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 = \Drupal::time()->getRequestTime() - 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);
    }

}