<? php

namespace Drupal\file\Plugin\Field\FieldFormatter;

use Drupal\Core\Field\FieldDefinitionInterface;
use Drupal\Core\Field\FieldItemInterface;
use Drupal\Core\Field\FieldItemListInterface;
use Drupal\Core\Field\FormatterBase;
use Drupal\Core\Form\FormStateInterface;
use Drupal\Core\Url;

/**
 * Base class for file formatters, which allow to link to the file download URL. 
 */
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. 
   */
  abstract protected function viewValue(FieldItemInterface $item);

  /**
   * {@inheritdoc}
   */
  public static function isApplicable(FieldDefinitionInterface $field_definition) {
    return $field_definition->getTargetEntityTypeId() === 'file';
  }

}
