class DateTimeComputed

class DateTimeComputed extends TypedData {
    
    /**
     * Cached computed date.
     *
     * @var \DateTime|null
     */
    protected $date = NULL;
    
    /**
     * {@inheritdoc}
     */
    public function __construct(DataDefinitionInterface $definition, $name = NULL, TypedDataInterface $parent = NULL) {
        parent::__construct($definition, $name, $parent);
        if (!$definition->getSetting('date source')) {
            throw new \InvalidArgumentException("The definition's 'date source' key has to specify the name of the date property to be computed.");
        }
    }
    
    /**
     * {@inheritdoc}
     */
    public function getValue() {
        if ($this->date !== NULL) {
            return $this->date;
        }
        
        /** @var \Drupal\Core\Field\FieldItemInterface $item */
        $item = $this->getParent();
        $value = $item->{$this->definition
            ->getSetting('date source')};
        // A date cannot be created from a NULL value.
        if ($value === NULL) {
            return NULL;
        }
        $datetime_type = $item->getFieldDefinition()
            ->getSetting('datetime_type');
        $storage_format = $datetime_type === DateTimeItem::DATETIME_TYPE_DATE ? DateTimeItemInterface::DATE_STORAGE_FORMAT : DateTimeItemInterface::DATETIME_STORAGE_FORMAT;
        try {
            $date = DrupalDateTime::createFromFormat($storage_format, $value, DateTimeItemInterface::STORAGE_TIMEZONE);
            if ($date instanceof DrupalDateTime && !$date->hasErrors()) {
                $this->date = $date;
                // If the format did not include an explicit time portion, then the
                // time will be set from the current time instead. For consistency, we
                // set the time to 12:00:00 UTC for date-only fields. This is used so
                // that the local date portion is the same, across nearly all time
                // zones.
                // @see \Drupal\Component\Datetime\DateTimePlus::setDefaultDateTime()
                // @see http://php.net/manual/datetime.createfromformat.php
                if ($datetime_type === DateTimeItem::DATETIME_TYPE_DATE) {
                    $this->date
                        ->setDefaultDateTime();
                }
            }
        } catch (\Exception $e) {
            // @todo Handle this.
        }
        return $this->date;
    }
    
    /**
     * {@inheritdoc}
     */
    public function setValue($value, $notify = TRUE) {
        $this->date = $value;
        // Notify the parent of any changes.
        if ($notify && isset($this->parent)) {
            $this->parent
                ->onChange($this->name);
        }
    }

}