Code quality check for `theme.json` related APIs

[oandregal]

This issue tracks the current state of theme.json related APIs from a quality point of view, so we can bring up, discuss and address the issues.

WordPress 6.2: shipped

Public API:

• Add new method wp_theme_has_theme_json. #45168 #45380

Performance:

• wp_theme_has_theme_json: cache output #45543
• wp_get_global_settings: cache output #45372 #45971 #45969
• wp_get_global_stylesheet: migrate from transient to object cache #45679
• wp_get_global_styles_svg_filters : migrate from transient to object cache #47460 (done in core as the function was removed from Gutenberg)
• While initially the cache was implemented as persistent across requests for the above methods, concerns were brought up to make it non-persistent and keep testing: #46150

Sync WordPress core <=> Gutenberg:

• Try: bundle WP_Theme_JSON class instead of inheriting per WordPress version #46579
• Try: bundle the WP_Theme_JSON_Resolver class instead of inheriting per WordPress version #46750

WordPress 6.3: shipped

Public API

• Add gutenberg_get_remote_theme_patterns function #49307
• block.json API: consider graduating __experimentalSelector and __experimentalDuotone from experimental to stable #45194
• wp_get_global_styles: for custom props, it should return the CSS format not the shortened internal one #49693
• wp_get_global_styles: return values instead of CSS Custom Properties #49712

WordPress 6.4: ongoing

Performance:

• get_root_layout_rules: remove unnecessary call to sanitize_title #53568
• Compute presets from theme.json: skip those without classes or variables #53574
• Font Face API: use gutenberg_get_global_settings instead of private API #53805
• Optimize compute_style_properties by reducing the number of iterations it performs #51983
• Try/improve get stylesheet performance #53351
• webfonts handler: use wp_get_global_settings instead of private API wordpress-develop#4980
• Try: reduce number of WP_Theme_JSON instances #53357
• Try: get_style_nodes: only return those that will be used #53575

Public API

• Add wp_get_theme_data_template_parts: create public API to access templateParts from theme.json wordpress-develop#4971
• Add wp_get_theme_data_custom_templates wordpress-develop#5031

Next

Algorithm:

• Iterate on absorbing block styles as part of the global styles mechanism #45198

Public API:

• Make sure all current uses cases of WP_Theme_JSON or WP_Theme_JSON_Resolver are covered by the addition of public methods.
• Collocate customTemplates and templateAreas metadata with the actual data they represent (templates and template parts provided by the theme as HTML) and remove them from theme.json. #42732 In testing #38984 we ran into an issue with valid HTML comments that don't belong to a block #47212
• Deprecate/Remove the wp_theme_json_get_style_nodes filter #45172 Filters, together with methods or the theme.json datum, are all part of the public API. We should aim to not expose private implementation details in the public API that prevent us from evolving it.
• wp_get_global_styles: resolver the reference to the value #49715

Performance

• Make theme.json caches persistent across requests: Control whether theme (e.g. theme.json) cache usage is enabled through a dedicated function #45912
• wp_get_global_styles: add object cache.
• Try caching stylesheets generated for the editor https://github.com/WordPress/gutenberg/pull/45679/files#r1024084613
• wp_add_global_styles_for_blocks: review status.
• Styles generation: there are two points for styles, get_stylesheet (renders the top-level styles) and get_styles_for_block (renders the block-level styles). Potential optimizations:
  • top-level styles: remove any operation that is not relevant for top-level.
  • block-level styles: look for ways to only process the theme.json nodes of the blocks in use. At the moment all are processed (see wp_add_global_styles_for_blocks), though only the blocks in use get enqueued.

[oandregal]

Added a section on experimental API that covers __experimentalSelector, __experimentalDuotone, and __experimentalStyle.

[oandregal]

#45168 is ready for review.

[oandregal]

Added a section about performance.