--- /dev/null
+<?php
+
+namespace Drupal\diff;
+
+use Drupal\Core\Config\ConfigFactoryInterface;
+use Drupal\Core\Routing\RouteMatchInterface;
+use Drupal\Core\Theme\ThemeNegotiatorInterface;
+
+/**
+ * Visual inline layout theme negotiator.
+ *
+ * @package Drupal\diff
+ */
+class VisualDiffThemeNegotiator implements ThemeNegotiatorInterface {
+
+ /**
+ * The config factory.
+ *
+ * @var \Drupal\Core\Config\ConfigFactoryInterface
+ */
+ protected $configFactory;
+
+ /**
+ * VisualDiffThemeNegotiator constructor.
+ *
+ * @param \Drupal\Core\Config\ConfigFactoryInterface $config_factory
+ * The config factory.
+ */
+ public function __construct(ConfigFactoryInterface $config_factory) {
+ $this->configFactory = $config_factory;
+ }
+
+ /**
+ * {@inheritdoc}
+ */
+ public function applies(RouteMatchInterface $routeMatch) {
+ if ($routeMatch->getParameter('filter') !== 'visual_inline') {
+ return FALSE;
+ }
+
+ if (!$this->isDiffRoute($routeMatch)) {
+ return FALSE;
+ }
+
+ if ($this->configFactory->get('diff.settings')->get('general_settings.visual_inline_theme') !== 'default') {
+ return FALSE;
+ }
+
+ return TRUE;
+ }
+
+ /**
+ * {@inheritdoc}
+ */
+ public function determineActiveTheme(RouteMatchInterface $route_match) {
+ return $this->configFactory->get('system.theme')->get('default');
+ }
+
+ /**
+ * Checks if route names for node or other entity are corresponding.
+ *
+ * @param \Drupal\Core\Routing\RouteMatchInterface $route_match
+ * Route match object.
+ *
+ * @return bool
+ * Return TRUE if route name is ok.
+ */
+ protected function isDiffRoute(RouteMatchInterface $route_match) {
+ $regex_pattern = '/^entity\..*\.revisions_diff$/';
+ return $route_match->getRouteName() === 'diff.revisions_diff' ||
+ preg_match($regex_pattern, $route_match->getRouteName());
+ }
+
+}