Provide an API to save the printed document to disk

+  /**
+   * Configure the print engine with the passed entities.
+   *
+   * @param array $entities
+   *   An array of entities.
+   * @param \Drupal\entity_print\Plugin\PrintEngineInterface $print_engine
+   *   The print engine.
+   * @param bool $use_default_css
+   *   TRUE if we want the default CSS included.
+   *
+   * @return \Drupal\entity_print\Renderer\RendererInterface
+   *   A print renderer.
+   */
+  protected function configurePrintEngine(array $entities, PrintEngineInterface $print_engine, $use_default_css) {

I was confused by the method name before I saw what's inside. Especially since it mentions a print engine but the variable used is $renderer. Maybe prepareRenderer() would make more sense?

Looks great now!

Looking pretty good.

1. +++ b/src/PrintBuilder.php
   @@ -89,4 +74,56 @@ class PrintBuilder implements PrintBuilderInterface {
   +    if (!$uri) {
   +      $uri = file_default_scheme() . '://' . $renderer->getFilename($entities) . '.' . $print_engine->getExportType()->getFileExtension();
   +    }
   

   is putting them in public:// by default safe? If someone misses the param of it's a falsey path, it writes them to public where entity access can't be checked?

2. +++ b/src/PrintBuilderInterface.php
   @@ -38,7 +38,27 @@ interface PrintBuilderInterface {
   +   * @param bool $force_download
   +   *   (optional) TRUE to try and force the document download.
   ...
   +  public function savePrintable(array $entities, PrintEngineInterface $print_engine, $uri, $force_download = FALSE, $use_default_css = TRUE);
   

   What does $force_download mean in the context of saving to disk?

When i tried to save a pdf to public files dir i was getting a empty pdf. May be the data is not being rendered. I tried to call render() over the output print object. And there was a error in prepareRenderer() which i ve corrected. Correct me if i'm wrong.

Divine timing Benjy, just when I needed it. Thanks a lot all contributers!

How to call this API directly

I do not want to tweak the module, I build an extension of EntityPrintController and made here the call to savePrintable. I only get it working via a route because of the abundance of parameters. Is there a way to call savePrintable directly, without routing??

Thanks Benyi
to make a PDF of a node I can do the following now:
$print_engine = \Drupal::service('plugin.manager.entity_print.print_engine')->createSelectedInstance('pdf');
$print_builder = \Drupal::service('entity_print.print_builder');
$nodes = \Drupal::entityTypeManager()->getStorage('node')->load($node_id);
$uri = $print_builder->savePrintable([$nodes], $print_engine);

Awesome! Perfect timing. This is exactly what we needed.