class FormController

abstract class FormController {
    use DependencySerializationTrait;
    
    /**
     * The argument resolver.
     *
     * @var \Symfony\Component\HttpKernel\Controller\ArgumentResolverInterface
     */
    protected $argumentResolver;
    
    /**
     * The form builder.
     *
     * @var \Drupal\Core\Form\FormBuilderInterface
     */
    protected $formBuilder;
    
    /**
     * Constructs a new \Drupal\Core\Controller\FormController object.
     *
     * @param \Symfony\Component\HttpKernel\Controller\ArgumentResolverInterface $argument_resolver
     *   The argument resolver.
     * @param \Drupal\Core\Form\FormBuilderInterface $form_builder
     *   The form builder.
     */
    public function __construct(ArgumentResolverInterface $argument_resolver, FormBuilderInterface $form_builder) {
        $this->argumentResolver = $argument_resolver;
        $this->formBuilder = $form_builder;
    }
    
    /**
     * Invokes the form and returns the result.
     *
     * @param \Symfony\Component\HttpFoundation\Request $request
     *   The request object.
     * @param \Drupal\Core\Routing\RouteMatchInterface $route_match
     *   The route match.
     *
     * @return array
     *   The render array that results from invoking the controller.
     */
    public function getContentResult(Request $request, RouteMatchInterface $route_match) {
        $form_arg = $this->getFormArgument($route_match);
        $form_object = $this->getFormObject($route_match, $form_arg);
        // Add the form and form_state to trick the getArguments method of the
        // controller resolver.
        $form_state = new FormState();
        $request->attributes
            ->set('form', []);
        $request->attributes
            ->set('form_state', $form_state);
        $args = $this->argumentResolver
            ->getArguments($request, [
            $form_object,
            'buildForm',
        ]);
        $request->attributes
            ->remove('form');
        $request->attributes
            ->remove('form_state');
        // Remove $form and $form_state from the arguments, and re-index them.
        unset($args[0], $args[1]);
        $form_state->addBuildInfo('args', array_values($args));
        return $this->formBuilder
            ->buildForm($form_object, $form_state);
    }
    
    /**
     * Extracts the form argument string from a request.
     *
     * Depending on the type of form the argument string may be stored in a
     * different request attribute.
     *
     * One example of a route definition is given below.
     * @code
     *   defaults:
     *     _form: Drupal\example\Form\ExampleForm
     * @endcode
     *
     * @param \Drupal\Core\Routing\RouteMatchInterface $route_match
     *   The route match object from which to extract a form definition string.
     *
     * @return string
     *   The form definition string.
     */
    protected abstract function getFormArgument(RouteMatchInterface $route_match);
    
    /**
     * Returns the object used to build the form.
     *
     * @param \Drupal\Core\Routing\RouteMatchInterface $route_match
     *   The route match.
     * @param string $form_arg
     *   Either a class name or a service ID.
     *
     * @return \Drupal\Core\Form\FormInterface
     *   The form object to use.
     */
    protected abstract function getFormObject(RouteMatchInterface $route_match, $form_arg);

}