Block API: Add attribute-sourced block attributes on server

src/wp-includes/class-wp-block-type.php

@@ -532,6 +532,87 @@ public function prepare_attributes_for_render( $attributes ) {
 		return $attributes;
 	}
 
+	/**
+	 * Gets attributes from a parsed block.
+	 *
+	 * This method will return an array of the block attributes that includes:
+	 *   - Block attributes serialized in the comment delimiters from the initial parse.
+	 *   - Sourced attributes from the "attribute" source.
+	 *
+	 * This method does not perform and validation of attribute types or default values.
+	 *
+	 * @since TBD
+	 *
+	 * @param array $parsed_block Original parsed array representation of block.
+	 * @return array Block attributes.
+	 */
+	public function get_attributes_from_parsed_block( $parsed_block ) {
+		$attributes = isset( $parsed_block['attrs'] ) ?
+			$parsed_block['attrs'] :
+			array();
+
+		// If there are no attribute definitions for the block type, skip
+		// processing and return verbatim.
+		if ( ! isset( $this->attributes ) ) {
+			return $attributes;
+		}
+
+		// Sourced attributes are extracted from the block's HTML. If there's no
+		// HTML, there's nothing to do.
+		if ( '' === $parsed_block['innerHTML'] ) {
+			return $attributes;
+		}
+
+		foreach ( $this->attributes as $attribute_name => $attribute_definition ) {
+			if (
+				! isset( $attribute_definition['source'] ) ||
+				'attribute' !== $attribute_definition['source'] ||
+				! ( isset( $attribute_definition['attribute'] ) && is_string( $attribute_definition['attribute'] ) ) ||
+				! isset( $attribute_definition['selector'] ) ||
+
+				// @todo what to do if it's in serialized attributes already? Skip for now.
+				isset( $attributes[ $attribute_name ] )
+			) {
+				continue;
+			}
+
+			$processor = WP_HTML_Processor::create_fragment( $parsed_block['innerHTML'] );
+			if ( null === $processor ) {
+				continue;
+			}
+
+			$selector = $attribute_definition['selector'];
+
+			// This is a workaround for a known unsupported selector in a core block.
+			if ( 'a:not([download])' === $selector ) {
+				$selector = 'a';
+			}
+
+			foreach ( $processor->select_all( $selector ) as $_ ) {
+				// This is a workaround for a known unsupported selector in a core block.
+				if (
+					'a:not([download])' === $attribute_definition['selector'] &&
+					null !== $processor->get_attribute( 'download' )
+				) {
+					continue;
+				}
+
+				$value = $processor->get_attribute( $attribute_definition['attribute'] );
+
+				// @todo Should matched selectors without the attribute include null?
+				// if ( null === $value ) {
+				//   continue 2;
+				// }
+
+				// @todo another function validates value types, is that sufficient?
+				$attributes[ $attribute_name ] = $value;
+
+			}
+		}
+
+		return $attributes;
+	}
+
 	/**
 	 * Sets block type properties.
 	 *

src/wp-includes/class-wp-block.php

@@ -186,12 +186,14 @@ public function __construct( $block, $available_context = array(), $registry = n
 	 */
 	public function __get( $name ) {
 		if ( 'attributes' === $name ) {
-			$this->attributes = isset( $this->parsed_block['attrs'] ) ?
-				$this->parsed_block['attrs'] :
-				array();
-
 			if ( ! is_null( $this->block_type ) ) {
-				$this->attributes = $this->block_type->prepare_attributes_for_render( $this->attributes );
+				$this->attributes = $this->block_type->prepare_attributes_for_render(
+					$this->block_type->get_attributes_from_parsed_block( $this->parsed_block )
+				);
+			} else {
+				$this->attributes = isset( $this->parsed_block['attrs'] ) ?
+					$this->parsed_block['attrs'] :
+					array();
 			}
 
 			return $this->attributes;