private static function init() {
            add_shortcode('single_user', array($this, 'register_single_user_shortcode' ) );
        }

        public function register_user_shortcode($atts) {

            //Settings (data get)
            $santized_atts = shortcode_atts( array(
                'id'        => 1,
                'imagesize' => 'thumbnail',
                'walker'    => ''
            ), $atts );

            //Get data
            $data = grum_get_single_user($santized_atts);

            //Walk
            return $this->shortcode_walker ( array('walker' => $santized_atts['walker'] ) );

        }

        private function shortcode_walker ($args) {
            $object = $args['walker'];
            if ( !isset( $args['walker'] ) && empty( $args['walker'] ) ){
                $walker = new GertrudUserManagerWalker( $args );
                return $walker->walk( $data );
            } else {
                $walker = new $object( $args );
                return $walker->walk( $data );
            }
        }