我是第一次在codeigniter中使用REST API。我在API中有以下函数

public function cource_topics_post() {
        $username = $this->input->post('username');
        //$value=$this->uri->segment(3);
        $this->response(array('status' => 1, 'username' =>  $username ));       
    }

但是

$username = $this->input->post('username');

不起作用需要帮助
我的控制器如下

<?php

header("Access-Control-Allow-Origin: *");
require(APPPATH . '/libraries/REST_Controller.php');

class Admin extends REST_Controller {
    function __construct() {
        parent::__construct();
        $this->load->helper('file');
        $this->load->helper(array('form', 'url'));
    }

    public function cource_topics_post() {
        $username = $this->post('username');
        $password = $this->post('password');
        //$value=$this->uri->segment(3);
        $user = "faizan";
        $pass = "faizan";
        if($username == $user && $password == $pass){
            $this->response(array('status' => 1, 'logged in as' =>  $username ));
        }
        else{
            $this->response(array('status' => 0, 'Could not login' => 'incorrect username password'));
        }
    }

}

我正在使用rest_controller。