Experiment: Auto-inserting blocks on the frontend

lib/experimental/blocks.php

@@ -100,3 +100,77 @@ function gutenberg_register_metadata_attribute( $args ) {
 	return $args;
 }
 add_filter( 'register_block_type_args', 'gutenberg_register_metadata_attribute' );
+
+/**
+ * Auto-insert a block as another block's first or last inner block.
+ *
+ * @param array $parsed_block The block being rendered.
+ */
+function gutenberg_auto_insert_child_block( $parsed_block ) {
+	// TODO: Implement an API for users to set the following two parameters.
+	$block_name     = 'core/comment-template';
+	$block_position = 'last-child';
+
+	if ( $block_name === $parsed_block['blockName'] ) {
+		// parse_blocks( '<!-- wp:avatar {"size":40,"style":{"border":{"radius":"10px"}}} /-->' )[0].
+		$inserted_block = array(
+			'blockName'    => 'core/avatar',
+			'attrs'        => array(
+				'size'  => 40,
+				'style' => array(
+					'border' => array( 'radius' => '10px' ),
+				),
+			),
+			'innerHTML'    => '',
+			'innerContent' => array(),
+		);
+
+		if ( 'first-child' === $block_position ) {
+			array_unshift( $parsed_block['innerBlocks'], $inserted_block );
+			// Since WP_Block::render() iterates over `inner_content` (rather than `inner_blocks`)
+			// when rendering blocks, we also need to prepend the new block to that array.
+			array_unshift( $parsed_block['innerContent'], $inserted_block );
+		} elseif ( 'last-child' === $block_position ) {
+			array_push( $parsed_block['innerBlocks'], $inserted_block );
+			// Since WP_Block::render() iterates over `inner_content` (rather than `inner_blocks`)
+			// when rendering blocks, we also need to append the new block to that array.
+			array_push( $parsed_block['innerContent'], $inserted_block );
+		}
+	}
+	return $parsed_block;
+}
+add_filter( 'render_block_data', 'gutenberg_auto_insert_child_block', 10, 1 );
+
+/**
+ * Auto-insert blocks relative to a given block.
+ *
+ * @param string $block_content The block content.
+ * @param array  $block         The full block, including name and attributes.
+ */
+function gutenberg_auto_insert_blocks( $block_content, $block ) {
+	// TODO: Implement an API for users to set the following two parameters.
+	$block_name     = 'core/post-content';
+	$block_position = 'after';
+
+	// Can we avoid infinite loops?
+
+	if ( $block_name === $block['blockName'] ) {
+		$inserted_block_markup = <<<END
+<!-- wp:social-links -->
+<ul class="wp-block-social-links"><!-- wp:social-link {"url":"https://wordpress.org","service":"wordpress"} /--></ul>
+<!-- /wp:social-links -->'
+END;
+
+		$inserted_blocks  = parse_blocks( $inserted_block_markup );
+		$inserted_content = render_block( $inserted_blocks[0] );
+
+		if ( 'before' === $block_position ) {
+			$block_content = $inserted_content . $block_content;
+		} elseif ( 'after' === $block_position ) {
+			$block_content = $block_content . $inserted_content;
+		}
+	}
+
+	return $block_content;
+}
+add_filter( 'render_block', 'gutenberg_auto_insert_blocks', 10, 2 );

phpunit/blocks/render-comment-template-test.php

@@ -142,6 +142,9 @@ public function test_inner_block_inserted_by_render_block_data_is_retained() {
 			return $parsed_block;
 		};
 
+		// Remove auto-insertion filter so it won't collide.
+		remove_filter( 'render_block_data', 'gutenberg_auto_insert_child_block' );
+
 		add_filter( 'render_block_data', $render_block_data_callback, 10, 1 );
 		$parsed_blocks = parse_blocks(
 			'<!-- wp:comments --><!-- wp:comment-template --><!-- wp:comment-content /--><!-- /wp:comment-template --><!-- /wp:comments -->'
@@ -154,6 +157,8 @@ public function test_inner_block_inserted_by_render_block_data_is_retained() {
 		);
 		$block->render();
 		remove_filter( 'render_block_data', $render_block_data_callback );
+		// Add back auto-insertion filter.
+		add_filter( 'render_block_data', 'gutenberg_auto_insert_child_block', 10, 1 );
 
 		$this->assertSame( 5, $render_block_callback->get_call_count() );