Issue #3323856 by robphillips: Autocomplete controller to lookup routes.

dynamic_path_aliases.routing.yml

+dynamic_path_aliases.autocomplete:
+  path: "/path-rewrite/autocomplete"
+  defaults:
+    _controller: Drupal\dynamic_path_aliases\Controller\PathRewriteAutocompleteController::autocomplete
+    _format: json
+  requirements:
+    _permission: "administer dynamic path rewrites"

src/Controller/PathRewriteAutocompleteController.php

+<?php
+
+namespace Drupal\dynamic_path_aliases\Controller;
+
+use Drupal\Component\Utility\Xss;
+use Drupal\Core\Controller\ControllerBase;
+use Drupal\Core\Database\Connection;
+use Symfony\Component\DependencyInjection\ContainerInterface;
+use Symfony\Component\HttpFoundation\JsonResponse;
+use Symfony\Component\HttpFoundation\Request;
+
+/**
+ * Provides path rewrite route autocomplete controller.
+ */
+class PathRewriteAutocompleteController extends ControllerBase {
+
+  /**
+   * The database connection.
+   *
+   * @var \Drupal\Core\Database\Connection
+   */
+  protected Connection $connection;
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function create(ContainerInterface $container) {
+    $instance = parent::create($container);
+    $instance->connection = $container->get('database');
+    return $instance;
+  }
+
+  /**
+   * Autocomplete callback.
+   *
+   * @param \Symfony\Component\HttpFoundation\Request $request
+   *   The request.
+   *
+   * @return \Symfony\Component\HttpFoundation\JsonResponse
+   *   Returns JSON response of results.
+   */
+  public function autocomplete(Request $request): JsonResponse {
+    if (!($input = Xss::filter($request->query->get('q')))) {
+      return new JsonResponse([]);
+    }
+    $query = $this->connection->select('router')
+      ->fields('router', ['name', 'path'])
+      ->range(0, 10);
+    $input = '%' . $query->escapeLike($input) . '%';
+    $query->condition($query->orConditionGroup()
+      ->condition('name', $input, 'LIKE')
+      ->condition('path', $input, 'LIKE'));
+    $query->orderBy('path');
+
+    $items = [];
+    foreach ($query->execute() as $result) {
+      $items[] = [
+        'value' => $result->name,
+        'label' => $result->path,
+      ];
+    }
+    return new JsonResponse(array_values($items));
+  }
+
+}

src/PathRewriteForm.php

@@ -78,6 +78,7 @@ class PathRewriteForm extends EntityForm {
       '#type' => 'textfield',
       '#title' => $this->t('Route name'),
       '#description' => $this->t('Route to rewrite its path. Search by route name or path.'),
+      '#autocomplete_route_name' => 'dynamic_path_aliases.autocomplete',
       '#required' => TRUE,
       '#default_value' => $this->entity->get('route_name'),
     ];