class TextareaWithSummaryWidget

class TextareaWithSummaryWidget extends TextareaWidget {
    
    /**
     * {@inheritdoc}
     */
    public static function defaultSettings() {
        return [
            'rows' => '9',
            'summary_rows' => '3',
            'placeholder' => '',
            'show_summary' => FALSE,
        ] + parent::defaultSettings();
    }
    
    /**
     * {@inheritdoc}
     */
    public function settingsForm(array $form, FormStateInterface $form_state) {
        $element = parent::settingsForm($form, $form_state);
        $element['summary_rows'] = [
            '#type' => 'number',
            '#title' => t('Summary rows'),
            '#default_value' => $this->getSetting('summary_rows'),
            '#description' => $element['rows']['#description'],
            '#required' => TRUE,
            '#min' => 1,
        ];
        $element['show_summary'] = [
            '#type' => 'checkbox',
            '#title' => t('Always show the summary field'),
            '#default_value' => $this->getSetting('show_summary'),
        ];
        return $element;
    }
    
    /**
     * {@inheritdoc}
     */
    public function settingsSummary() {
        $summary = parent::settingsSummary();
        $summary[] = t('Number of summary rows: @rows', [
            '@rows' => $this->getSetting('summary_rows'),
        ]);
        if ($this->getSetting('show_summary')) {
            $summary[] = t('Summary field will always be visible');
        }
        return $summary;
    }
    
    /**
     * {@inheritdoc}
     */
    public function formElement(FieldItemListInterface $items, $delta, array $element, array &$form, FormStateInterface $form_state) {
        $element = parent::formElement($items, $delta, $element, $form, $form_state);
        $display_summary = $items[$delta]->summary || $this->getFieldSetting('display_summary');
        $required = empty($form['#type']) && $this->getFieldSetting('required_summary');
        $element['summary'] = [
            '#type' => $display_summary ? 'textarea' : 'value',
            '#default_value' => $items[$delta]->summary,
            '#title' => t('Summary'),
            '#rows' => $this->getSetting('summary_rows'),
            '#description' => !$required ? $this->t('Leave blank to use trimmed value of full text as the summary.') : '',
            '#attributes' => [
                'class' => [
                    'js-text-summary',
                    'text-summary',
                ],
            ],
            '#prefix' => '<div class="js-text-summary-wrapper text-summary-wrapper">',
            '#suffix' => '</div>',
            '#weight' => -10,
            '#required' => $required,
        ];
        if (!$this->getSetting('show_summary') && !$required) {
            $element['summary']['#attached']['library'][] = 'text/drupal.text';
        }
        return $element;
    }
    
    /**
     * {@inheritdoc}
     */
    public function errorElement(array $element, ConstraintViolationInterface $violation, array $form, FormStateInterface $form_state) {
        $element = parent::errorElement($element, $violation, $form, $form_state);
        return $element === FALSE ? FALSE : $element[$violation->arrayPropertyPath[0]];
    }

}