大家知道CodeIgniter框架式MVC分层的，通常大家把业务逻辑写到Controller中，而Model只负责和数据库打交道。

但是随着业务越来越复杂，controller越来越臃肿，举一个简单的例子，比如说用户下订单，这必然会有一系列的操作：更新购物车、添加订单记录、会员添加积分等等，且下订单的过程可能在多种场景出现，如果这样的代码放controller中则很臃肿难以复用，如果放model会让持久层和业务层耦合。现在公司的项目就是，很多人将一些业务逻辑写到model中去了，model中又调其它model，也就是业务层和持久层相互耦合。这是极其不合理的，会让model难以维护，且方法难以复用。

是不是可以考虑在controller和model中加一个业务层service，由它来负责业务逻辑，封装好的调用接口可以被controller复用。

这样各层的任务就明确了：
Model(DAO)：数据持久层的工作，对数据库的操作都封装在这。
Service：业务逻辑层，负责业务模块的逻辑应用设计，controller中就可以调用service的接口实现业务逻辑处理，提高了通用的业务逻辑的复用性，设计到具体业务实现会调用Model的接口。
Controller：控制层，负责具体业务流程控制，这里调用service层，将数据返回到视图
View：负责前端页面展示，与Controller紧密联系。

<?php
class MY_Service
{
    public function __construct()
    {
        log_message('debug', "Service Class Initialized");
        
    }
 
    function __get($key)
    {
        $CI = & get_instance();
        return $CI->$key;
    }
}

<?php 
class MY_Loader extends CI_Loader
{
    /**
	 * List of loaded sercices
	 *
	 * @var array
	 * @access protected
	 */
	protected $_ci_services = array();
	/**
	 * List of paths to load sercices from
	 *
	 * @var array
	 * @access protected
	 */
	protected $_ci_service_paths		= array();
    
    /**
     * Constructor
     * 
     * Set the path to the Service files
     */
    public function __construct()
    {
        
        parent::__construct();
        load_class('Service','core');
        $this->_ci_service_paths = array(APPPATH);
    }
    
    /**
     * Service Loader
     * 
     * This function lets users load and instantiate classes.
	 * It is designed to be called from a user's app controllers.
	 *
	 * @param	string	the name of the class
	 * @param	mixed	the optional parameters
	 * @param	string	an optional object name
	 * @return	void
     */
    public function service($service = '', $params = NULL, $object_name = NULL)
    {
        if(is_array($service))
        {
            foreach($service as $class)
            {
                $this->service($class, $params);
            }
            
            return;
        }
        
        if($service == '' or isset($this->_ci_services[$service])) {
            return FALSE;
        }
 
        if(! is_null($params) && ! is_array($params)) {
            $params = NULL;
        }
        
        $subdir = '';
 
        // Is the service in a sub-folder? If so, parse out the filename and path.
        if (($last_slash = strrpos($service, '/')) !== FALSE)
        {
                // The path is in front of the last slash
                $subdir = substr($service, 0, $last_slash + 1);
 
                // And the service name behind it
                $service = substr($service, $last_slash + 1);
        }
        
        foreach($this->_ci_service_paths as $path)
        {
            
            $filepath = $path .'service/'.$subdir.$service.'.php';
            
            if ( ! file_exists($filepath))
            {
                continue;
            }
            
            include_once($filepath);
            
            $service = strtolower($service);
 
            if (empty($object_name))
            {
                $object_name = $service;
            }
            
            $service = ucfirst($service);
            $CI = &get_instance();
            if($params !== NULL)
            {
                $CI->$object_name = new $service($params);
            }
            else
            {
                $CI->$object_name = new $service();
            }
            
            $this->_ci_services[] = $object_name;
            
            return;
        }
    }
}

<?php
class User extends CI_Controller
{
    public function __construct() 
    {
		
        parent::__construct();
        $this->load->service('user_service');
    }
    
    public function login()
    {
        $name = 'codetc.com';
        $psw = 'password';
        print_r($this->user_service->login($name, $psw));
    }
}

<?php
class User_service extends MY_Service
{
    public function __construct()
    {
        parent::__construct();
        $this->load->model('user_model');
    }
    
    public function login($name, $password)
    {
        $user = $this->user_model->get_user_by_where($name, $password);
        
        //.....
        //.....
        //.....
        
        return $user;
    }
}

<?php
class User_model extends CI_Model
{
    public function __construct()
    {
        parent::__construct();
    }
    
    public function get_user_by_where($name, $password)
    {
        
        //$this->db
        //......
        //......
        return array('id' => 1, 'name' => 'mckee');
    }
}