class FormTestStoragePageCacheForm

class FormTestStoragePageCacheForm extends FormBase {
    
    /**
     * {@inheritdoc}
     */
    public function getFormId() {
        return 'form_test_storage_page_cache';
    }
    
    /**
     * {@inheritdoc}
     */
    public function buildForm(array $form, FormStateInterface $form_state) {
        $form['title'] = [
            '#type' => 'textfield',
            '#title' => 'Title',
            '#required' => TRUE,
        ];
        $form['test_build_id_old'] = [
            '#type' => 'item',
            '#title' => 'Old build id',
            '#markup' => 'No old build id',
        ];
        $form['submit'] = [
            '#type' => 'submit',
            '#value' => 'Save',
        ];
        $form['rebuild'] = [
            '#type' => 'submit',
            '#value' => 'Rebuild',
            '#submit' => [
                [
                    $this,
                    'form_test_storage_page_cache_rebuild',
                ],
            ],
        ];
        $form['#after_build'] = [
            [
                $this,
                'form_test_storage_page_cache_old_build_id',
            ],
        ];
        return $form;
    }
    
    /**
     * Form element #after_build callback: output the old form build-id.
     */
    public function form_test_storage_page_cache_old_build_id($form) {
        if (isset($form['#build_id_old'])) {
            $form['test_build_id_old']['#plain_text'] = $form['#build_id_old'];
        }
        return $form;
    }
    
    /**
     * Form submit callback: Rebuild the form and continue.
     */
    public function form_test_storage_page_cache_rebuild($form, FormStateInterface $form_state) {
        $form_state->setRebuild();
    }
    
    /**
     * {@inheritdoc}
     */
    public function validateForm(array &$form, FormStateInterface $form_state) {
        // Test using form cache when re-displaying a form due to validation
        // errors.
        if ($form_state->hasAnyErrors()) {
            $form_state->setCached();
        }
    }
    
    /**
     * {@inheritdoc}
     */
    public function submitForm(array &$form, FormStateInterface $form_state) {
        // Nothing must happen.
    }

}