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'));
    }

}