willow-custom-template-single.php

<?php

// factory.php

$page = new q\theme\child\controller\page();
$page->hooks();

// page.php

namespace q\theme\child\controller;

class page {

	function __construct(){

		// ... empty ##

	}

	function hooks(){

		// add template view ##
		\add_filter( 'willow/view/custom', function( $array ){
			return array_merge(
				[ 	
					'page.willow'      => [ 
						'template'			=> 'page.willow',
						'name'  			=> 'Theme : Page',
					],
				],
				$array
			);
		});

	}

}