Flutter Scroll Physics that always ends on a whole page

page_aligned_scroll_physics.dart

class PageAlignedScrollPhysics extends ScrollPhysics {
  final double pageSize;

  PageAlignedScrollPhysics({@required this.pageSize, ScrollPhysics parent}) : super(parent: parent);

  @override
  PageAlignedScrollPhysics applyTo(ScrollPhysics ancestor) {
    return PageAlignedScrollPhysics(pageSize: pageSize, parent: buildParent(ancestor));
  }

  @override
  Simulation createBallisticSimulation(ScrollMetrics position, double velocity) {
    if (position.outOfRange) return super.createBallisticSimulation(position, velocity);

    final frictionSimulation = FrictionSimulation(0.135, position.pixels, velocity * 0.91);
    double endPosition = frictionSimulation.finalX;
    if (endPosition <= position.minScrollExtent || endPosition >= position.maxScrollExtent)
      return super.createBallisticSimulation(position, velocity);

    final page = (endPosition / pageSize).roundToDouble().clamp(0.0, double.maxFinite);
    endPosition = page * pageSize;
    if ((endPosition - position.pixels).abs() < tolerance.distance) return null;

    if (velocity.abs() < tolerance.velocity) {
      return ScrollSpringSimulation(
        spring,
        position.pixels,
        endPosition,
        velocity,
        tolerance: tolerance,
      );
    }

    velocity = velocity.abs().clamp(200.0, double.maxFinite) * velocity.sign;
    return FrictionSimulation.through(
      position.pixels,
      endPosition,
      velocity,
      tolerance.velocity * velocity.sign,
    );
  }
}