Template Part Block: Use get_block_file_template for rendering

[ockham]

What?

Change the Template Part block's render method (render_block_core_template_part) to use get_block_file_template rather than a number of lower-level functions.

Why?

Higher-level functions such as get_block_file_template apply filters (such as the eponymous get_block_file_template) that otherwise aren't invoked. (This is relevant e.g. for #51449.)

Furthermore, it can be argued that if a high-level function is available to what otherwise requires calling a number of lower-level functions, it's preferable to use the high-level function (unless there are side effects and/or performance issues, of course) 😄

How?

By using get_block_file_template (which returns a WP_Block_Template object) in the code branch that loads a block template from a theme file, thus supplanting calls to the lower-level _get_block_template_file and _inject_theme_attribute_in_block_template_content functions.

Note that get_block_file_template invokes both of them in its function body. (_inject_theme_attribute_in_block_template_content is called via _build_block_template_result_from_file).

Testing Instructions

Verify that Template Parts still work as before:

• On the frontend, with an unmodified block theme;
• In the editor -- loading, modifying, and saving;
• On the frontend, with modifications to the template part.

[felixarntz]

Thanks @ockham!

[spacedmonkey]

The new function doesn't seem to
Support parent themes. Did you test with a parent theme?

[ockham]

The new function doesn't seem to Support parent themes. Did you test with a parent theme?

I just did, and it seemed to work fine in my testing. (I made a quick child theme of TT3 locally and verified that it picked up the parent's template parts; I then added a modified header.html template part to the child and verified that it overrode the parent's.)

I don't quite see why get_block_file_template wouldn't support child themes 🤔 -- after all, it's using _get_block_template_file to locate the file, just like we used to do directly in the block's render method. But please LMK if I'm missing something!

[ockham]

Since this PR has approval and parent themes seem to be working fine for me, I'll go ahead and merge (in order to unblock #51449).

Happy to tweak or revert in case we find that it's breaking something after all!

[johnbillion]

This introduced a regression: #56013