WordPress: add page template from custom plugin

php

<?php

/**
 * Plugin Name: Page templates from plugin
 * Plugin URI: 
 * Description: Page templates from plugin
 * Version: 1.0.0
 * Author: Pierre Lebedel
 * Author URI: https://www.pierrelebedel.fr
 */

class PageTemplatesFromPlugin {

    private static $instance;

    public $dirpath;
    public $templatespath;
    public $templates;

    public static function getInstance()
    {
        if( is_null(self::$instance) ){
            self::$instance = new PageTemplatesFromPlugin();
        }
        return self::$instance;
    }

    private function __construct() 
    {
        $this->dirpath = plugin_dir_path(__FILE__);
        $this->templatespath = $this->dirpath.'templates/';
        $this->templates = array(
            'new-page-template.php' => "New template 1",
            'new-page-template-bis.php' => "New template 2",
        );

		    add_filter('theme_page_templates', array($this, 'mergeTemplates'));
        add_filter('save_post_page', array($this, 'savePostPage'), 10, 3);
		    add_filter('template_include', array($this, 'useTemplate'));
	} 

	public function mergeTemplates($posts_templates=array())
    {
		$posts_templates = array_merge($posts_templates, $this->templates);
		return $posts_templates;
	}

  public function savePostPage($post_id, $post, $update)
  {
      if( array_key_exists($post->page_template, $this->templates) ){
          update_post_meta($post->ID, '_wp_page_template', $post->page_template);
		  }
  }

	public function useTemplate($template)
  {
		global $post;

		if( !$post ){
			return $template;
		}

    $post_page_template = get_post_meta($post->ID, '_wp_page_template', true);

		if( !array_key_exists($post_page_template, $this->templates) ){
			return $template;
		}

		if(file_exists($this->templatespath.$post_page_template)){
			return $this->templatespath.$post_page_template;
		}

		return $template;
	}

}

add_action('plugins_loaded', array('PageTemplatesFromPlugin', 'getInstance') );