Elements: Add styles to the footer before the block is rendered

lib/block-supports/elements.php

@@ -6,14 +6,23 @@
  */
 
 /**
- * Render the elements stylesheet.
+ * Get the elements class names.
+ *
+ * @param array $block Block object.
+ * @return string      The unique class name.
+ */
+function gutenberg_get_elements_class_name( $block ) {
+	return 'wp-elements-' . md5( serialize( $block ) );
+}
+
+/**
+ * Update the block content with elements class names.
  *
  * @param  string $block_content Rendered block content.
  * @param  array  $block         Block object.
  * @return string                Filtered block content.
  */
 function gutenberg_render_elements_support( $block_content, $block ) {
-
 	if ( ! $block_content ) {
 		return $block_content;
 	}
@@ -40,17 +49,7 @@ function gutenberg_render_elements_support( $block_content, $block ) {
 		return $block_content;
 	}
 
-	$class_name = wp_unique_id( 'wp-elements-' );
-
-	if ( strpos( $link_color, 'var:preset|color|' ) !== false ) {
-		// Get the name from the string and add proper styles.
-		$index_to_splice = strrpos( $link_color, '|' ) + 1;
-		$link_color_name = substr( $link_color, $index_to_splice );
-		$link_color      = "var(--wp--preset--color--$link_color_name)";
-	}
-	$link_color_declaration = esc_html( safecss_filter_attr( "color: $link_color" ) );
-
-	$style = ".$class_name a{" . $link_color_declaration . ';}';
+	$class_name = gutenberg_get_elements_class_name( $block );
 
 	// Like the layout hook this assumes the hook only applies to blocks with a single wrapper.
 	// Retrieve the opening tag of the first HTML element.
@@ -72,11 +71,61 @@ function gutenberg_render_elements_support( $block_content, $block ) {
 		$content              = substr_replace( $block_content, ' class="' . $class_name . '"', $first_element_offset + strlen( $first_element ) - 1, 0 );
 	}
 
+	return $content;
+}
+
+/**
+ * Render the elements stylesheet.
+ *
+ * In the case of nested blocks we want the parent element styles to be rendered before their descendants.
+ * This solves the issue of an element (e.g.: link color) being styled in both the parent and a descendant:
+ * we want the descendant style to take priority, and this is done by loading it after, in DOM order.
+ *
+ * @param string|null $pre_render   The pre-rendered content. Default null.
+ * @param array       $block The block being rendered.
+ *
+ * @return null
+ */
+function gutenberg_render_elements_support_styles( $pre_render, $block ) {
+	$block_type                    = WP_Block_Type_Registry::get_instance()->get_registered( $block['blockName'] );
+	$skip_link_color_serialization = gutenberg_should_skip_block_supports_serialization( $block_type, 'color', 'link' );
+	if ( $skip_link_color_serialization ) {
+		return null;
+	}
+
+	$link_color = null;
+	if ( ! empty( $block['attrs'] ) ) {
+		$link_color = _wp_array_get( $block['attrs'], array( 'style', 'elements', 'link', 'color', 'text' ), null );
+	}
+
+	/*
+	* For now we only care about link color.
+	* This code in the future when we have a public API
+	* should take advantage of WP_Theme_JSON_Gutenberg::compute_style_properties
+	* and work for any element and style.
+	*/
+	if ( null === $link_color ) {
+		return null;
+	}
+
+	$class_name = gutenberg_get_elements_class_name( $block );
+
+	if ( strpos( $link_color, 'var:preset|color|' ) !== false ) {
+		// Get the name from the string and add proper styles.
+		$index_to_splice = strrpos( $link_color, '|' ) + 1;
+		$link_color_name = substr( $link_color, $index_to_splice );
+		$link_color      = "var(--wp--preset--color--$link_color_name)";
+	}
+	$link_color_declaration = esc_html( safecss_filter_attr( "color: $link_color" ) );
+
+	$style = ".$class_name a{" . $link_color_declaration . ';}';
+
 	gutenberg_enqueue_block_support_styles( $style );
 
-	return $content;
+	return null;
 }
 
 // Remove WordPress core filter to avoid rendering duplicate elements stylesheet.
 remove_filter( 'render_block', 'wp_render_elements_support', 10, 2 );
 add_filter( 'render_block', 'gutenberg_render_elements_support', 10, 2 );
+add_filter( 'pre_render_block', 'gutenberg_render_elements_support_styles', 10, 2 );

phpunit/class-elements-test.php

@@ -13,7 +13,7 @@ class Gutenberg_Elements_Test extends WP_UnitTestCase {
 	 * @return string                String where the unique id classes were replaced with "wp-elements-1".
 	 */
 	private static function make_unique_id_one( $string ) {
-		return preg_replace( '/wp-elements-\d+/', 'wp-elements-1', $string );
+		return preg_replace( '/wp-elements-[a-zA-Z0-9]+/', 'wp-elements-1', $string );
 	}
 
 	/**
@@ -103,4 +103,5 @@ public function test_anchor_paragraph_link_color() {
 			'<p id="anchor" class="wp-elements-1">Hello <a href="http://www.wordpress.org/">WordPress</a>!</p>'
 		);
 	}
+
 }