Entity iterator with generator

EntityIterator.php

<?php

/**
 * @file
 * Contains \Drupal\Core\Entity\EntityIterator.
 */

namespace Drupal\Core\Entity;

/**
 * Provides an Iterator class for dealing with large amounts of entities
 * but not loading them all into memory.
 */
class EntityIterator implements \IteratorAggregate, \Countable {

  /**
   * The entity storage controller to load entities.
   *
   * @var \Drupal\Core\Entity\EntityStorageInterface
   */
  protected $entityStorage;

  /**
   * An array of entity IDs to iterate over.
   *
   * @var array
   */
  protected $entityIds;

  /**
   * The Amount of cached entities to store before clearing the static cache.
   *
   * @var int
   */
  protected $cacheLimit;

  /**
   * Constructs an entity iterator object.
   *
   * @param \Drupal\Core\Entity\EntityStorageInterface $entity_controller
   * @param array $ids
   * @param int $cache_limit
   */
  public function __construct(EntityStorageInterface $entity_storage_controller, array $ids, $cache_limit = 50) {
    $this->entityStorage = $entity_storage_controller;
    // Make sure we don't use a keyed array.
    $this->entityIds = array_values($ids);
    $this->cacheLimit = (int) $cache_limit;
  }

  /**
   * Implements \Countable::count().
   */
  public function count() {
    return count($this->entityIds);
  }

  /**
   * Implements \IteratorAggregate::GetIterator()
   */
  public function getIterator() {
    foreach (array_chunk($this->entityIds, $this->cacheLimit) as $ids_chunk) {
      foreach ($this->loadEntities($ids_chunk) as $id => $entity) {
        yield $id => $entity;
      }
    }
  }

  /**
   * Loads a set of entities.
   *
   * This depends on the cacheLimit property.
   */
  protected function loadEntities(array $ids) {
    // Reset any previously loaded entities then load the current set of IDs.
    $this->entityStorage->resetCache();
    return $this->entityStorage->loadMultiple($ids);
  }

}