Dynamic Routes can be defined in routing.yml

Please refer New routing system to know more about Drupal 7 vs Drupal 8. This change record explains the dynamic routing changes since initial implementation in Drupal 8.

Previously, if you wanted to create routes dynamically, you'd need to create a class that extends RouteSubscriberBase and create a service definition tagged with 'event_subscriber' for it to be picked up.

Now you can declare a dynamic routing method directly in your routing.yml file that returns an array of \Symfony\Component\Routing\Route objects or a \Symfony\Component\Routing\RouteCollection object.

Example snippet from my_module.routing.yml:

route_callbacks:
  - '\Drupal\mymodule\Routing\SomeClass::routes'

Before

Takes a RouteCollection and modifies it, returning nothing.
Only $collection->add() should be used here, but all methods are available.

protected function routes(RouteCollection $collection) {
  $directory_path = file_stream_wrapper_get_instance_by_scheme('public')->getDirectoryPath();
  $route = new Route(
    '/' . $directory_path . '/styles/{image_style}/{scheme}',
    array(
      '_controller' => 'Drupal\image\Controller\ImageStyleDownloadController::deliver',
    ),
    array(
      '_access' => 'TRUE',
    )
  );
  $collection->add('image.style_public', $route);
}

After

No parameters, returns a traversable keyed by route name, containing Route objects.
This can either be an associative array keyed by route name of Route objects:

public function routes() {
  $routes = array();
  $routes['mymodule.route_name'] = new Route(
    '/mymodule/route',
    array(
      '_form' => '\Drupal\mymodule\Form\MyForm',
    ),
    array(
      '_permission'  => 'access content',
    )
  );
  return $routes;
}

Or return a RouteCollection object:

public function routes() {
  $route_collection = new RouteCollection();
  $route = new Route(
    '/mymodule/route',
    array(
      '_form' => '\Drupal\mymodule\Form\MyForm',
    ),
    array(
      '_permission'  => 'access content',
    )
  );
  $route_collection->add('mymodule.route_name', $route);
  return $route_collection;
}

If a RouteCollection needs to be modified outside of adding routes, an EventSubscriber for RoutingEvents::ALTER should be used.