--- /dev/null
+<?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();
+ }
+ }
+
+}