Drupal 8.0.0 beta 12. More info: https://www.drupal.org/node/2514176

core/modules/node/src/ParamConverter/NodePreviewConverter.php

@@ -0,0 +1,57 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\node\ParamConverter\NodePreviewConverter.
+ */
+
+namespace Drupal\node\ParamConverter;
+
+use Drupal\Core\Entity\EntityManagerInterface;
+use Drupal\user\PrivateTempStoreFactory;
+use Symfony\Component\Routing\Route;
+use Drupal\Core\ParamConverter\ParamConverterInterface;
+
+/**
+ * Provides upcasting for a node entity in preview.
+ */
+class NodePreviewConverter implements ParamConverterInterface {
+
+  /**
+   * Stores the tempstore factory.
+   *
+   * @var \Drupal\user\PrivateTempStoreFactory
+   */
+  protected $tempStoreFactory;
+
+  /**
+   * Constructs a new NodePreviewConverter.
+   *
+   * @param \Drupal\user\PrivateTempStoreFactory $temp_store_factory
+   *   The factory for the temp store object.
+   */
+  public function __construct(PrivateTempStoreFactory $temp_store_factory) {
+    $this->tempStoreFactory = $temp_store_factory;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function convert($value, $definition, $name, array $defaults) {
+    $store = $this->tempStoreFactory->get('node_preview');
+    if ($form_state = $store->get($value)) {
+      return $form_state->getFormObject()->getEntity();
+    }
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function applies($definition, $name, Route $route) {
+    if (!empty($definition['type']) && $definition['type'] == 'node_preview') {
+      return TRUE;
+    }
+    return FALSE;
+  }
+
+}