Themes: Introduces block-based template parts for Classic themes.

Allows Classic / Hybrid themes to use block-based template parts without using complete block-based templates.

* Exposes the Site Editor's template parts UI
* Adds Appearance > "Template Parts" menu
* Enabled within the theme via adding a theme support for `'block-template-parts'`
{{{#!php
add_theme_support( 'block-template-parts' );
}}}

This is a backport from Gutenberg.[WordPress/gutenberg#42729 See WordPress/gutenberg PR 42729].

Follow-up to [52330], [52069], [52178].

Props mamaduka, fabiankaegy, poena, scruffian, manfcarlo, bernhard-reiter, hellofromTonya.
See #56467.
Built from https://develop.svn.wordpress.org/trunk@54176

wp-admin/menu.php

@@ -214,12 +214,20 @@
 	);
 }
 
+if ( ! wp_is_block_theme() && current_theme_supports( 'block-template-parts' ) ) {
+	$submenu['themes.php'][6] = array(
+		__( 'Template Parts' ),
+		'edit_theme_options',
+		'site-editor.php?postType=wp_template_part',
+	);
+}
+
 $customize_url = add_query_arg( 'return', urlencode( remove_query_arg( wp_removable_query_args(), wp_unslash( $_SERVER['REQUEST_URI'] ) ) ), 'customize.php' );
 
 // Hide Customize link on block themes unless a plugin or theme
 // is using 'customize_register' to add a setting.
 if ( ! wp_is_block_theme() || has_action( 'customize_register' ) ) {
-	$position = wp_is_block_theme() ? 7 : 6;
+	$position = ( wp_is_block_theme() || current_theme_supports( 'block-template-parts' ) ) ? 7 : 6;
 
 	$submenu['themes.php'][ $position ] = array( __( 'Customize' ), 'customize', esc_url( $customize_url ), '', 'hide-if-no-customize' );
 }

wp-admin/site-editor.php

@@ -19,10 +19,15 @@
 	);
 }
 
-if ( ! wp_is_block_theme() ) {
+if ( ! ( current_theme_supports( 'block-template-parts' ) || wp_is_block_theme() ) ) {
 	wp_die( __( 'The theme you are currently using is not compatible with Full Site Editing.' ) );
 }
 
+$is_template_part_editor = isset( $_GET['postType'] ) && 'wp_template_part' === sanitize_key( $_GET['postType'] );
+if ( ! wp_is_block_theme() && ! $is_template_part_editor ) {
+	wp_die( __( 'The theme you are currently using is not compatible with the Site Editor.' ) );
+}
+
 /**
  * Do a server-side redirection if missing `postType` and `postId`
  * query args when visiting Site Editor.
@@ -64,14 +69,24 @@ static function( $classes ) {
 
 $block_editor_context = new WP_Block_Editor_Context( array( 'name' => 'core/edit-site' ) );
 $custom_settings      = array(
-	'siteUrl'                  => site_url(),
-	'postsPerPage'             => get_option( 'posts_per_page' ),
-	'styles'                   => get_block_editor_theme_styles(),
-	'defaultTemplateTypes'     => $indexed_template_types,
-	'defaultTemplatePartAreas' => get_allowed_block_template_part_areas(),
-	'__unstableHomeTemplate'   => $home_template,
+	'siteUrl'                   => site_url(),
+	'postsPerPage'              => get_option( 'posts_per_page' ),
+	'styles'                    => get_block_editor_theme_styles(),
+	'defaultTemplateTypes'      => $indexed_template_types,
+	'defaultTemplatePartAreas'  => get_allowed_block_template_part_areas(),
+	'supportsLayout'            => WP_Theme_JSON_Resolver::theme_has_support(),
+	'supportsTemplatePartsMode' => ! wp_is_block_theme() && current_theme_supports( 'block-template-parts' ),
+	'__unstableHomeTemplate'    => $home_template,
 );
 
+/**
+ * Home template resolution is not needed when block template parts are supported.
+ * Set the value to `true` to satisfy the editor initialization guard clause.
+ */
+if ( $custom_settings['supportsTemplatePartsMode'] ) {
+	$custom_settings['__unstableHomeTemplate'] = true;
+}
+
 // Add additional back-compat patterns registered by `current_screen` et al.
 $custom_settings['__experimentalAdditionalBlockPatterns']          = WP_Block_Patterns_Registry::get_instance()->get_all_registered( true );
 $custom_settings['__experimentalAdditionalBlockPatternCategories'] = WP_Block_Pattern_Categories_Registry::get_instance()->get_all_registered( true );

wp-includes/functions.php

@@ -5261,7 +5261,7 @@ function wp_widgets_add_menu() {
 	}
 
 	$menu_name = __( 'Widgets' );
-	if ( wp_is_block_theme() ) {
+	if ( wp_is_block_theme() || current_theme_supports( 'block-template-parts' ) ) {
 		$submenu['themes.php'][] = array( $menu_name, 'edit_theme_options', 'widgets.php' );
 	} else {
 		$submenu['themes.php'][7] = array( $menu_name, 'edit_theme_options', 'widgets.php' );

wp-includes/theme.php

@@ -3843,6 +3843,13 @@ function create_initial_theme_features() {
 			'show_in_rest' => true,
 		)
 	);
+	register_theme_feature(
+		'block-template-parts',
+		array(
+			'description'  => __( 'Whether a theme uses block-based template parts.' ),
+			'show_in_rest' => true,
+		)
+	);
 	register_theme_feature(
 		'custom-background',
 		array(

wp-includes/version.php

@@ -16,7 +16,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '6.1-alpha-54175';
+$wp_version = '6.1-alpha-54176';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.