class FormTestStorageForm

class FormTestStorageForm extends FormBase {
    
    /**
     * {@inheritdoc}
     */
    public function getFormId() {
        return 'form_test_storage_form';
    }
    
    /**
     * {@inheritdoc}
     */
    public function buildForm(array $form, FormStateInterface $form_state) {
        if ($form_state->isRebuilding()) {
            $form_state->setUserInput([]);
        }
        // Initialize
        $storage = $form_state->getStorage();
        $session = $this->getRequest()
            ->getSession();
        if (empty($storage)) {
            $user_input = $form_state->getUserInput();
            if (empty($user_input)) {
                $session->set('constructions', 0);
            }
            // Put the initial thing into the storage
            $storage = [
                'thing' => [
                    'title' => 'none',
                    'value' => '',
                ],
            ];
            $form_state->setStorage($storage);
        }
        // Count how often the form is constructed.
        $counter = $session->get('constructions');
        $session->set('constructions', ++$counter);
        $this->messenger()
            ->addStatus("Form constructions: " . $counter);
        $form['title'] = [
            '#type' => 'textfield',
            '#title' => 'Title',
            '#default_value' => $storage['thing']['title'],
            '#required' => TRUE,
        ];
        $form['value'] = [
            '#type' => 'textfield',
            '#title' => 'Value',
            '#default_value' => $storage['thing']['value'],
            '#element_validate' => [
                '::elementValidateValueCached',
            ],
        ];
        $form['continue_button'] = [
            '#type' => 'button',
            '#value' => 'Reset',
        ];
        $form['continue_submit'] = [
            '#type' => 'submit',
            '#value' => 'Continue submit',
            '#submit' => [
                '::continueSubmitForm',
            ],
        ];
        $form['submit'] = [
            '#type' => 'submit',
            '#value' => 'Save',
        ];
        // @todo Remove this in https://www.drupal.org/node/2524408, because form
        //   cache immutability is no longer necessary, because we no longer cache
        //   forms during safe HTTP methods. In the meantime, because
        //   Drupal\system\Tests\Form still has test coverage for a poisoned form
        //   cache following a GET request, trick $form_state into caching the form
        //   to keep that test working until we either remove it or change it in
        //   that issue.
        if ($this->getRequest()
            ->get('immutable')) {
            $form_state->addBuildInfo('immutable', TRUE);
            if ($this->getRequest()
                ->get('cache') && $this->getRequest()
                ->isMethodCacheable()) {
                $form_state->setRequestMethod('FAKE');
                $form_state->setCached();
            }
        }
        return $form;
    }
    
    /**
     * {@inheritdoc}
     */
    public function validateForm(array &$form, FormStateInterface $form_state) {
        if ($this->getRequest()
            ->get('cache')) {
            // Manually activate caching, so we can test that the storage keeps working
            // when it's enabled.
            $form_state->setCached();
        }
    }
    
    /**
     * Form element validation handler for 'value' element.
     *
     * Tests updating of cached form storage during validation.
     */
    public function elementValidateValueCached($element, FormStateInterface $form_state) {
        // If caching is enabled and we receive a certain value, change the storage.
        // This presumes that another submitted form value triggers a validation error
        // elsewhere in the form. Form API should still update the cached form storage
        // though.
        if ($this->getRequest()
            ->get('cache') && $form_state->getValue('value') == 'change_title') {
            $form_state->set([
                'thing',
                'changed',
            ], TRUE);
        }
    }
    
    /**
     * {@inheritdoc}
     */
    public function continueSubmitForm(array &$form, FormStateInterface $form_state) {
        $form_state->set([
            'thing',
            'title',
        ], $form_state->getValue('title'));
        $form_state->set([
            'thing',
            'value',
        ], $form_state->getValue('value'));
        $form_state->setRebuild();
    }
    
    /**
     * {@inheritdoc}
     */
    public function submitForm(array &$form, FormStateInterface $form_state) {
        $this->messenger()
            ->addStatus("Title: " . Html::escape($form_state->getValue('title')));
        $this->messenger()
            ->addStatus("Form constructions: " . $this->getRequest()
            ->getSession()
            ->get('constructions'));
        if ($form_state->has([
            'thing',
            'changed',
        ])) {
            $this->messenger()
                ->addStatus("The thing has been changed.");
        }
        $form_state->setRedirect('<front>');
    }

}