class DataReferenceBase

abstract class DataReferenceBase extends TypedData implements DataReferenceInterface {
    
    /**
     * The referenced data.
     *
     * @var \Drupal\Core\TypedData\TypedDataInterface
     */
    protected $target;
    
    /**
     * {@inheritdoc}
     */
    public function getTarget() {
        return $this->target;
    }
    
    /**
     * {@inheritdoc}
     */
    public function getValue() {
        if ($target = $this->getTarget()) {
            return $target->getValue();
        }
    }
    
    /**
     * {@inheritdoc}
     */
    public function setValue($value, $notify = TRUE) {
        $this->target = $this->getTypedDataManager()
            ->create($this->definition
            ->getTargetDefinition(), $value);
        // Notify the parent of any changes.
        if ($notify && isset($this->parent)) {
            $this->parent
                ->onChange($this->name);
        }
    }
    
    /**
     * {@inheritdoc}
     */
    public function getString() {
        return (string) $this->getType() . ':' . $this->getTargetIdentifier();
    }

}