Background
I am trying to add page templates from inside of a plugin. For this question, I have trimmed my code down into a test plugin which has two files, the PHP main plugin file, and a PHP template file.
wp-plugins/test-plugin/test-plugin.php
wp-plugins/test-plugin/templates/test-template.php
The plugin has two pieces. First off, I tap into the template_include filter, and I return the path to the template file (test-template.php).
Next, I have a new extension of Walker_Page, called Walker_Page_New in this example. In the file, it is a word for word copy of Walker_Page.
Current Code
test-plugin.php
<?php
/**
* Plugin Name: Test Plugin
* Version: 1.0
* Author: Andy Mercer
* Author URI: http://www.andymercer.net
* License: GPL2
*/
add_filter( 'template_include', 'test_get_template' );
function test_get_template( $template ) {
$template_path = plugin_dir_path( __FILE__ ) . 'templates/test-template.php';
if ( file_exists( $template_path ) ) {
$template = $template_path;
}
return $template;
}
class Walker_Page_New extends Walker_Page {
// THE CODE IN HERE IS AN EXACT COPY OF WALKER_PAGE
// I AM NOT ENTERING IT ALL IN THIS QUESTION BECAUSE IT'S A COUPLE HUNDRED LINES OF CODE
}
test-template.php
<!DOCTYPE HTML>
<html xmlns="http://www.w3.org/1999/xhtml" lang="en">
<head>
...head stuff...
</head>
<body>
<?php if (have_posts()) : while (have_posts()) : the_post(); ?>
<div>
<?php $ancestors = get_ancestors( get_the_ID(), 'page' );
wp_list_pages([
'title_li' => '',
'sort_column' => 'menu_order',
'child_of' => $ancestors[0],
'depth' => 2,
'walker' => 'Walker_Page_New',
]); ?>
</div>
<div>
<?php the_title(); ?>
<?php the_content() ?>
</div>
<?php endwhile; endif; ?>
<?php wp_footer(); ?>
</body>
</html>
Problem
When I then load the page, I only get this error:
Fatal error: Uncaught Error: Using $this when not in object context in C:...\wp-includes\class-wp-walker.php:199
What is triggering this error is the call to wp_list_pages() with a custom Walker. When I remove the Walker, I am fine and everything works as expected.
Research
From looking around, the only specific mention of this I have found is semi-related here: https://github.com/Automattic/amp-wp/issues/412#issuecomment-240871878, where it's stated that using template_include will cause this:
e.g. we would no longer have the $this context in the templates
Question
Is it expected that using template_include will break stuff? Should I use template_redirect instead?
