class EntityWrapper

class EntityWrapper extends TypedData implements IteratorAggregate, ComplexDataInterface, ContextAwareInterface {

  /**
   * The name.
   *
   * @var string
   */
  protected $name;

  /**
   * The parent data structure.
   *
   * @var mixed
   */
  protected $parent;

  /**
   * The referenced entity type.
   *
   * @var string
   */
  protected $entityType;

  /**
   * The entity ID if no 'id source' is used.
   *
   * @var string
   */
  protected $id;

  /**
   * Implements TypedDataInterface::__construct().
   */
  public function __construct(array $definition) {
    $this->definition = $definition + array(
      'constraints' => array(),
    );
    $this->entityType = isset($this->definition['constraints']['entity type']) ? $this->definition['constraints']['entity type'] : NULL;
  }

  /**
   * Implements TypedDataInterface::getValue().
   */
  public function getValue() {
    $source = $this
      ->getIdSource();
    $id = $source ? $source
      ->getValue() : $this->id;
    return $id ? entity_load($this->entityType, $id) : NULL;
  }

  /**
   * Helper to get the typed data object holding the source entity ID.
   *
   * @return \Drupal\Core\TypedData\TypedDataInterface|FALSE
   */
  protected function getIdSource() {
    return !empty($this->definition['settings']['id source']) ? $this->parent
      ->get($this->definition['settings']['id source']) : FALSE;
  }

  /**
   * Implements TypedDataInterface::setValue().
   *
   * Both the entity ID and the entity object may be passed as value.
   */
  public function setValue($value) {

    // Support passing in the entity object.
    if ($value instanceof EntityInterface) {
      $this->entityType = $value
        ->entityType();
      $value = $value
        ->id();
    }
    elseif (isset($value) && !(is_scalar($value) && !empty($this->definition['constraints']['entity type']))) {
      throw new InvalidArgumentException('Value is no valid entity.');
    }
    $source = $this
      ->getIdSource();
    if ($source) {
      $source
        ->setValue($value);
    }
    else {
      $this->id = $value;
    }
  }

  /**
   * Implements TypedDataInterface::getString().
   */
  public function getString() {
    $entity = $this
      ->getValue();
    return $entity ? $entity
      ->label() : '';
  }

  /**
   * Implements TypedDataInterface::validate().
   */
  public function validate($value = NULL) {

    // TODO: Implement validate() method.
  }

  /**
   * Implements IteratorAggregate::getIterator().
   */
  public function getIterator() {
    $entity = $this
      ->getValue();
    return $entity ? $entity
      ->getIterator() : new ArrayIterator(array());
  }

  /**
   * Implements ComplexDataInterface::get().
   */
  public function get($property_name) {
    $entity = $this
      ->getValue();

    // @todo: Allow navigating through the tree without data as well.
    return $entity ? $entity
      ->get($property_name) : NULL;
  }

  /**
   * Implements ComplexDataInterface::set().
   */
  public function set($property_name, $value) {
    $this
      ->get($property_name)
      ->setValue($value);
  }

  /**
   * Implements ContextAwareInterface::getName().
   */
  public function getName() {
    return $this->name;
  }

  /**
   * Implements ContextAwareInterface::setName().
   */
  public function setName($name) {
    $this->name = $name;
  }

  /**
   * Implements ContextAwareInterface::getParent().
   */
  public function getParent() {
    return $this->parent;
  }

  /**
   * Implements ContextAwareInterface::setParent().
   */
  public function setParent($parent) {
    $this->parent = $parent;
  }

  /**
   * Implements ComplexDataInterface::getProperties().
   */
  public function getProperties($include_computed = FALSE) {
    $entity = $this
      ->getValue();
    return $entity ? $entity
      ->getProperties($include_computed) : array();
  }

  /**
   * Implements ComplexDataInterface::getPropertyDefinition().
   */
  public function getPropertyDefinition($name) {
    $definitions = $this
      ->getPropertyDefinitions();
    return isset($definitions[$name]) ? $definitions[$name] : FALSE;
  }

  /**
   * Implements ComplexDataInterface::getPropertyDefinitions().
   */
  public function getPropertyDefinitions() {

    // @todo: Support getting definitions if multiple bundles are specified.
    return entity_get_controller($this->entityType)
      ->getFieldDefinitions($this->definition['constraints']);
  }

  /**
   * Implements ComplexDataInterface::getPropertyValues().
   */
  public function getPropertyValues() {
    $entity = $this
      ->getValue();
    return $entity ? $entity
      ->getPropertyValues() : array();
  }

  /**
   * Implements ComplexDataInterface::setPropertyValues().
   */
  public function setPropertyValues($values) {
    if ($entity = $this
      ->getValue()) {
      $entity
        ->setPropertyValues($values);
    }
  }

  /**
   * Implements ComplexDataInterface::isEmpty().
   */
  public function isEmpty() {
    return (bool) $this
      ->getValue();
  }

}