class ContainerDemo

class ContainerDemo extends DemoBase {
    
    /**
     * {@inheritdoc}
     */
    public function buildForm(array $form, FormStateInterface $form_state) {
        $form['description'] = [
            '#type' => 'item',
            '#markup' => $this->t('This form example demonstrates container elements: details, fieldset and container.'),
        ];
        // Details containers replace D7's collapsible field sets.
        $form['author'] = [
            '#type' => 'details',
            '#title' => 'Author Info (type = details)',
        ];
        $form['author']['name'] = [
            '#type' => 'textfield',
            '#title' => $this->t('Name'),
        ];
        $form['author']['pen_name'] = [
            '#type' => 'textfield',
            '#title' => $this->t('Pen Name'),
        ];
        // Conventional field set.
        $form['book'] = [
            '#type' => 'fieldset',
            '#title' => $this->t('Book Info (type = fieldset)'),
        ];
        $form['book']['title'] = [
            '#type' => 'textfield',
            '#title' => $this->t('Title'),
        ];
        $form['book']['publisher'] = [
            '#type' => 'textfield',
            '#title' => $this->t('Publisher'),
        ];
        // Containers have no visual display but wrap any contained elements in a
        // div tag.
        $form['accommodation'] = [
            '#type' => 'container',
        ];
        $form['accommodation']['title'] = [
            '#type' => 'html_tag',
            '#tag' => 'p',
            '#value' => $this->t('Special Accommodations (type = container)'),
        ];
        $form['accommodation']['diet'] = [
            '#type' => 'textfield',
            '#title' => $this->t('Dietary Restrictions'),
        ];
        $form['actions'] = [
            '#type' => 'actions',
        ];
        $form['actions']['submit'] = [
            '#type' => 'submit',
            '#value' => $this->t('Submit'),
        ];
        return $form;
    }
    
    /**
     * {@inheritdoc}
     */
    public function getFormId() {
        return 'form_api_example_container_demo';
    }

}