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.
 */
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();
    }
  }

}