HtmlFormController.php

<?php

namespace Drupal\Core\Controller;

use Drupal\Core\Form\FormBuilderInterface;
use Drupal\Core\Routing\RouteMatchInterface;
use Drupal\Core\DependencyInjection\ClassResolverInterface;
use Symfony\Component\HttpKernel\Controller\ArgumentResolverInterface;

/**
 * Wrapping controller for forms that serve as the main page body.
 */
class HtmlFormController extends FormController {
    
    /**
     * The class resolver.
     *
     * @var \Drupal\Core\DependencyInjection\ClassResolverInterface
     */
    protected $classResolver;
    
    /**
     * Constructs a new \Drupal\Core\Controller\HtmlFormController object.
     *
     * @param \Symfony\Component\HttpKernel\Controller\ArgumentResolverInterface $argument_resolver
     *   The argument resolver.
     * @param \Drupal\Core\Form\FormBuilderInterface $form_builder
     *   The form builder.
     * @param \Drupal\Core\DependencyInjection\ClassResolverInterface $class_resolver
     *   The class resolver.
     */
    public function __construct(ArgumentResolverInterface $argument_resolver, FormBuilderInterface $form_builder, ClassResolverInterface $class_resolver) {
        parent::__construct($argument_resolver, $form_builder);
        $this->classResolver = $class_resolver;
    }
    
    /**
     * {@inheritdoc}
     */
    protected function getFormArgument(RouteMatchInterface $route_match) {
        return $route_match->getRouteObject()
            ->getDefault('_form');
    }
    
    /**
     * {@inheritdoc}
     */
    protected function getFormObject(RouteMatchInterface $route_match, $form_arg) {
        return $this->classResolver
            ->getInstanceFromDefinition($form_arg);
    }

}