trait MediaElementFieldFormatterTrait in MediaElement 8

trait MediaElementFieldFormatterTrait {

  /**
   * {@inheritdoc}
   */
  public static function defaultSettings() {
    return parent::defaultSettings() + [
      'download_link' => FALSE,
      'download_text' => '',
      'preload' => 'none',
    ];
  }

  /**
   * {@inheritdoc}
   */
  public function settingsForm(array $form, FormStateInterface $form_state) {
    return parent::settingsForm($form, $form_state) + [
      'preload' => [
        '#type' => 'select',
        '#title' => $this
          ->t('Preload'),
        '#description' => $this
          ->t('The preload attribute specifies if and how the author thinks that the video should be loaded when the page loads.'),
        '#default_value' => $this
          ->getSetting('preload'),
        '#options' => [
          'auto' => 'auto',
          'metadata' => 'metadata',
          'none' => 'none',
        ],
      ],
      'download_link' => [
        '#title' => $this
          ->t('Download Link'),
        '#type' => 'checkbox',
        '#default_value' => $this
          ->getSetting('download_link'),
      ],
      'download_text' => [
        '#title' => $this
          ->t('Download Text'),
        '#type' => 'textfield',
        '#default_value' => $this
          ->getSetting('download_text'),
        '#states' => [
          'visible' => [
            ':input[name*="download_link"]' => [
              'checked' => TRUE,
            ],
          ],
        ],
      ],
    ];
  }

  /**
   * {@inheritdoc}
   */
  public function settingsSummary() {
    $summary = parent::settingsSummary();
    $for_download = $this->settings['download_link'];
    $preload = $this->settings['preload'];
    $summary[] = $this
      ->t('Provide Download Link: %download', [
      '%download' => $for_download ? $this
        ->t('yes') : $this
        ->t('no'),
    ]);
    if ($for_download) {
      $summary[] = $this
        ->t('Download Link Text: %link_text', [
        '%link_text' => $this->settings['download_text'],
      ]);
    }
    $summary[] = $this
      ->t('Preload: %preload', [
      '%preload' => $preload,
    ]);
    return $summary;
  }

  /**
   * {@inheritdoc}
   */
  public function viewElements(FieldItemListInterface $items, $langcode) {
    $elements = parent::viewElements($items, $langcode);
    $library_source = \Drupal::config('mediaelement.settings')
      ->get('library_settings.library_source');

    // Attach the MediaElement library to the elements as well as settings.
    foreach ($elements as &$element) {
      $element['#attributes']
        ->setAttribute('preload', $this->settings['preload']);
      $element['#download_link'] = $this->settings['download_link'];
      $element['#download_text'] = $this->settings['download_text'];
      $element['#attached']['library'][] = "mediaelement/mediaelement_{$library_source}";
    }
    return $elements;
  }

}