1. override
GridChildPlacement getChildPlacement(GridSpecification specification, int index, Object placementData)

Override this method to control where children are placed in the grid.

During layout, the grid calls this function for each child, passing the placementData associated with that child as context. The returned GridChildPlacement is then used to determine the size and position of that child within the grid.

Source 

@override
GridChildPlacement getChildPlacement(GridSpecification specification, int index, Object placementData) {
  final int columnCount = specification.columnOffsets.length - 1;
  return new GridChildPlacement(
    column: index % columnCount,
    row: index ~/ columnCount
  );
}