public static function get_instance()
 {
     if (!self::$instance) {
         self::$instance = new self();
     }
     return self::$instance;
 }
 /**
  * Class constructor
  */
 private function __construct()
 {
     define('WP_STREAM_NOTIFICATIONS_DIR', WP_STREAM_EXTENSIONS_DIR . 'notifications/');
     // Has trailing slash
     define('WP_STREAM_NOTIFICATIONS_URL', WP_STREAM_URL . 'extensions/notifications/');
     // Has trailing slash
     define('WP_STREAM_NOTIFICATIONS_INC_DIR', WP_STREAM_NOTIFICATIONS_DIR . 'includes/');
     // Has trailing slash
     if (!apply_filters('wp_stream_notifications_load', true)) {
         return;
     }
     add_action('plugins_loaded', array($this, 'load'));
     // Register post type
     require_once WP_STREAM_NOTIFICATIONS_INC_DIR . 'class-wp-stream-notifications-post-type.php';
     WP_Stream_Notifications_Post_Type::get_instance();
 }