[Site Editor]: Add fallback template content on creation

lib/compat/wordpress-6.1/block-template-utils.php

@@ -296,3 +296,63 @@ function gutenberg_build_block_template_result_from_post( $post ) {
 	}
 	return $template;
 }
+
+/**
+ * Helper function to get the Template Hierarchy for a given slug.
+ * We need to Handle special cases here like `front-page`, `singular` and `archive` templates.
+ *
+ * Noting that we always add `index` as the last fallback template.
+ *
+ * @param string  $slug            The template slug to be created.
+ * @param boolean $is_custom       Indicates if a template is custom or part of the template hierarchy.
+ * @param string  $template_prefix The template prefix for the created template. This is used to extract the main template type ex. in `taxonomy-books` we extract the `taxonomy`.
+ *
+ * @return array<string> The template hierarchy.
+ */
+function get_template_hierarchy( $slug, $is_custom = false, $template_prefix = '' ) {
+	if ( 'index' === $slug ) {
+		return array( 'index' );
+	}
+	if ( $is_custom ) {
+		return array( 'page', 'singular', 'index' );
+	}
+	if ( 'front-page' === $slug ) {
+		return array( 'front-page', 'home', 'index' );
+	}
+	$template_hierarchy = array( $slug );
+	// Most default templates don't have `$template_prefix` assigned.
+	if ( $template_prefix ) {
+		list($type) = explode( '-', $template_prefix );
+		// We need these checks because we always add the `$slug` above.
+		if ( ! in_array( $template_prefix, array( $slug, $type ), true ) ) {
+			$template_hierarchy[] = $template_prefix;
+		}
+		if ( $slug !== $type ) {
+			$template_hierarchy[] = $type;
+		}
+	}
+	// Handle `archive` template.
+	if (
+		str_starts_with( $slug, 'author' ) ||
+		str_starts_with( $slug, 'taxonomy' ) ||
+		str_starts_with( $slug, 'category' ) ||
+		str_starts_with( $slug, 'tag' ) ||
+		'date' === $slug
+	) {
+		$template_hierarchy[] = 'archive';
+	}
+	// Handle `single` template.
+	if ( 'attachment' === $slug ) {
+		$template_hierarchy[] = 'single';
+	}
+	// Handle `singular` template.
+	if (
+		str_starts_with( $slug, 'single' ) ||
+		str_starts_with( $slug, 'page' ) ||
+		'attachment' === $slug
+	) {
+		$template_hierarchy[] = 'singular';
+	}
+	$template_hierarchy[] = 'index';
+	return $template_hierarchy;
+};

lib/compat/wordpress-6.1/class-gutenberg-rest-templates-controller.php

@@ -10,6 +10,62 @@
  * Base Templates REST API Controller.
  */
 class Gutenberg_REST_Templates_Controller extends WP_REST_Templates_Controller {
+
+	/**
+	 * Registers the controllers routes.
+	 *
+	 * @return void
+	 */
+	public function register_routes() {
+		// Get fallback template content.
+		register_rest_route(
+			$this->namespace,
+			'/' . $this->rest_base . '/lookup',
+			array(
+				array(
+					'methods'             => WP_REST_Server::READABLE,
+					'callback'            => array( $this, 'get_template_fallback' ),
+					'permission_callback' => array( $this, 'get_items_permissions_check' ),
+					'args'                => array(
+						'slug'            => array(
+							'description' => __( 'The slug of the template to get the fallback for', 'gutenberg' ),
+							'type'        => 'string',
+						),
+						'is_custom'       => array(
+							'description' => __( ' Indicates if a template is custom or part of the template hierarchy', 'gutenberg' ),
+							'type'        => 'boolean',
+						),
+						'template_prefix' => array(
+							'description' => __( 'The template prefix for the created template. This is used to extract the main template type ex. in `taxonomy-books` we extract the `taxonomy`', 'gutenberg' ),
+							'type'        => 'string',
+						),
+					),
+				),
+			)
+		);
+		parent::register_routes();
+	}
+
+	/**
+	 * Returns the fallback template for a given slug.
+	 *
+	 * @param WP_REST_Request $request The request instance.
+	 *
+	 * @return WP_REST_Response|WP_Error
+	 */
+	public function get_template_fallback( $request ) {
+		if ( empty( $request['slug'] ) ) {
+			return new WP_Error(
+				'rest_invalid_param',
+				__( 'Invalid slug.', 'gutenberg' ),
+				array( 'status' => 400 )
+			);
+		}
+		$hierarchy         = get_template_hierarchy( $request['slug'], $request['is_custom'], $request['template_prefix'] );
+		$fallback_template = resolve_block_template( $request['slug'], $hierarchy, '' );
+		return rest_ensure_response( $fallback_template );
+	}
+
 	/**
 	 * Returns a list of templates.
 	 *

packages/edit-site/src/components/add-new-template/add-custom-template-modal.js

@@ -192,9 +192,18 @@ function AddCustomTemplateModal( { onClose, onSelect, entityForSuggestions } ) {
 							isBlock
 							as={ Button }
 							onClick={ () => {
-								const { slug, title, description } =
-									entityForSuggestions.template;
-								onSelect( { slug, title, description } );
+								const {
+									slug,
+									title,
+									description,
+									templatePrefix,
+								} = entityForSuggestions.template;
+								onSelect( {
+									slug,
+									title,
+									description,
+									templatePrefix,
+								} );
 							} }
 						>
 							<Text as="span" weight={ 600 }>

packages/edit-site/src/components/add-new-template/new-template.js

@@ -1,6 +1,8 @@
 /**
  * WordPress dependencies
  */
+import apiFetch from '@wordpress/api-fetch';
+import { addQueryArgs } from '@wordpress/url';
 import {
 	DropdownMenu,
 	MenuGroup,
@@ -91,7 +93,19 @@ export default function NewTemplate( { postType } ) {
 
 	async function createTemplate( template, isWPSuggestion = true ) {
 		try {
-			const { title, description, slug } = template;
+			const { title, description, slug, templatePrefix } = template;
+			let templateContent = template.content;
+			// Try to find fallback content from existing templates.
+			if ( ! templateContent ) {
+				const fallbackTemplate = await apiFetch( {
+					path: addQueryArgs( '/wp/v2/templates/lookup', {
+						slug,
+						is_custom: ! isWPSuggestion,
+						template_prefix: templatePrefix,
+					} ),
+				} );
+				templateContent = fallbackTemplate.content;
+			}
 			const newTemplate = await saveEntityRecord(
 				'postType',
 				'wp_template',
@@ -101,6 +115,7 @@ export default function NewTemplate( { postType } ) {
 					slug: slug.toString(),
 					status: 'publish',
 					title,
+					content: templateContent,
 					// This adds a post meta field in template that is part of `is_custom` value calculation.
 					is_wp_suggestion: isWPSuggestion,
 				},

packages/edit-site/src/components/add-new-template/utils.js

@@ -152,7 +152,10 @@ export const usePostTypeMenuItems = ( onClickMenuItem ) => {
 				);
 			}
 			const menuItem = defaultTemplateType
-				? { ...defaultTemplateType }
+				? {
+						...defaultTemplateType,
+						templatePrefix: templatePrefixes[ slug ],
+				  }
 				: {
 						slug: generalTemplateSlug,
 						title: menuItemTitle,
@@ -167,6 +170,7 @@ export const usePostTypeMenuItems = ( onClickMenuItem ) => {
 						icon: icon?.startsWith( 'dashicons-' )
 							? icon.slice( 10 )
 							: post,
+						templatePrefix: templatePrefixes[ slug ],
 				  };
 			const hasEntities = postTypesInfo?.[ slug ]?.hasEntities;
 			// We have a different template creation flow only if they have entities.
@@ -210,6 +214,7 @@ export const usePostTypeMenuItems = ( onClickMenuItem ) => {
 									title,
 									description,
 									slug: `${ templatePrefixes[ slug ] }-${ suggestion.slug }`,
+									templatePrefix: templatePrefixes[ slug ],
 								};
 							},
 						},
@@ -317,7 +322,10 @@ export const useTaxonomiesMenuItems = ( onClickMenuItem ) => {
 				);
 			}
 			const menuItem = defaultTemplateType
-				? { ...defaultTemplateType }
+				? {
+						...defaultTemplateType,
+						templatePrefix: templatePrefixes[ slug ],
+				  }
 				: {
 						slug: generalTemplateSlug,
 						title: menuItemTitle,
@@ -327,6 +335,7 @@ export const useTaxonomiesMenuItems = ( onClickMenuItem ) => {
 							labels.singular_name
 						),
 						icon: blockMeta,
+						templatePrefix: templatePrefixes[ slug ],
 				  };
 			const hasEntities = taxonomiesInfo?.[ slug ]?.hasEntities;
 			// We have a different template creation flow only if they have entities.
@@ -369,6 +378,7 @@ export const useTaxonomiesMenuItems = ( onClickMenuItem ) => {
 									title,
 									description,
 									slug: `${ templatePrefixes[ slug ] }-${ suggestion.slug }`,
+									templatePrefix: templatePrefixes[ slug ],
 								};
 							},
 						},
@@ -455,7 +465,7 @@ export function useAuthorMenuItem( onClickMenuItem ) {
 		( { slug } ) => slug === 'author'
 	);
 	if ( authorInfo.user?.hasEntities ) {
-		authorMenuItem = { ...authorMenuItem };
+		authorMenuItem = { ...authorMenuItem, templatePrefix: 'author' };
 		authorMenuItem.onClick = ( template ) => {
 			onClickMenuItem( {
 				type: 'root',
@@ -494,6 +504,7 @@ export function useAuthorMenuItem( onClickMenuItem ) {
 							title,
 							description,
 							slug: `author-${ suggestion.slug }`,
+							templatePrefix: 'author',
 						};
 					},
 				},