FormTestRedirectForm.php

<?php

namespace Drupal\form_test\Form;

use Drupal\Core\Form\FormBase;
use Drupal\Core\Form\FormStateInterface;
use Drupal\Core\Url;

/**
 * Form builder to detect form redirect.
 *
 * @internal
 */
class FormTestRedirectForm extends FormBase {
    
    /**
     * {@inheritdoc}
     */
    public function getFormId() {
        return 'form_test_redirect';
    }
    
    /**
     * {@inheritdoc}
     */
    public function buildForm(array $form, FormStateInterface $form_state) {
        $form['redirection'] = [
            '#type' => 'checkbox',
            '#title' => t('Use redirection'),
        ];
        $form['destination'] = [
            '#type' => 'textfield',
            '#title' => t('Redirect destination'),
            '#states' => [
                'visible' => [
                    ':input[name="redirection"]' => [
                        'checked' => TRUE,
                    ],
                ],
            ],
        ];
        $form['submit'] = [
            '#type' => 'submit',
            '#value' => t('Submit'),
        ];
        return $form;
    }
    
    /**
     * {@inheritdoc}
     */
    public function submitForm(array &$form, FormStateInterface $form_state) {
        if (!$form_state->isValueEmpty('redirection')) {
            if (!$form_state->isValueEmpty('destination')) {
                // The destination is a random URL, so we can't use routed URLs.
                // @todo Revist this in https://www.drupal.org/node/2418219.
                $form_state->setRedirectUrl(Url::fromUserInput('/' . $form_state->getValue('destination')));
            }
        }
        else {
            $form_state->disableRedirect();
        }
    }

}