getScrollControllerOffset function

double getScrollControllerOffset({ required double dx, required double dy, required Offset delta, required Offset offset, required Offset scroll, required Size itemSize, required Size viewSize, required Axis direction, required ScrollPhysics physics, required ScrollController scrollController, })

Computing scrolling

Implementation

double getScrollControllerOffset({
  required double dx,
  required double dy,
  required Offset delta,
  required Offset offset,
  required Offset scroll,
  required Size itemSize,
  required Size viewSize,
  required Axis direction,
  required ScrollPhysics physics,
  required ScrollController scrollController,
}) {
  if (physics == const NeverScrollableScrollPhysics()) {
    return 0.0;
  }

  final tx = dx + scroll.dx;
  final ty = dy + scroll.dy;
  final left = tx - offset.dx - itemSize.width / 2 - 16.0;
  final top = ty - offset.dy - itemSize.height / 2 - 16.0;

  final inx1 = left > scroll.dx;
  final iny1 = top > scroll.dy;
  final inx2 = left + itemSize.width < viewSize.width;
  final iny2 = top + itemSize.height < viewSize.height;
  final scrollExtent = scrollController.position.maxScrollExtent;
  final scollOffset = scrollController.offset;

  if (direction == Axis.vertical) {
    if (!iny1 && scollOffset > 0 && delta.dy <= 0) {
      return -min(scollOffset, 5.0);
    }

    if (!iny2 && scollOffset < scrollExtent && delta.dy >= 0) {
      return min(scrollExtent - scollOffset, 5.0);
    }
  }

  if (direction == Axis.horizontal) {
    if (!inx1 && scollOffset > 0 && delta.dx <= 0) {
      return -min(scollOffset, 5.0);
    }

    if (!inx2 && scollOffset < scrollExtent && delta.dx >= 0) {
      return min(scrollExtent - scollOffset, 5.0);
    }
  }

  return 0.0;
}