Page Template Class to include in plugin

page-templates-for-plugins.php

<?php

class 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 = array(
'filename.php' => 'Template Name',
'filename.php' => 'Template Name #2',
);

}

/**
* 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 );

// 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 ) {

// 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 = plugin_dir_path( __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;

}

}

?>