How to alter routes defined in route_callbacks?

Usually I alter static routes in routes subscriber:

  protected function alterRoutes(RouteCollection $collection) {
    $route = $collection->get('gutenberg.media.load_media');
    if ($route) {
      $route->setDefault('_controller', '\Drupal\gutenberg_adv\Controller\MediaController::loadMedia');
    }
  }

However I this does not work for routes defined in routes_callback, so this does not work for search route for example:

$route = $collection->get('search.view_node_search');
if ($route) {
  $route->setDefault('_controller',
      '\Drupal\wi_misc\Controller\SearchController::view');
}

So how to alter search route controller?

Try to change the weight of the execution of the alterRoutes function. You can override the getSubscribedEvents function:

public static function getSubscribedEvents(): array
{
    return [
        RoutingEvents::ALTER => [
            'onAlterRoutes', 1
        ],
    ];
}

Notice that i call onAlterRoutes instead of alterRoutes. This is because in the parent class this is a wrapper for the called function:

public static function getSubscribedEvents() {
    $events[RoutingEvents::ALTER] = 'onAlterRoutes';
    return $events;
}

The higher your number (= weight) is, the earlier it gets executed (or the other way around). I would try with some high and some low numbers and check the result.

I have never tried it with this specific eventbut i used this method to solve other problems with events in some RouteSubscriber.

If you use drush to clear your cache (which you should after changing code in a RouteSubscriber) then you can also use var_dump($route); to ensure your route does exist and is loaded correctly. The dump will get displayed in the cmd.