class DateRangeItem

class DateRangeItem extends DateTimeItem {
  
  /**
   * Value for the 'datetime_type' setting: store a date and time.
   */
  const DATETIME_TYPE_ALLDAY = 'allday';
  
  /**
   * {@inheritdoc}
   */
  public static function propertyDefinitions(FieldStorageDefinitionInterface $field_definition) {
    $properties['value'] = DataDefinition::create('datetime_iso8601')->setLabel(t('Start date value'))
      ->setRequired(TRUE);
    $properties['start_date'] = DataDefinition::create('any')->setLabel(t('Computed start date'))
      ->setDescription(t('The computed start DateTime object.'))
      ->setComputed(TRUE)
      ->setClass(DateTimeComputed::class)
      ->setSetting('date source', 'value');
    $properties['end_value'] = DataDefinition::create('datetime_iso8601')->setLabel(t('End date value'))
      ->setRequired(TRUE);
    $properties['end_date'] = DataDefinition::create('any')->setLabel(t('Computed end date'))
      ->setDescription(t('The computed end DateTime object.'))
      ->setComputed(TRUE)
      ->setClass(DateTimeComputed::class)
      ->setSetting('date source', 'end_value');
    return $properties;
  }
  
  /**
   * {@inheritdoc}
   */
  public static function schema(FieldStorageDefinitionInterface $field_definition) {
    $schema = parent::schema($field_definition);
    $schema['columns']['value']['description'] = 'The start date value.';
    $schema['columns']['end_value'] = [
      'description' => 'The end date value.',
    ] + $schema['columns']['value'];
    $schema['indexes']['end_value'] = [
      'end_value',
    ];
    return $schema;
  }
  
  /**
   * {@inheritdoc}
   */
  public function storageSettingsForm(array &$form, FormStateInterface $form_state, $has_data) {
    $element = parent::storageSettingsForm($form, $form_state, $has_data);
    $element['datetime_type']['#options'][static::DATETIME_TYPE_ALLDAY] = $this->t('All Day');
    return $element;
  }
  
  /**
   * {@inheritdoc}
   */
  public static function generateSampleValue(FieldDefinitionInterface $field_definition) {
    $type = $field_definition->getSetting('datetime_type');
    // Just pick a date in the past year. No guidance is provided by this Field
    // type.
    $start = REQUEST_TIME - mt_rand(0, 86400 * 365) - 86400;
    $end = $start + 86400;
    if ($type == static::DATETIME_TYPE_DATETIME) {
      $values['value'] = gmdate(DateTimeItemInterface::DATETIME_STORAGE_FORMAT, $start);
      $values['end_value'] = gmdate(DateTimeItemInterface::DATETIME_STORAGE_FORMAT, $end);
    }
    else {
      $values['value'] = gmdate(DateTimeItemInterface::DATE_STORAGE_FORMAT, $start);
      $values['end_value'] = gmdate(DateTimeItemInterface::DATE_STORAGE_FORMAT, $end);
    }
    return $values;
  }
  
  /**
   * {@inheritdoc}
   */
  public function isEmpty() {
    $start_value = $this->get('value')
      ->getValue();
    $end_value = $this->get('end_value')
      ->getValue();
    return ($start_value === NULL || $start_value === '') && ($end_value === NULL || $end_value === '');
  }
  
  /**
   * {@inheritdoc}
   */
  public function onChange($property_name, $notify = TRUE) {
    // Enforce that the computed date is recalculated.
    if ($property_name == 'value') {
      $this->start_date = NULL;
    }
    elseif ($property_name == 'end_value') {
      $this->end_date = NULL;
    }
    parent::onChange($property_name, $notify);
  }

}