Issue #3468145: Allow search page for UI configuration

Closes #3468145

modules/ui/src/Controller/SearchPage.php

+<?php
+
+namespace Drupal\search_api_decoupled_ui\Controller;
+
+use Drupal\Core\Controller\ControllerBase;
+use Drupal\Core\Http\Exception\CacheableNotFoundHttpException;
+use Drupal\search_api_decoupled_ui\SearchApiEndpointUiInterface;
+use Symfony\Component\DependencyInjection\ContainerInterface;
+
+/**
+ * Class SearchPage for dedicated "coupled" search page.
+ */
+class SearchPage extends ControllerBase {
+
+  /**
+   * The block manager.
+   *
+   * @var \Drupal\Core\Block\BlockManagerInterface
+   */
+  protected $blockManager;
+
+  /**
+   * {@inheritDoc}
+   */
+  public static function create(ContainerInterface $container) {
+    $instance = parent::create($container);
+    $instance->blockManager = $container->get('plugin.manager.block');
+    return $instance;
+  }
+
+  /**
+   * Gets the dedicated search page title.
+   *
+   * @param \Drupal\search_api_decoupled_ui\SearchApiEndpointUiInterface $search_api_endpoint
+   *   The search api endpoint.
+   *
+   * @return string
+   *   The page title.
+   */
+  public function title(SearchApiEndpointUiInterface $search_api_endpoint) {
+    if ($search_api_endpoint->withSearchPage()) {
+      return $search_api_endpoint->label();
+    }
+    return $this->t('Page not found');
+  }
+
+  /**
+   * Gets the dedicated search page content.
+   *
+   * @param \Drupal\search_api_decoupled_ui\SearchApiEndpointUiInterface $search_api_endpoint
+   *   The search api endpoint.
+   *
+   * @return array
+   *   The page content as render array.
+   */
+  public function content(SearchApiEndpointUiInterface $search_api_endpoint) {
+    if (!$search_api_endpoint->withSearchPage()) {
+      throw new CacheableNotFoundHttpException($search_api_endpoint);
+    }
+    $search_block = $this->blockManager->createInstance('search_api_endpoint:' . $search_api_endpoint->id());
+    return [
+      '#theme' => 'block',
+      '#configuration' => $search_block->getConfiguration(),
+      '#plugin_id' => $search_block->getPluginId(),
+      '#base_plugin_id' => $search_block->getBaseId(),
+      '#derivative_plugin_id' => $search_block->getDerivativeId(),
+      'content' => $search_block->build(),
+    ];
+  }
+
+}