Theme JSON resolver: read theme.json files from the styles/ folder only once

[ramonjd]

What?

Cache reading theme.json files from styles/ folder.

Syncs the following Core patch to Gutenberg:

• Read only once the theme.json files from the styles/ folder. wordpress-develop#6843

Why?

From WordPress/wordpress-develop#6843:

I've realized that, unlike the theme.json files defined by core & theme, we don't cache these — hence we end up reading them a few times from the filesystem.

How?

From WordPress/wordpress-develop#6843:

By using the existing read_json_file function instead of using wp_json_file_decode directly.

Testing Instructions

From WordPress/wordpress-develop#6843:

1. Verify the theme style variations are working as expected.

Go to "Site Editor > Styles" and apply one of them. Verify the changes are reflected in the frontend.

Do the same from the global styles sidebar in the site editor.

2. Verify the block style variations defined via theme.json are working as expected.

Create a partial.json file within the styles/ folder with the following contents:

{
        "$schema": "https://schemas.wp.org/trunk/theme.json",
        "version": 2,
        "title": "Partial",
        "blockTypes": [ "core/group" ],
        "styles": {
                "color": {
                        "background": "aliceblue"
                }
        }
}

Go to any editor, add a group block, and verify there is a "Partial" style variation (Block Settings > Styles). Apply the variation and save the changes. Verify the contents are the expected (background color is aliceblue) — also in the frontend.

Screenshots or screencast

[github-actions]

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Co-authored-by: ramonjd <ramonopoly@git.wordpress.org>
Co-authored-by: aaronrobertshaw <aaronrobertshaw@git.wordpress.org>

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[github-actions]

Flaky tests detected in 0d79048.
Some tests passed with failed attempts. The failures may not be related to this commit but are still reported for visibility. See the documentation for more information.

🔍 Workflow run URL: https://github.com/WordPress/gutenberg/actions/runs/9558154045
📝 Reported issues:

• [Flaky Test] Redirection after template creation #60678 in /test/e2e/specs/site-editor/site-editor-url-navigation.spec.js
• [Flaky Test] submenu click on the arrow interactions #61884 in /test/e2e/specs/editor/blocks/navigation-frontend-interactivity.spec.js

[aaronrobertshaw]

LGTM 👍

Tests as advertised.

✅ Theme style variations are available as before
✅ Block style variations are also correct
✅ Works with child theme definitions

Theme Style Variations	Block Style Variations

[ramonjd]

From WordPress/wordpress-develop#6843 (review)

I wonder if we can also avoid the potential of processing the same decoded files multiple times as well?

I briefly played around with caching decoded, processed variation files in a local class property.

I took "processed" to mean:

• run through static::translate, then
• using the output to generate a new WP_Theme_JSON_Gutenberg object

Pretty naive implementation, by using the variation file path as a key, similar to theme_json_file_cache.

Seems to work with very limited testing, but probably deserves its own PR with test cases. Also, the question of whether to extend caching to other theme data. Caching is hard. 😄

[aaronrobertshaw]

I briefly played around with caching decoded, processed variation files in a local class property.

Sounds similar to the original PR: #62610

I'm sure we can narrow down a solution. This is a good first step though.