class BaseFieldFileFormatterBase

abstract class BaseFieldFileFormatterBase extends FormatterBase {
    
    /**
     * {@inheritdoc}
     */
    public static function defaultSettings() {
        $settings['link_to_file'] = FALSE;
        return $settings;
    }
    
    /**
     * {@inheritdoc}
     */
    public function settingsForm(array $form, FormStateInterface $form_state) {
        $form = parent::settingsForm($form, $form_state);
        $form['link_to_file'] = [
            '#title' => $this->t('Link this field to the file download URL'),
            '#type' => 'checkbox',
            '#default_value' => $this->getSetting('link_to_file'),
        ];
        return $form;
    }
    
    /**
     * {@inheritdoc}
     */
    public function viewElements(FieldItemListInterface $items, $langcode) {
        $elements = [];
        $url = NULL;
        // Add support to link to the entity itself.
        if ($this->getSetting('link_to_file')) {
            // @todo Wrap in file_url_transform_relative(). This is currently
            // impossible. See below.
            $url = file_create_url($items->getEntity()->uri->value);
        }
        foreach ($items as $delta => $item) {
            $view_value = $this->viewValue($item);
            if ($url) {
                $elements[$delta] = [
                    '#type' => 'link',
                    '#title' => $view_value,
                    '#url' => Url::fromUri($url),
                    // @todo Remove the 'url.site' cache context by using a relative file
                    // URL (file_url_transform_relative()). This is currently impossible
                    // because #type => link requires a Url object, and Url objects do not
                    // support relative URLs: they require fully qualified URLs. Fix in
                    // https://www.drupal.org/node/2646744.
'#cache' => [
                        'contexts' => [
                            'url.site',
                        ],
                    ],
                ];
            }
            else {
                $elements[$delta] = is_array($view_value) ? $view_value : [
                    '#markup' => $view_value,
                ];
            }
        }
        return $elements;
    }
    
    /**
     * Generate the output appropriate for one field item.
     *
     * @param \Drupal\Core\Field\FieldItemInterface $item
     *   One field item.
     *
     * @return mixed
     *   The textual output generated.
     */
    protected abstract function viewValue(FieldItemInterface $item);
    
    /**
     * {@inheritdoc}
     */
    public static function isApplicable(FieldDefinitionInterface $field_definition) {
        return $field_definition->getTargetEntityTypeId() === 'file';
    }

}