class UserNameFormatter

class UserNameFormatter extends FormatterBase {
    
    /**
     * {@inheritdoc}
     */
    public static function defaultSettings() {
        $options = parent::defaultSettings();
        $options['link_to_entity'] = TRUE;
        return $options;
    }
    
    /**
     * {@inheritdoc}
     */
    public function settingsForm(array $form, FormStateInterface $form_state) {
        $form = parent::settingsForm($form, $form_state);
        $form['link_to_entity'] = [
            '#type' => 'checkbox',
            '#title' => $this->t('Link to the user'),
            '#default_value' => $this->getSetting('link_to_entity'),
        ];
        return $form;
    }
    
    /**
     * {@inheritdoc}
     */
    public function viewElements(FieldItemListInterface $items, $langcode) {
        $elements = [];
        foreach ($items as $delta => $item) {
            
            /** @var \Drupal\user\UserInterface $user */
            if ($user = $item->getEntity()) {
                if ($this->getSetting('link_to_entity')) {
                    $elements[$delta] = [
                        '#theme' => 'username',
                        '#account' => $user,
                        '#link_options' => [
                            'attributes' => [
                                'rel' => 'user',
                            ],
                        ],
                        '#cache' => [
                            'tags' => $user->getCacheTags(),
                        ],
                    ];
                }
                else {
                    $elements[$delta] = [
                        '#markup' => $user->getDisplayName(),
                        '#cache' => [
                            'tags' => $user->getCacheTags(),
                        ],
                    ];
                }
            }
        }
        return $elements;
    }
    
    /**
     * {@inheritdoc}
     */
    public static function isApplicable(FieldDefinitionInterface $field_definition) {
        return $field_definition->getTargetEntityTypeId() === 'user' && $field_definition->getName() === 'name';
    }

}