Editor: Warn about empty templates on the frontend for logged in users.

Adds a new function, `wp_render_empty_block_template_warning`, that renders a warning for logged-in users when a block template is empty.

Reviewed by get_dave, richtabor.
Props vcanales, mikachan, peterwilsoncc, richtabor, get_dave, mrfoxtalbot, matveb, arielmaidana, seifradwane, annezazu.
Fixes #62053.

git-svn-id: https://develop.svn.wordpress.org/trunk@59449 602fd350-edb4-49c9-b593-d223f7449a82

Author: mikachan

src/wp-includes/block-template.php

@@ -17,6 +17,32 @@ function _add_template_loader_filters() {
 	}
 }
 
+/**
+ * Renders a warning screen for empty block templates.
+ *
+ * @since 6.8.0
+ *
+ * @param WP_Block_Template $block_template The block template object.
+ * @return string The warning screen HTML.
+ */
+function wp_render_empty_block_template_warning( $block_template ) {
+	wp_enqueue_style( 'wp-empty-template-alert' );
+	return sprintf(
+		/* translators: %1$s: Block template title. %2$s: Empty template warning message. %3$s: Edit template link. %4$s: Edit template button label. */
+		'<div id="wp-empty-template-alert">
+			<h2>%1$s</h2>
+			<p>%2$s</p>
+			<a href="%3$s" class="wp-element-button">
+				%4$s
+			</a>
+		</div>',
+		esc_html( $block_template->title ),
+		__( 'This page is blank because the template is empty. You can reset or customize it in the Site Editor.' ),
+		get_edit_post_link( $block_template->wp_id, 'site-editor' ),
+		__( 'Edit template' )
+	);
+}
+
 /**
  * Finds a block template with equal or higher specificity than a given PHP template file.
  *
@@ -68,13 +94,18 @@ function locate_block_template( $template, $type, array $templates ) {
 	if ( $block_template ) {
 		$_wp_current_template_id = $block_template->id;
 
-		if ( empty( $block_template->content ) && is_user_logged_in() ) {
-			$_wp_current_template_content =
-			sprintf(
-				/* translators: %s: Template title */
-				__( 'Empty template: %s' ),
-				$block_template->title
-			);
+		if ( empty( $block_template->content ) ) {
+			if ( is_user_logged_in() ) {
+				$_wp_current_template_content = wp_render_empty_block_template_warning( $block_template );
+			} else {
+				if ( $block_template->has_theme_file ) {
+					// Show contents from theme template if user is not logged in.
+					$theme_template = _get_block_template_file( 'wp_template', $block_template->slug );
+					$_wp_current_template_content = file_get_contents( $theme_template['path'] );
+				} else {
+					$_wp_current_template_content = $block_template->content;
+				}
+			}
 		} elseif ( ! empty( $block_template->content ) ) {
 			$_wp_current_template_content = $block_template->content;
 		}

src/wp-includes/css/wp-empty-template-alert.css

@@ -0,0 +1,23 @@
+#wp-empty-template-alert {
+    display: flex;
+    padding: var(--wp--style--root--padding-right, 2rem);
+    min-height: 60vh;
+    flex-direction: column;
+    align-items: center;
+    justify-content: center;
+    gap: var(--wp--style--block-gap, 2rem);
+}
+
+#wp-empty-template-alert > * {
+     max-width: 400px;
+}
+
+#wp-empty-template-alert h2,
+#wp-empty-template-alert p {
+    margin: 0;
+    text-align: center;
+}
+
+#wp-empty-template-alert a {
+    margin-top: 1rem;
+}

src/wp-includes/script-loader.php

@@ -1627,6 +1627,7 @@ function wp_default_styles( $styles ) {
 	$styles->add( 'wp-pointer', "/wp-includes/css/wp-pointer$suffix.css", array( 'dashicons' ) );
 	$styles->add( 'customize-preview', "/wp-includes/css/customize-preview$suffix.css", array( 'dashicons' ) );
 	$styles->add( 'wp-embed-template-ie', "/wp-includes/css/wp-embed-template-ie$suffix.css" );
+	$styles->add( 'wp-empty-template-alert', "/wp-includes/css/wp-empty-template-alert$suffix.css" );
 	$styles->add_data( 'wp-embed-template-ie', 'conditional', 'lte IE 8' );
 
 	// External libraries and friends.
@@ -1809,6 +1810,7 @@ function wp_default_styles( $styles ) {
 		'customize-preview',
 		'login',
 		'site-health',
+		'wp-empty-template-alert',
 		// Includes CSS.
 		'buttons',
 		'admin-bar',