class MY_Controller extends CI_Controller
{
    public function __construct()
    {
        parent::__construct();
        /*判断是否登录，判断当前URL是否是auth/login*/
        if ( ! $this->tank_auth->is_logged_in()
                && ( $this->router->fetch_class() != 'auth' && $this->router->fetch_method() != 'login'))
        {
            $redirect = $this->uri->uri_string();

            if ( $_SERVER['QUERY_STRING'])
            {
                $redirect .= '?' . $_SERVER['QUERY_STRING'];
            }
            /*跳转到用户登陆页面，指定Login后跳转的URL*/
            redirect('auth/login?redirect='.$redirect);
        }   
    }
}

class User extends MY_Controller
{
    function login()
    {
        if ($this->tank_auth->is_logged_in()) { // logged in
            redirect('/');
        } else {
            //other codes here......
            /*判断是否有redirect信息*/
            $data['redirect'] = isset($_GET['redirect']) ? $_GET['redirect'] : '/';
            if ($this->form_validation->run()) {  // validation ok
                if ($this->tank_auth->login(
                        $this->form_validation->set_value('login'),
                        $this->form_validation->set_value('password'),
                        $this->form_validation->set_value('remember'),
                        $data['login_by_username'],
                        $data['login_by_email'])) { // success
                    redirect($data['redirect']);
                } else {
                    //error handling
                }
            }
            $this->load->view("login_form")
        }
    }
/*
Note: 在login_form中注意，提交表单的form地址：

*/
}