class EntityReference

class EntityReference extends DataReferenceBase {
    
    /**
     * The entity ID.
     *
     * @var int|string
     */
    protected $id;
    
    /**
     * Gets the definition of the referenced entity.
     *
     * @return \Drupal\Core\Entity\TypedData\EntityDataDefinitionInterface
     *   The reference target's definition.
     */
    public function getTargetDefinition() {
        return $this->definition
            ->getTargetDefinition();
    }
    
    /**
     * Checks whether the target entity has not been saved yet.
     *
     * @return bool
     *   TRUE if the entity is new, FALSE otherwise.
     */
    public function isTargetNew() {
        // If only an ID is given, the reference cannot be a new entity.
        return !isset($this->id) && isset($this->target) && $this->target
            ->getValue()
            ->isNew();
    }
    
    /**
     * {@inheritdoc}
     */
    public function getTarget() {
        if (!isset($this->target) && isset($this->id)) {
            // If we have a valid reference, return the entity's TypedData adapter.
            $entity = \Drupal::entityTypeManager()->getStorage($this->getTargetDefinition()
                ->getEntityTypeId())
                ->load($this->id);
            $this->target = isset($entity) ? $entity->getTypedData() : NULL;
        }
        return $this->target;
    }
    
    /**
     * {@inheritdoc}
     */
    public function getTargetIdentifier() {
        if (isset($this->id)) {
            return $this->id;
        }
        elseif ($entity = $this->getValue()) {
            return $entity->id();
        }
    }
    
    /**
     * {@inheritdoc}
     */
    public function setValue($value, $notify = TRUE) {
        unset($this->target);
        unset($this->id);
        // Both the entity ID and the entity object may be passed as value. The
        // reference may also be unset by passing NULL as value.
        if (!isset($value)) {
            $this->target = NULL;
        }
        elseif ($value instanceof EntityInterface) {
            $this->target = $value->getTypedData();
        }
        elseif (!is_scalar($value) || $this->getTargetDefinition()
            ->getEntityTypeId() === NULL) {
            throw new \InvalidArgumentException('Value is not a valid entity.');
        }
        else {
            $this->id = $value;
        }
        // Notify the parent of any changes.
        if ($notify && isset($this->parent)) {
            $this->parent
                ->onChange($this->name);
        }
    }
    
    /**
     * {@inheritdoc}
     */
    public function getString() {
        if ($entity = $this->getValue()) {
            return $entity->label();
        }
        return '';
    }

}