Scroll page physics that only scrolls to next pages

limiting_scroll_physics.dart

class LimitingScrollPhysics extends PageScrollPhysics {
  LimitingScrollPhysics({ScrollPhysics parent, @required this.controller})
      : super(parent: parent);

  final ScrollController controller;

  @override
  LimitingScrollPhysics applyTo(ScrollPhysics ancestor) {
    return LimitingScrollPhysics(
        parent: buildParent(ancestor), controller: controller);
  }

  @override
  double applyBoundaryConditions(ScrollMetrics position, double value) {
    if (controller.hasClients &&
        controller.position.userScrollDirection == ScrollDirection.idle) {
      return super.applyBoundaryConditions(position, value);
    }

    final page = (position.pixels / position.viewportDimension).floor();
    if (value < page * position.viewportDimension) {
      return value - position.pixels;
    } else {
      return super.applyBoundaryConditions(position, value);
    }
  }
}