1. override
bool shouldRelayout(FixedColumnCountGridDelegate oldDelegate)

Override this method to return true when the children need to be laid out.

Source 

@override
bool shouldRelayout(FixedColumnCountGridDelegate oldDelegate) {
  return columnCount != oldDelegate.columnCount
      || tileAspectRatio != oldDelegate.tileAspectRatio
      || super.shouldRelayout(oldDelegate);
}