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' => $this->t('Placeholder'),
      '#default_value' => $this->getSetting('placeholder'),
      '#description' => $this->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[] = $this->t('Placeholder: @placeholder', [
        '@placeholder' => $placeholder,
      ]);
    }
    else {
      $summary[] = $this->t('No placeholder');
    }
    return $summary;
  }
  
  /**
   * {@inheritdoc}
   */
  public function formElement(FieldItemListInterface $items, $delta, array $element, array &$form, FormStateInterface $form_state) {
    $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'];
  }

}