class NumberWidget

class NumberWidget extends WidgetBase {
    
    /**
     * {@inheritdoc}
     */
    public static function defaultSettings() {
        return [
            'placeholder' => '',
        ] + parent::defaultSettings();
    }
    
    /**
     * {@inheritdoc}
     */
    public function settingsForm(array $form, FormStateInterface $form_state) {
        $element['placeholder'] = [
            '#type' => 'textfield',
            '#title' => t('Placeholder'),
            '#default_value' => $this->getSetting('placeholder'),
            '#description' => t('Text that will be shown inside the field until a value is entered. This hint is usually a sample value or a brief description of the expected format.'),
        ];
        return $element;
    }
    
    /**
     * {@inheritdoc}
     */
    public function settingsSummary() {
        $summary = [];
        $placeholder = $this->getSetting('placeholder');
        if (!empty($placeholder)) {
            $summary[] = t('Placeholder: @placeholder', [
                '@placeholder' => $placeholder,
            ]);
        }
        else {
            $summary[] = t('No placeholder');
        }
        return $summary;
    }
    
    /**
     * {@inheritdoc}
     */
    public function formElement(FieldItemListInterface $items, $delta, array $element, array &$form, FormStateInterface $form_state) {
        $value = isset($items[$delta]->value) ? $items[$delta]->value : NULL;
        $field_settings = $this->getFieldSettings();
        $element += [
            '#type' => 'number',
            '#default_value' => $value,
            '#placeholder' => $this->getSetting('placeholder'),
        ];
        // Set the step for floating point and decimal numbers.
        switch ($this->fieldDefinition
            ->getType()) {
            case 'decimal':
                $element['#step'] = pow(0.1, $field_settings['scale']);
                break;
            case 'float':
                $element['#step'] = 'any';
                break;
        }
        // Set minimum and maximum.
        if (is_numeric($field_settings['min'])) {
            $element['#min'] = $field_settings['min'];
        }
        if (is_numeric($field_settings['max'])) {
            $element['#max'] = $field_settings['max'];
        }
        // Add prefix and suffix.
        if ($field_settings['prefix']) {
            $prefixes = explode('|', $field_settings['prefix']);
            $element['#field_prefix'] = FieldFilteredMarkup::create(array_pop($prefixes));
        }
        if ($field_settings['suffix']) {
            $suffixes = explode('|', $field_settings['suffix']);
            $element['#field_suffix'] = FieldFilteredMarkup::create(array_pop($suffixes));
        }
        return [
            'value' => $element,
        ];
    }
    
    /**
     * {@inheritdoc}
     */
    public function errorElement(array $element, ConstraintViolationInterface $violation, array $form, FormStateInterface $form_state) {
        return $element['value'];
    }

}