Correct smooth scroll for Flutter Web - by Simon Lightfoot - 14th July 2022

smooth_scroll.dart

// MIT License
//
// Copyright (c) 2022 Simon Lightfoot
//
// Permission is hereby granted, free of charge, to any person obtaining a copy
// of this software and associated documentation files (the "Software"), to deal
// in the Software without restriction, including without limitation the rights
// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
// copies of the Software, and to permit persons to whom the Software is
// furnished to do so, subject to the following conditions:
//
// The above copyright notice and this permission notice shall be included in all
// copies or substantial portions of the Software.
//
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
// SOFTWARE.
//
import 'package:flutter/material.dart';

class SmoothScrollController extends ScrollController {
  SmoothScrollController({
    super.initialScrollOffset,
    super.keepScrollOffset,
    super.debugLabel,
    this.velocityMultiplier = 5,
  });

  final double velocityMultiplier;

  @override
  ScrollPosition createScrollPosition(
      ScrollPhysics physics, ScrollContext context, ScrollPosition? oldPosition) {
    return SmoothScrollPositionWithSingleContext(
      velocityMultiplier: velocityMultiplier,
      physics: physics,
      context: context,
      initialPixels: initialScrollOffset,
      keepScrollOffset: keepScrollOffset,
      oldPosition: oldPosition,
      debugLabel: debugLabel,
    );
  }
}

class SmoothScrollPositionWithSingleContext extends ScrollPositionWithSingleContext {
  SmoothScrollPositionWithSingleContext({
    required this.velocityMultiplier,
    required super.physics,
    required super.context,
    super.initialPixels,
    super.keepScrollOffset,
    super.oldPosition,
    super.debugLabel,
  });

  final double velocityMultiplier;

  @override
  void pointerScroll(double delta) {
    double velocity = ((activity?.velocity ?? 0.0) + (delta * velocityMultiplier))
        .clamp(-physics.maxFlingVelocity, physics.maxFlingVelocity);
    goBallistic(velocity);
  }
}