1. override
FixedColumnCountGridDelegate createDelegate()

The delegate that controls the layout of the children.

Source 

@override
FixedColumnCountGridDelegate createDelegate() {
  return new FixedColumnCountGridDelegate(
    columnCount: columnCount,
    columnSpacing: columnSpacing,
    rowSpacing: rowSpacing,
    tileAspectRatio: tileAspectRatio,
    padding: padding
  );
}