Add support for adding templates and template parts for plugins

[costasovo]

What problem does this address?

Currently, templates are defined within the theme (source theme) and can be edited in the site editor (source custom). There are some use cases where plugins could also use the possibility to add a template. WooCommerce blocks plugin already adds templates for Product Catalog page, Product Page, etc. At MailPoet, we are considering using templates and templates parts for email templates.
WooCommerce Blocks developers documented how they added the templates. As you can see from the docs, adding a template is currently cumbersome.

What is your proposed solution?

This could be solved by adding an API for registering a template. I don't know the internals in detail, so I struggle with proposing the solution.

I was thinking about a function like:

/**
 * @param WP_Block_Template
 */
function register_block_template( WP_Block_Template $template ) {}

or maybe just

function register_block_template( array $args ) {}

The $args would cover parameters for defining the block template.

It could be used for both templates and template_parts.

[gigitux]

It isn't easy for a plugin to register a custom block template. I created a POC (I used @wordpress/create-block for the boilerplate), to show it: https://github.com/gigitux/poc-register-template-plugin/blob/main/poc-register-template-plugin.php.

Also, this is a production class that handles Block Templates for WooCommerce Blocks:

https://github.com/woocommerce/woocommerce-blocks/blob/2b79d3e87dffd5b38d5910ebcf39057e267d190d/src/BlockTemplatesController.php

As you can see, it is complex registering and handles custom block templates for a plugin.

Also, the current POC is bugged because it doesn't load the custom block template from the database after the user saves a new version of the template.

Following my exploration, it is necessary using three filters to register a block template:

• pre_get_block_template
• pre_get_block_file_template
• get_block_templates

Also, it is necessary to write custom logic to read the directory where the templates are placed and parse the filename to get the template's name.

It would be great to have a better way/specific API to register block templates.

[fabiankaegy]

I think this ticket may be a duplicate of #41401

I'm going to leave it open for now since it already has some valuable context here but want to make both sides aware of one another :)