public function addCallback($callback, $driver = null)
{
if (!empty($driver)) {
if (is_array($driver)) {
if (!in_array($this->index->driver, $driver)) {
return $this;
}
} else {
if ($driver != $this->index->driver) {
return $this;
}
}
}
$this->callbacks[] = $callback;
return $this;
}