main.dart

import 'package:flutter/material.dart';
import 'dart:math' as math;

main() => runApp(MaterialApp(home: MyHomePage()));

class MyHomePage extends StatelessWidget {

  final itemWidth = 120.0;
  final itemHorizontalMargin = 4.0;

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Flutter 4 Fun'),
      ),
      body: Center(
        child: SizedBox(
          height: 200.0,
          child: ListView.builder(
            scrollDirection: Axis.horizontal,
            physics: SnappingListScrollPhysic(itemWidth: itemWidth + (itemHorizontalMargin * 2)),
            itemBuilder: (context, i) {
              return Container(
                width: itemWidth,
                height: 200,
                margin: EdgeInsets.symmetric(horizontal: itemHorizontalMargin),
                decoration: BoxDecoration(
                  color: Colors.purple,
                  borderRadius: BorderRadius.all(Radius.circular(4.0)),
                ),
                child: Center(
                  child: Text(
                    '$i',
                    style: TextStyle(
                      color: Colors.white,
                      fontWeight: FontWeight.bold,
                      fontSize: 22,
                    ),
                  ),
                ),
              );
            },
          ),
        ),
      ),
    );
  }
}

class SnappingListScrollPhysic extends ScrollPhysics {
  final double itemWidth;

  const SnappingListScrollPhysic({
    @required this.itemWidth,
    ScrollPhysics parent,
  }) : super(parent: parent);

  @override
  SnappingListScrollPhysic applyTo(ScrollPhysics ancestor) => SnappingListScrollPhysic(
    parent: buildParent(ancestor),
    itemWidth: itemWidth,
  );

  double _getItem(ScrollPosition position) => (position.pixels) / itemWidth;

  double _getPixels(ScrollPosition position, double item) =>
      math.min(item * itemWidth, position.maxScrollExtent);

  double _getTargetPixels(ScrollPosition position, Tolerance tolerance, double velocity) {
    double item = _getItem(position);
    if (velocity < -tolerance.velocity) {
      item -= 0.5;
    } else if (velocity > tolerance.velocity) {
      item += 0.5;
    }
    return _getPixels(position, item.roundToDouble());
  }

  @override
  Simulation createBallisticSimulation(ScrollMetrics position, double velocity) {
    // If we're out of range and not headed back in range, defer to the parent
    // ballistics, which should put us back in range at a page boundary.
    if ((velocity <= 0.0 && position.pixels <= position.minScrollExtent) ||
        (velocity >= 0.0 && position.pixels >= position.maxScrollExtent)) {
      return super.createBallisticSimulation(position, velocity);
    }
    final Tolerance tolerance = this.tolerance;
    final double target = _getTargetPixels(position, tolerance, velocity);
    if (target != position.pixels) {
      return ScrollSpringSimulation(spring, position.pixels, target, velocity,
          tolerance: tolerance);
    }
    return null;
  }

  @override
  bool get allowImplicitScrolling => false;
}