Register WordPress page templates outside the theme

DG_PageTemplater.php

<?php
/**
 * This class can be used to register a template outside WordPress theme.
 *
 * NOTE: This class is fork of the original class which was hardcoded to look for 
 * the template in the plugin directory. After the modifications i added 
 * the possiblity new templates to be registered using hook.
 *
 * @author gdarko
 *
 */
class DG_PageTemplater {

	/**
	 * A reference to an instance of this class.
	 */
	private static $instance;

	/**
	 * The array of templates that this plugin tracks.
	 */
	protected $templates;

	/**
	 * Returns an instance of this class.
	 */
	public static function get_instance() {

		if ( null == self::$instance ) {
			self::$instance = new PageTemplater();
		}

		return self::$instance;

	}

	/**
	 * Initializes the plugin by setting filters and administration functions.
	 */
	private function __construct() {

		$this->templates = array();


		// Add a filter to the attributes metabox to inject template into the cache.
		if ( version_compare( floatval( get_bloginfo( 'version' ) ), '4.7', '<' ) ) {

			// 4.6 and older
			add_filter(
				'page_attributes_dropdown_pages_args',
				array( $this, 'register_project_templates' )
			);

		} else {

			// Add a filter to the wp 4.7 version attributes metabox
			add_filter(
				'theme_page_templates', array( $this, 'add_new_template' )
			);

		}

		// Add a filter to the save post to inject out template into the page cache
		add_filter(
			'wp_insert_post_data',
			array( $this, 'register_project_templates' )
		);


		// Add a filter to the template include to determine if the page has our
		// template assigned and return it's path
		add_filter(
			'template_include',
			array( $this, 'view_project_template' )
		);


		// Add your templates to this array
		$this->templates = apply_filters( 'dg_templates', $this->templates );
	}

	/**
	 * Adds our template to the page dropdown for v4.7+
	 *
	 */
	public function add_new_template( $posts_templates ) {
		$posts_templates = array_merge( $posts_templates, $this->templates );

		return $posts_templates;
	}

	/**
	 * Adds our template to the pages cache in order to trick WordPress
	 * into thinking the template file exists where it doens't really exist.
	 */
	public function register_project_templates( $atts ) {

		// Create the key used for the themes cache
		$cache_key = 'page_templates-' . md5( get_theme_root() . '/' . get_stylesheet() );

		// Retrieve the cache list.
		// If it doesn't exist, or it's empty prepare an array
		$templates = wp_get_theme()->get_page_templates();
		if ( empty( $templates ) ) {
			$templates = array();
		}

		// New cache, therefore remove the old one
		wp_cache_delete( $cache_key, 'themes' );

		// Now add our template to the list of templates by merging our templates
		// with the existing templates array from the cache.
		$templates = array_merge( $templates, $this->templates );

		bd_log_write( $templates );

		// Add the modified cache to allow WordPress to pick it up for listing
		// available templates
		wp_cache_add( $cache_key, $templates, 'themes', 1800 );

		return $atts;

	}

	/**
	 * Checks if the template is assigned to the page
	 */
	public function view_project_template( $template ) {
		// Return the search template if we're searching (instead of the template for the first result)
		if ( is_search() ) {
			return $template;
		}

		// Get global post
		global $post;

		// Return template if post is empty
		if ( ! $post ) {
			return $template;
		}

		// Return default template if we don't have a custom one defined
		if ( ! isset( $this->templates[ get_post_meta(
				$post->ID, '_wp_page_template', true
			) ] ) ) {
			return $template;
		}
    
		$file = get_post_meta( $post->ID, '_wp_page_template', true );

		// Just to be safe, we check if the file exist first
		if ( file_exists( $file ) ) {
			return $file;
		} else {
			echo $file;
		}

		// Return template
		return $template;
	}
}
add_action( 'plugins_loaded', array( 'DG_PageTemplater', 'get_instance' ) );

functions.php

<?php
//This is the functions php file from where you will register templates
//This file in most cases will be location in a plugin.

//Require the page templater class
require_once dirname(__FILE__) . 'DG_PageTemplater.php';

/**
 * Registers new template with the Page Templater script
 *
 * @param $templates
 *
 * @return array
 */
function dg_frontend_template_register( $templates ) {
	$templates = array(
		'/full/path/to/page-template-lemonades.php' => 'Lemonades Template',
	);
	return $templates;
}
add_filter( 'dg_templates', 'dg_frontend_template_register' );