FormTestStatePersistForm.php

<?php

namespace Drupal\form_test\Form;

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

/**
 * Form constructor for testing form state persistence.
 *
 * @internal
 */
class FormTestStatePersistForm extends FormBase {
    
    /**
     * {@inheritdoc}
     */
    public function getFormId() {
        return 'form_test_state_persist';
    }
    
    /**
     * {@inheritdoc}
     */
    public function buildForm(array $form, FormStateInterface $form_state) {
        $form['title'] = [
            '#type' => 'textfield',
            '#title' => 'title',
            '#default_value' => 'DEFAULT',
            '#required' => TRUE,
        ];
        $form_state->set('value', 'State persisted.');
        $form['submit'] = [
            '#type' => 'submit',
            '#value' => t('Submit'),
        ];
        return $form;
    }
    
    /**
     * {@inheritdoc}
     */
    public function submitForm(array &$form, FormStateInterface $form_state) {
        $this->messenger()
            ->addStatus($form_state->get('value'));
        $form_state->setRebuild();
    }

}