顾名思义，装载器就是加载元素的，使用CI时，经常加载的有：

$this->load->library()
$this->load->view()
$this->load->model()
$this->load->database()
$this->load->helper()
$this->load->config()
$this->load->add_package_path()

复制代码 代码如下:
/**
 * Loader Class
 *
 * 用户加载views和files，常见的函数有model(),view(),library(),helper()
 *
 * Controller的好助手，$this->load =& load_class('Loader', 'core');，加载了loader，Controller就无比强大了
 */
class CI_Loader {
 protected $_ci_ob_level;
 protected $_ci_view_paths  = array();
 protected $_ci_library_paths = array();
 protected $_ci_model_paths  = array();
 protected $_ci_helper_paths  = array();
 protected $_base_classes  = array(); // Set by the controller class
 protected $_ci_cached_vars  = array();
 protected $_ci_classes   = array();
 protected $_ci_loaded_files  = array();
 
 protected $_ci_models   = array();
 protected $_ci_helpers   = array();
 protected $_ci_varmap   = array('unit_test' => 'unit',
           'user_agent' => 'agent');
 public function __construct()
 {      
                //获取缓冲嵌套级别
  $this->_ci_ob_level  = ob_get_level();
  //library路径
                $this->_ci_library_paths = array(APPPATH, BASEPATH);
                //helper路径
  $this->_ci_helper_paths = array(APPPATH, BASEPATH);
                //model路径
  $this->_ci_model_paths = array(APPPATH);
                //view路径
  $this->_ci_view_paths = array(APPPATH.'views/' => TRUE);
  log_message('debug', "Loader Class Initialized");
 }
 // --------------------------------------------------------------------
 /**
  * 初始化Loader
  *
  */
 public function initialize()
 {
  $this->_ci_classes = array();
  $this->_ci_loaded_files = array();
  $this->_ci_models = array();
                //将is_loaded（common中记录加载核心类函数）加载的核心类交给_base_classes
  $this->_base_classes =& is_loaded();
                //加载autoload.php配置中文件
  $this->_ci_autoloader();
  return $this;
 }
 // --------------------------------------------------------------------
 /**
  * 检测类是否加载
  */
 public function is_loaded($class)
 {
  if (isset($this->_ci_classes[$class]))
  {
   return $this->_ci_classes[$class];
  }
  return FALSE;
 }
 // --------------------------------------------------------------------
 /**
  * 加载Class
  */
 public function library($library = '', $params = NULL, $object_name = NULL)
 {
  if (is_array($library))
  {
   foreach ($library as $class)
   {
    $this->library($class, $params);
   }
   return;
  }
                //如果$library为空或者已经加载。。。
  if ($library == '' OR isset($this->_base_classes[$library]))
  {
   return FALSE;
  }
  if ( ! is_null($params) && ! is_array($params))
  {
   $params = NULL;
  }
  $this->_ci_load_class($library, $params, $object_name);
 }
 // --------------------------------------------------------------------
 /**
  * 加载和实例化model
  */
 public function model($model, $name = '', $db_conn = FALSE)
 {
                //CI支持数组加载多个model
  if (is_array($model))
  {
   foreach ($model as $babe)
   {
    $this->model($babe);
   }
   return;
  }
  if ($model == '')
  {
   return;
  }
  $path = '';
  // 是否存在子目录
  if (($last_slash = strrpos($model, '/')) !== FALSE)
  {
   // The path is in front of the last slash
   $path = substr($model, 0, $last_slash + 1);
   // And the model name behind it
   $model = substr($model, $last_slash + 1);
  }
  if ($name == '')
  {
   $name = $model;
  }
  if (in_array($name, $this->_ci_models, TRUE))
  {
   return;
  }
  $CI =& get_instance();
  if (isset($CI->$name))
  {
   show_error('The model name you are loading is the name of a resource that is already being used: '.$name);
  }
  $model = strtolower($model); //model文件名全小写
  foreach ($this->_ci_model_paths as $mod_path)
  {
   if ( ! file_exists($mod_path.'models/'.$path.$model.'.php'))
   {
    continue;
   }
   if ($db_conn !== FALSE AND ! class_exists('CI_DB'))
   {
    if ($db_conn === TRUE)
    {
     $db_conn = '';
    }
    $CI->load->database($db_conn, FALSE, TRUE);
   }
   if ( ! class_exists('CI_Model'))
   {
    load_class('Model', 'core');
   }
   require_once($mod_path.'models/'.$path.$model.'.php');
   $model = ucfirst($model);
   $CI->$name = new $model();
                        //保存在Loader::_ci_models中，以后可以用它来判断某个model是否已经加载过。
   $this->_ci_models[] = $name;
   return;
  }
  // couldn't find the model
  show_error('Unable to locate the model you have specified: '.$model);
 }
 // --------------------------------------------------------------------
 /**
  * 数据库Loader
  */
 public function database($params = '', $return = FALSE, $active_record = NULL)
 {
  // Grab the super object
  $CI =& get_instance();
  // 是否需要加载db
  if (class_exists('CI_DB') AND $return == FALSE AND $active_record == NULL AND isset($CI->db) AND is_object($CI->db))
  {
   return FALSE;
  }
  require_once(BASEPATH.'database/DB.php');
  if ($return === TRUE)
  {
   return DB($params, $active_record);
  }
  // Initialize the db variable.  Needed to prevent
  // reference errors with some configurations
  $CI->db = '';
  // Load the DB class
  $CI->db =& DB($params, $active_record);
 }
 // --------------------------------------------------------------------
 /**
  * 加载数据库工具类
  */
 public function dbutil()
 {
  if ( ! class_exists('CI_DB'))
  {
   $this->database();
  }
  $CI =& get_instance();
  // for backwards compatibility, load dbforge so we can extend dbutils off it
  // this use is deprecated and strongly discouraged
  $CI->load->dbforge();
  require_once(BASEPATH.'database/DB_utility.php');
  require_once(BASEPATH.'database/drivers/'.$CI->db->dbdriver.'/'.$CI->db->dbdriver.'_utility.php');
  $class = 'CI_DB_'.$CI->db->dbdriver.'_utility';
  $CI->dbutil = new $class();
 }
 // --------------------------------------------------------------------
 /**
  * Load the Database Forge Class
  *
  * @return string
  */
 public function dbforge()
 {
  if ( ! class_exists('CI_DB'))
  {
   $this->database();
  }
  $CI =& get_instance();
  require_once(BASEPATH.'database/DB_forge.php');
  require_once(BASEPATH.'database/drivers/'.$CI->db->dbdriver.'/'.$CI->db->dbdriver.'_forge.php');
  $class = 'CI_DB_'.$CI->db->dbdriver.'_forge';
  $CI->dbforge = new $class();
 }
 // --------------------------------------------------------------------
 /**
  * 加载视图文件
  */
 public function view($view, $vars = array(), $return = FALSE)
 {
  return $this->_ci_load(array('_ci_view' => $view, '_ci_vars' => $this->_ci_object_to_array($vars), '_ci_return' => $return));
 }
 // --------------------------------------------------------------------
 /**
  * 加载普通文件
  */
 public function file($path, $return = FALSE)
 {
  return $this->_ci_load(array('_ci_path' => $path, '_ci_return' => $return));
 }
 // --------------------------------------------------------------------
 /**
  * 设置变量
  *
  * Once variables are set they become available within
  * the controller class and its "view" files.
  *
  */
 public function vars($vars = array(), $val = '')
 {
  if ($val != '' AND is_string($vars))
  {
   $vars = array($vars => $val);
  }
  $vars = $this->_ci_object_to_array($vars);
  if (is_array($vars) AND count($vars) > 0)
  {
   foreach ($vars as $key => $val)
   {
    $this->_ci_cached_vars[$key] = $val;
   }
  }
 }
 // --------------------------------------------------------------------
 /**
  * 检查并获取变量
  */
 public function get_var($key)
 {
  return isset($this->_ci_cached_vars[$key]) "/;*\s*\", "; ", str_replace('<"Unable to load the requested class: ".$class);
     show_error("Unable to load the requested class: ".$class);
    }
    // Safety:  Was the class already loaded by a previous call" class already loaded. Second attempt ignored.");
     return;
    }
    include_once($baseclass);
    include_once($subclass);
    $this->_ci_loaded_files[] = $subclass;
                                //实例化类
    return $this->_ci_init_class($class, config_item('subclass_prefix'), $params, $object_name);
   }
   // 如果不是扩展，和上面类似
   $is_duplicate = FALSE;
   foreach ($this->_ci_library_paths as $path)
   {
    $filepath = $path.'libraries/'.$subdir.$class.'.php';
    // Does the file exist" class already loaded. Second attempt ignored.");
     return;
    }
    include_once($filepath);
    $this->_ci_loaded_files[] = $filepath;
    return $this->_ci_init_class($class, '', $params, $object_name);
   }
  } // END FOREACH
  // 如果还没有找到该class，最后的尝试是该class会不会在同名的子目录下
  if ($subdir == '')
  {
   $path = strtolower($class).'/'.$class;
   return $this->_ci_load_class($path, $params);
  }
  // 加载失败，报错
  if ($is_duplicate == FALSE)
  {
   log_message('error', "Unable to load the requested class: ".$class);
   show_error("Unable to load the requested class: ".$class);
  }
 }
 // --------------------------------------------------------------------
 /**
  * 实例化已经加载的类
  */
 protected function _ci_init_class($class, $prefix = '', $config = FALSE, $object_name = NULL)
 {
  // 是否有类的配置信息
  if ($config === NULL)
  {
   // Fetch the config paths containing any package paths
   $config_component = $this->_ci_get_component('config');
   if (is_array($config_component->_config_paths))
   {
    // Break on the first found file, thus package files
    // are not overridden by default paths
    foreach ($config_component->_config_paths as $path)
    {
     // We test for both uppercase and lowercase, for servers that
     // are case-sensitive with regard to file names. Check for environment
     // first, global next
     if (defined('ENVIRONMENT') AND file_exists($path .'config/'.ENVIRONMENT.'/'.strtolower($class).'.php'))
     {
      include($path .'config/'.ENVIRONMENT.'/'.strtolower($class).'.php');
      break;
     }
     elseif (defined('ENVIRONMENT') AND file_exists($path .'config/'.ENVIRONMENT.'/'.ucfirst(strtolower($class)).'.php'))
     {
      include($path .'config/'.ENVIRONMENT.'/'.ucfirst(strtolower($class)).'.php');
      break;
     }
     elseif (file_exists($path .'config/'.strtolower($class).'.php'))
     {
      include($path .'config/'.strtolower($class).'.php');
      break;
     }
     elseif (file_exists($path .'config/'.ucfirst(strtolower($class)).'.php'))
     {
      include($path .'config/'.ucfirst(strtolower($class)).'.php');
      break;
     }
    }
   }
  }
  if ($prefix == '')
  {       //system下library
   if (class_exists('CI_'.$class))
   {
    $name = 'CI_'.$class;
   }
   elseif (class_exists(config_item('subclass_prefix').$class))
   {       //扩展library
    $name = config_item('subclass_prefix').$class;
   }
   else
   {
    $name = $class;
   }
  }
  else
  {
   $name = $prefix.$class;
  }
  // Is the class name valid"Non-existent class: ".$name);
   show_error("Non-existent class: ".$class);
  }
  // Set the variable name we will assign the class to
  // Was a custom class name supplied?  If so we'll use it
  $class = strtolower($class);
  if (is_null($object_name))
  {
   $classvar = ( ! isset($this->_ci_varmap[$class])) ? $class : $this->_ci_varmap[$class];
  }
  else
  {
   $classvar = $object_name;
  }
  // Save the class name and object name
  $this->_ci_classes[$class] = $classvar;
  // 将初始化的类的实例给CI超级句柄
  $CI =& get_instance();
  if ($config !== NULL)
  {
   $CI->$classvar = new $name($config);
  }
  else
  {
   $CI->$classvar = new $name;
  }
 }
 // --------------------------------------------------------------------
 /**
  * 自动加载器
         *
         * autoload.php配置的自动加载文件有：
         *  | 1. Packages
            | 2. Libraries
            | 3. Helper files
            | 4. Custom config files
            | 5. Language files
            | 6. Models
  */
 private function _ci_autoloader()
 {
  if (defined('ENVIRONMENT') AND file_exists(APPPATH.'config/'.ENVIRONMENT.'/autoload.php'))
  {
   include(APPPATH.'config/'.ENVIRONMENT.'/autoload.php');
  }
  else
  {
   include(APPPATH.'config/autoload.php');
  }
  if ( ! isset($autoload))
  {
   return FALSE;
  }
  // 自动加载packages,也就是将package_path加入到library，model，helper，config
  if (isset($autoload['packages']))
  {
   foreach ($autoload['packages'] as $package_path)
   {
    $this->add_package_path($package_path);
   }
  }
  // 加载config文件
  if (count($autoload['config']) > 0)
  {
   $CI =& get_instance();
   foreach ($autoload['config'] as $key => $val)
   {
    $CI->config->load($val);
   }
  }
  // 加载helper和language
  foreach (array('helper', 'language') as $type)
  {
   if (isset($autoload[$type]) AND count($autoload[$type]) > 0)
   {
    $this->$type($autoload[$type]);
   }
  }
  // 这个好像是为了兼容以前版本的
  if ( ! isset($autoload['libraries']) AND isset($autoload['core']))
  {
   $autoload['libraries'] = $autoload['core'];
  }
  // 加载libraries
  if (isset($autoload['libraries']) AND count($autoload['libraries']) > 0)
  {
   // 加载db
   if (in_array('database', $autoload['libraries']))
   {
    $this->database();
    $autoload['libraries'] = array_diff($autoload['libraries'], array('database'));
   }
   // 加载所有其他libraries
   foreach ($autoload['libraries'] as $item)
   {
    $this->library($item);
   }
  }
  // Autoload models
  if (isset($autoload['model']))
  {
   $this->model($autoload['model']);
  }
 }
 // --------------------------------------------------------------------
 /**
  * 返回由对象属性组成的关联数组
  */
 protected function _ci_object_to_array($object)
 {
  return (is_object($object)) ? get_object_vars($object) : $object;
 }
 // --------------------------------------------------------------------
 /**
  * 获取CI某个组件的实例
  */
 protected function &_ci_get_component($component)
 {
  $CI =& get_instance();
  return $CI->$component;
 }
 // --------------------------------------------------------------------
 /**
  * 处理文件名，这个函数主要是返回正确文件名
  */
 protected function _ci_prep_filename($filename, $extension)
 {
  if ( ! is_array($filename))
  {
   return array(strtolower(str_replace('.php', '', str_replace($extension, '', $filename)).$extension));
  }
  else
  {
   foreach ($filename as $key => $val)
   {
    $filename[$key] = strtolower(str_replace('.php', '', str_replace($extension, '', $val)).$extension);
   }
   return $filename;
  }
 }
}