public static function get_instance()
 {
     if (is_null(self::$instance)) {
         self::$instance = new self();
     }
     return self::$instance;
 }
 function YITH_Product_Slider_Type()
 {
     if (!defined('YWCPS_PREMIUM')) {
         return YITH_Product_Slider_Type::get_instance();
     } else {
         return YITH_Product_Slider_Type_Premium::get_instance();
     }
 }
 public function __construct()
 {
     parent::__construct();
     add_action('admin_init', array($this, 'add_tab_metabox'), 1);
     add_filter('manage_edit-' . $this->_post_type_name . '_columns', array($this, 'edit_columns'));
     add_action('manage_' . $this->_post_type_name . '_posts_custom_column', array($this, 'custom_columns'), 10, 2);
     //Custom Tab Message
     add_filter('post_updated_messages', array($this, 'custom_tab_messages'));
     //register metabox to tab manager
     add_filter('yit_fw_metaboxes_type_args', array($this, 'add_custom_product_slider_metaboxes'));
 }