/**
   * A cid to pass to cache_set() and cache_get().
   */
  private $cid;

  /**
   * A bin to pass to cache_set() and cache_get().
   */
  private $bin;

  /**
   * An array of keys to add to the cache at the end of the request.
   */
  protected $keysToPersist = array();

  /**
   * Storage for the data itself.
   */
  protected $storage = array();

  /**
   * Constructor.
   *
   * param $cid
   *   The cid for the array being cached.
   * param $bin
   *   The bin to cache the array.
   */
  public function __construct($cid, $bin) {
    $this->cid = $cid;
    $this->bin = $bin;

    if ($cached = cache_get($this->cid, $this->bin)) {
     $this->storage = $cached->data;
    }
  }

  public function offsetExists($offset) {
    return $this->offsetGet($offset) !== NULL;
  }

  public function offsetGet($offset) {
    if (isset($this->storage[$offset]) || array_key_exists($offset, $this->storage)) {
      return $this->storage[$offset];
    }
    else {
      return $this->resolveCacheMiss($offset);
    }
  }

  public function offsetSet($offset, $value) {
    $this->storage[$offset] = $value;
  }

  public function offsetUnset($offset) {
    unset($this->storage[$offset]);
  }