OneForm.php

<?php

namespace Drupal\ctools_wizard_test\Form;

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

/**
 * Simple wizard step form.
 */
class OneForm extends FormBase {
    
    /**
     * Returns a unique string identifying the form.
     *
     * @return string
     *   The unique string identifying the form.
     */
    public function getFormId() {
        return 'ctools_wizard_test_one_form';
    }
    
    /**
     * Form constructor.
     *
     * @param array $form
     *   An associative array containing the structure of the form.
     * @param \Drupal\Core\Form\FormStateInterface $form_state
     *   The current state of the form.
     *
     * @return array
     *   The form structure.
     */
    public function buildForm(array $form, FormStateInterface $form_state) {
        $cached_values = $form_state->getTemporaryValue('wizard');
        $form['one'] = [
            '#title' => $this->t('One'),
            '#type' => 'textfield',
            '#default_value' => !empty($cached_values['one']) ? $cached_values['one'] : '',
        ];
        $form['dynamic'] = [
            '#title' => $this->t('Dynamic value'),
            '#type' => 'item',
            '#markup' => !empty($cached_values['dynamic']) ? $cached_values['dynamic'] : '',
        ];
        return $form;
    }
    
    /**
     * Form submission handler.
     *
     * @param array $form
     *   An associative array containing the structure of the form.
     * @param \Drupal\Core\Form\FormStateInterface $form_state
     *   The current state of the form.
     */
    public function submitForm(array &$form, FormStateInterface $form_state) {
        $keys = [
            'one',
        ];
        $cached_values = $form_state->getTemporaryValue('wizard');
        foreach ($keys as $key) {
            $cached_values[$key] = $form_state->getValue($key);
        }
        $form_state->setTemporaryValue('wizard', $cached_values);
        $this->messenger()
            ->addMessage($this->t('Dynamic value submitted: @value', [
            '@value' => $cached_values['dynamic'],
        ]));
    }

}