codeigniter设置消息回叫功能消息不显示

I am just to learn Codeigniter, in basically i create registration form and validate thier using form validation library and i am using single array to use set rules method and set message below is my code.

$field_data=array(array(
                        'field'=>'username',
                        'label'=>'Userame',
                        'rules'=>'required|callback_check_existing_username1'
                        ),
                  array(
                        'field'=>'email',
                        'label'=>'Email',
                        'rules'=>'required|callback_check_existing_email1'
                        )
                   );
$message=array(
               array(
                    'rules'=>'callback_check_existing_username1',
                    'message'=>'Username Exist'
                    ),
                    array(
                        'rules'=>'callback_check_existing_email1',
                        'message'=>'Email Exist'
                    )
            );
            $this->form_validation->set_rules($field_data);
            $this->form_validation->set_message($message);

above is my code the rules is working and my problem is call back function message is not displaying.