Programmatically get a full page html rendering of a route/page

We provide our content via APIs. At times we use the Views Restful approach, and at times the JSONAPI.

We need to add an API field on nodes that is a full HTML rendering of that node's page according to the theme (technically I just need ... but I'll take the whole doc if I can get it).

I have tried a number of approaches:

I tried using the renderer service. It renders full html from the html.html.twig template but items such as blocks on the page are missing. I guess it doesn't have all the context it needs.

$view_builder = \Drupal::entityTypeManager()->getViewBuilder('node');
$content = $view_builder->view($node);
$build = [
  '#type' => 'html',
  'page' => [
    '#type' => 'page',
    '#theme' => 'page',
    '#title' => $node->get("title")->value,
    'content' => $content,
  ],
];
$page = \Drupal::service('renderer')->renderPlain($build);

Very similarly I tried using twig_render_template. It similarly renders full html but items such as blocks on the page are missing.

$markup = twig_render_template(drupal_get_path('theme', 'neato') . '/templates/base/html.html.twig', array(
  'page' => [
    '#type' => 'page',
    '#theme' => 'page',
    '#title' => $node->get("title")->value,
    'content' => $content,
  ],
  // Needed to prevent notices when Twig debugging is enabled.
  'theme_hook_original' => 'not-applicable',
));
$body = (string) $markup;

As a separate approach, I tried to make a 'subrequest'. With this approach, I get the rendered HTML but it causes fatal early rendering errors such as "A stray renderRoot() invocation is causing bubbling of attached assets to break."

$kernel = \Drupal::service('http_kernel.basic');
$sub_request = \Symfony\Component\HttpFoundation\Request::create("/node/".$value->_entity->id(), 'GET');
$subResponse = $kernel->handle($sub_request, \Symfony\Component\HttpKernel\HttpKernelInterface::SUB_REQUEST);
$html = $subResponse->getContent();

I even tried to mimic the full drupal 'bootstrap'

$autoloader = require '/app/web/autoload.php';
$sub_request = Request::create("/node/".$node->id(), 'GET');
$site_path = DrupalKernel::findSitePath($sub_request);
$kernel = DrupalKernel::createFromRequest($sub_request, $autoloader, 'prod');
$sub_response = $kernel->handle($sub_request, HttpKernelInterface::SUB_REQUEST);
$html = $sub_response->getContent();

I am open to any and all pointers and suggestions. I really appreciate it.

We went a similar path to @johnSmith's rec of using file_get_contents(). Thanks!

TL;DR; When nodes are saved, we use httpClient to make a 2nd request to the site. That response is saved in a field and included in the API. Thx!!

Details

• A new field was added to the associated content types. This field will be used to store the rendered page HTML.

• A custom module was written. When nodes of the associated content types are created or updated, a subsequent request is made using httpClient. This request is made to the node page. That HTML response is saved in the new field.

  //We have saved the configured domain name in which to make this render request to in Drupal's config.

  $config = \Drupal::service('config.factory')->getEditable('myApi.static_server_settings');

  $domain = $config->get('myApi.theme_domain');

  //Request this node's rendered page

  $nid = $node->id();

  $client = \Drupal::httpClient();

  $request = $client->get($domain.'/node/'.$nid);

  $render = (string) $request->getBody();

• Our API was extended to include this field. As a fallback, when the API runs, if the new field is empty that same subsequent httpClient request is made on the fly.

• We built a batch process that updates the 'rendered page' field for all the content. This can be run if the theme is updated. We may eventually trigger it from our theme-building scripts/gulp/CI stuff.