class DecimalFormatter

class DecimalFormatter extends NumericFormatterBase {
  
  /**
   * {@inheritdoc}
   */
  public static function defaultSettings() {
    return [
      'thousand_separator' => '',
      'decimal_separator' => '.',
      'scale' => 2,
      'prefix_suffix' => TRUE,
    ] + parent::defaultSettings();
  }
  
  /**
   * {@inheritdoc}
   */
  public function settingsForm(array $form, FormStateInterface $form_state) {
    $elements = parent::settingsForm($form, $form_state);
    $elements['decimal_separator'] = [
      '#type' => 'select',
      '#title' => $this->t('Decimal marker'),
      '#options' => [
        '.' => $this->t('Decimal point'),
        ',' => $this->t('Comma'),
      ],
      '#default_value' => $this->getSetting('decimal_separator'),
      '#weight' => 5,
    ];
    $elements['scale'] = [
      '#type' => 'number',
      '#title' => $this->t('Scale', [], [
        'context' => 'decimal places',
      ]),
      '#min' => 0,
      '#max' => 10,
      '#default_value' => $this->getSetting('scale'),
      '#description' => $this->t('The number of digits to the right of the decimal.'),
      '#weight' => 6,
    ];
    return $elements;
  }
  
  /**
   * {@inheritdoc}
   */
  protected function numberFormat($number) {
    return number_format($number, $this->getSetting('scale'), $this->getSetting('decimal_separator'), $this->getSetting('thousand_separator'));
  }

}