TRUE)
        {
            return;
        }

        // -----------------------------------
        // Set file path
        // -----------------------------------

        if ( ! isset($data['filepath']) OR ! isset($data['filename']))
        {
            return FALSE;
        }

        $filepath = APPPATH.$data['filepath'].'/'.$data['filename'];

        if ( ! file_exists($filepath))
        {
            return FALSE;
        }

        // -----------------------------------
        // Set class/function name
        // -----------------------------------

        $class     = FALSE;
        $function  = FALSE;
        $params        = '';

        if (isset($data['class']) AND $data['class'] != '')
        {
            $class = $data['class'];
        }

        if (isset($data['function']))
        {
            $function = $data['function'];
        }

        if (isset($data['params']))
        {
            $params = $data['params'];
        }

        if ($class === FALSE AND $function === FALSE)
        {
            return FALSE;
        }

        // -----------------------------------
        // Set the in_progress flag
        // -----------------------------------

        $this->in_progress = TRUE;

        // -----------------------------------
        // Call the requested class and/or function
        // -----------------------------------

        if ($class !== FALSE)
        {
            if ( ! class_exists($class))
            {
                require($filepath);
            }

            $HOOK = new $class;
            $HOOK->$function($params);
        }
        else
        {
            if ( ! function_exists($function))
            {
                require($filepath);
            }

            $function($params);
        }

        $this->in_progress = FALSE;
        return TRUE;
    }

}

// END CI_Hooks class

/* End of file Hooks.php */
/* Location: ./system/core/Hooks.php */

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