public static function instance()
 {
     if (is_null(self::$_instance)) {
         self::$_instance = new self();
     }
     return self::$_instance;
 }
 function send_drop()
 {
     check_ajax_referer('mdrop_nonce');
     if (!isset($_POST['message_id'])) {
         wp_send_json_error();
     }
     if (isset($_POST['message_id']) && !count($_POST['message_id'])) {
         wp_send_json_error();
     }
     $mail_ids = $_POST['message_id'];
     $mail_id = reset($_POST['message_id']);
     array_shift($mail_ids);
     $get_attachemt_ids = get_post_meta($mail_id, '_wp_attachment_id');
     $drop = MDROP_Dropbox::instance();
     foreach ($get_attachemt_ids as $key => $get_attachemt_id) {
         $drop->upload_file_dropbox($mail_id, $get_attachemt_id);
     }
     if ($mail_ids) {
         $runing_status = 1;
     } else {
         $runing_status = 0;
     }
     wp_send_json_success(array('runing_status' => $runing_status, 'message_id' => $mail_ids));
 }