sessionless-1.x-dev/tests/modules/sessionless_forms_test/src/Form/SessionlessFormsTestForm.php

<?php

declare(strict_types=1);
namespace Drupal\sessionless_forms_test\Form;

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

final class SessionlessFormsTestForm extends FormBase {

  public function getFormId() {
    return 'sessionless_forms_test_form';
  }

  public function buildForm(array $form, FormStateInterface $form_state) {
    $form['text'] = [
      '#type' => 'textfield',
    ];
    $form['actions']['submit'] = [
      '#type' => 'submit',
      '#value' => t('Submit'),
    ];
    // On the first build, there is no user input.
    // Form state can not be cached there, as it's a GET request.
    if ($form_state->getUserInput()) {
      if (!$form_state->get('form_state_test_value')) {
        // On the second build, set the value.
        $form_state->set('form_state_test_value', 'AliceInWonderland');
      }
      else {
        // On the third build, print what was cached.
        $testValue = $form_state->get('form_state_test_value');
        $form['test_value'] = [
          '#plain_text' => "TestValue:$testValue",
        ];
      }
    }
    return $form;
  }

  public function validateForm(array &$form, FormStateInterface $form_state) {
    $form_state->setCached();
    $form_state->setRebuild();
  }

  public function submitForm(array &$form, FormStateInterface $form_state) {}

}