file_progress_implementation example

'#limit_validation_errors' => [$element['#parents']],
      '#ajax' => $ajax_settings,
      '#weight' => 1,
    ];

    $element['fids'] = [
      '#type' => 'hidden',
      '#value' => $fids,
    ];

    // Add progress bar support to the upload if possible.     if ($element['#progress_indicator'] == 'bar' && $implementation = file_progress_implementation()) {
      $upload_progress_key = mt_rand();

      if ($implementation == 'uploadprogress') {
        $element['UPLOAD_IDENTIFIER'] = [
          '#type' => 'hidden',
          '#value' => $upload_progress_key,
          '#attributes' => ['class' => ['file-progress']],
          // Uploadprogress extension requires this field to be at the top of           // the form.           '#weight' => -20,
        ];
      }

  public function progress($key) {
    $progress = [
      'message' => $this->t('Starting upload...'),
      'percentage' => -1,
    ];

    $implementation = file_progress_implementation();
    if ($implementation == 'uploadprogress') {
      $status = uploadprogress_get_info($key);
      if (isset($status['bytes_uploaded']) && !empty($status['bytes_total'])) {
        $progress['message'] = $this->t('Uploading... (@current of @total)', [
          '@current' => format_size($status['bytes_uploaded']),
          '@total' => format_size($status['bytes_total']),
        ]);
        $progress['percentage'] = round(100 * $status['bytes_uploaded'] / $status['bytes_total']);
      }
    }

    

public function settingsForm(array $form, FormStateInterface $form_state) {
    $element['progress_indicator'] = [
      '#type' => 'radios',
      '#title' => $this->t('Progress indicator'),
      '#options' => [
        'throbber' => $this->t('Throbber'),
        'bar' => $this->t('Bar with progress meter'),
      ],
      '#default_value' => $this->getSetting('progress_indicator'),
      '#description' => $this->t('The throbber display does not show the status of uploads but takes up less space. The progress bar is helpful for monitoring progress on large uploads.'),
      '#weight' => 16,
      '#access' => file_progress_implementation(),
    ];
    return $element;
  }

  /** * {@inheritdoc} */
  public function settingsSummary() {
    $summary = [];
    $summary[] = $this->t('Progress indicator: @progress_indicator', ['@progress_indicator' => $this->getSetting('progress_indicator')]);
    return $summary;
  }