protected function _check_callbacks($event, $frame) {
    if (!isset($this->_callbacks)) {
      $this->_callbacks = $this->_dompdf->get_callbacks();
    }
    
    if (is_array($this->_callbacks) && isset($this->_callbacks[$event])) {
      $info = array(0 => $this->_canvas, "canvas" => $this->_canvas,
                    1 => $frame, "frame" => $frame);
      $fs = $this->_callbacks[$event];
      foreach ($fs as $f) {
        if (is_callable($f)) {
          if (is_array($f)) {
            $f[0]->$f[1]($info);
          } else {
            $f($info);
          }
        }
      }
    }
  }