Add hooks to indicate details of the template and template parts being rendered

[bobbingwide]

What problem does this address?

When developing a new theme it can be difficult to tell which template or template part is being rendered.
I added some debug information at the top of each of my templates to help identify the rendered template and template parts on the front end. eg

<!-- wp:html -->
<div class="WP_DEBUG">404.html</div>
<!-- /wp:html -->

If I wanted to check whether or not the template was being loaded from the file I would change the text in the div by adding a time stamp.

I subsequently attempted to develop a single block plugin ( https://github.com/bobbingwide/sb-debug-block ) that would semi-automate the information gathering. The block would not render anything when WP_DEBUG wasn't true.

In order to obtain the information I needed to display the debug information, I found I had to extend Gutenberg to add a couple of action hooks.
I propose that these action hooks become a standard part of Gutenberg/WordPress core.

These hooks will enable

1. The level of debugging / problem determination that I want to achieve.
2. The potential for server side rendering code to make decisions based upon the context.

What is your proposed solution?

Add two action hooks: rendering_template and rendering_template_part.

/lib/full-site-editing/template-loader.php

rendering_template will pass information about the selected template to the action hook.

function gutenberg_override_query_template( $template, $type, array $templates = array() ) {
        ...
        $block_template = gutenberg_resolve_template( $type, $templates );
     /**
     * Fires before the template is rendered.
     *
     * Allows extensions to follow template processing.
     * Called before the template is rendered.
     *
     * @param object $current_template The current template object.
     * @param string $type      Sanitized filename without extension.
     * @param array  $templates A list of template candidates, in descending order of priority.
     */
      do_action( 'rendering_template', $block_template, $type, $templates);

/packages/block-library/src/template-part/index.php

which becomes /build/block-library/blocks/template-part.php

Add two do_action hooks for rendering_template_part; one before the template part is rendered and the other after.
On the first call the attributes to the template part are passed.

 /**
     * Fires before/after the template part is rendered.
     *
     * Allows extensions to follow template part processing.
     * Called before and after each template part is rendered.
     *
     * @param array|null $attributes The template part attributes. Null after rendering.
     * @param array $seen_ids The current template part nesting.
     */
	do_action( 'rendering_template_part', $attributes, $seen_ids );

on the second call, the $attributes argument is null and the $seen_ids array no longer contains the template part that was rendered.

 /** This action is documented in template-part.php */
	do_action( 'rendering_template_part', null, $seen_ids );

This allows action hook function to track the template part nesting and un-nesting.

For more information on my prototyped solution see bobbingwide/sb-debug-block#2

[skorasaurus]

Sharing my support for this concept in general although I'm personally agnostic of the means to accomplish this at this point.

A few different plugins like show-current-template and what-the-file displays which template is being used in the wp-admin bar for the currently displayed page in php-based/"classic" themes like shown below.

show-current-template, query-monitor, and what-the-file do not require WP_DEBUG to be enabled; I will use them on my production sites if I need to quickly check that a template is being properly loaded.

show-current-template relies on the php function get_included_files which is not applicable for FSE, block-based themes.

The popular query-monitor plugin also includes this template information as well cc (@johnbillion who may be interested in this discussion)