Block CSS and insert block part into classic theme templates.

[jstuartj]

I'm building a theme framework that can fall back to classic PHP template when needed. The way I understand it, FSE/Block themes load CSS wp_head() not in the body. And we need to use do_blocks() prior executing wp_head() and then echo those blocks in body.

So, I've created the following functions to preload a single or an array of block parts. Render them with do_blocks(), and then parse them into an associative array. Which I can then use to insert them into at point in a classic theme's body.

Is there a better way to accomplish this? Besides building custom blocks and or pattern.

/**
 * Takes an array or single template part and parses dynamic blocks out of 
 * the `post_content` and renders them to an associative array by part slug. 
 * 
 * Used to pre-load block template content when using classic templates. WP 6.0 Block themes, 
 * load CSS in the head. Execute this function before using wp_head(), then echo it in the template
 * body using action: hgs_do_block_part. 
 * 
 * Example:
 * 
 *  Single Part: 
 *    $my_single_block_part = hgs_do_block_parts( 'header');
 *    wp_head();
 * 
 *  Array of Part: 
 *    $my_block_parts = hgs_do_block_parts(['header', 'classic-content','footer']);
 *    wp_head();
 * 
 * @param mixed  Array or single template_part of block template unique identifiers (i.e. part_slug).
 * @return array    Associative array array of post content [ part_slug = "part content" ]
 */
 function hgs_preload_block_parts( $part_slugs ) {
  $part_slugs = is_array( $part_slugs ) ? $part_slugs : [ $part_slugs ];
  foreach ( $part_slugs as $slug ) {
    $template_part = get_block_template( get_stylesheet() . '//' . $slug, 'wp_template_part' );   
    if ( ! is_null( $template_part) ) {
      $content = $template_part->content;
    } else { }
      $parts[ $slug ] = do_blocks($content);
  }
  return $parts;
}

/**
 * Outputs the contents of a template part from an array of parts generated by hgs_get_block_parts().
 * 
 * Example: do_action( 'hgs_do_block_part', 'my-block-part-slug', $hgs_block_parts);
 * 
 * @param string  Template unique identifiers (part_slug).
 * @param string  Associative array array of post content. [ part_slug = "part content" ]
 */
function hgs_do_block_part( $part = null, $block_parts = null ) {
  if ( empty($part) || empty($block_parts) ) { return; } 
  if ( array_key_exists( $part, $block_parts ) ) {
    echo $block_parts[$part];
  } else {
    echo do_blocks("<!-- wp:paragraph {} -->
      <p><strong>Error:</strong> No block template part found.</p>
      <!-- /wp:paragraph -->
      ");
  }
}
add_action( 'hgs_do_block_part', 'hgs_do_block_part', 10, 2 );`