class BaseFieldFileFormatterBase

abstract class BaseFieldFileFormatterBase extends FormatterBase {
    
    /**
     * The file URL generator.
     *
     * @var \Drupal\Core\File\FileUrlGeneratorInterface
     */
    protected $fileUrlGenerator;
    
    /**
     * Constructs a BaseFieldFileFormatterBase object.
     *
     * @param string $plugin_id
     *   The plugin ID for the formatter.
     * @param mixed $plugin_definition
     *   The plugin implementation definition.
     * @param \Drupal\Core\Field\FieldDefinitionInterface $field_definition
     *   The definition of the field to which the formatter is associated.
     * @param array $settings
     *   The formatter settings.
     * @param string $label
     *   The formatter label display setting.
     * @param string $view_mode
     *   The view mode.
     * @param array $third_party_settings
     *   Any third party settings.
     * @param \Drupal\Core\File\FileUrlGeneratorInterface $file_url_generator
     *   The file URL generator.
     */
    public function __construct($plugin_id, $plugin_definition, FieldDefinitionInterface $field_definition, array $settings, $label, $view_mode, array $third_party_settings, FileUrlGeneratorInterface $file_url_generator) {
        parent::__construct($plugin_id, $plugin_definition, $field_definition, $settings, $label, $view_mode, $third_party_settings);
        $this->fileUrlGenerator = $file_url_generator;
    }
    
    /**
     * {@inheritdoc}
     */
    public static function create(ContainerInterface $container, array $configuration, $plugin_id, $plugin_definition) {
        return new static($plugin_id, $plugin_definition, $configuration['field_definition'], $configuration['settings'], $configuration['label'], $configuration['view_mode'], $configuration['third_party_settings'], $container->get('file_url_generator'));
    }
    
    /**
     * {@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')) {
            $url = $this->fileUrlGenerator
                ->generate($items->getEntity()
                ->getFileUri());
        }
        foreach ($items as $delta => $item) {
            $view_value = $this->viewValue($item);
            if ($url) {
                $elements[$delta] = [
                    '#type' => 'link',
                    '#title' => $view_value,
                    '#url' => $url,
                ];
            }
            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';
    }

}