class SimpleTextFormatter

class SimpleTextFormatter extends FormatterBase {
    
    /**
     * {@inheritdoc}
     */
    public function viewElements(FieldItemListInterface $items, $langcode) {
        $elements = [];
        foreach ($items as $delta => $item) {
            $elements[$delta] = [
                // We wrap the fieldnote content up in a div tag.
'#type' => 'html_tag',
                '#tag' => 'div',
                // This text is auto-XSS escaped.  See docs for the html_tag element.
'#value' => $item->value,
                // Let's give the note a nice sticky-note CSS appearance.
'#attributes' => [
                    'class' => 'stickynote',
                ],
                // ..And this is the CSS for the stickynote.
'#attached' => [
                    'library' => [
                        'field_permission_example/fieldnote_sticky',
                    ],
                ],
            ];
        }
        return $elements;
    }

}