Flutter：为用户滚动增加摩擦力，而不是弹簧

Question

我有一个带有自定义滚动物理类的列表视图，它定义了我想要的列表的滚动和弹簧效果。我已经设法按照我想要的方式设置了弹簧阻尼，但我似乎找不到任何关于使用户的阻力更重的设置。

我的意思是当用户拖动时我希望列表感觉有张力所以用户需要比平时拖动得更远，我将处理移动到列表中的下一项使用自定义滚动物理。

我希望它感觉像在火车站的转弯风格。你的身体会受到很大的压力，一旦你通过转弯风格就会重新回到中心。

列表：

child: ListView.builder(
  cacheExtent: MediaQuery.of(context).size.height * 2,
  padding: EdgeInsets.only(top: 0),
  itemExtent: constraints.maxHeight -
      SizeConfig.blockSizeVertical * 40,
  itemCount: channelList?.length ?? 0,
  controller: _controller,
  physics: _physics,
  itemBuilder: (BuildContext context, int index) =>
      buildList(index),
),

自定义滚动类：

import 'package:flutter/material.dart';
import 'package:flutter/physics.dart';

class CustomScrollPhysics extends ScrollPhysics {
  final double itemDimension;
  static final SpringDescription customSpring =
      SpringDescription.withDampingRatio(
    mass: 4,
    stiffness: 150.0,
    ratio: 2.0,
  );

  @override
  double get dragStartDistanceMotionThreshold => 40;

  @override
  double get minFlingVelocity => double.infinity;

  @override
  double get maxFlingVelocity => double.infinity;

  @override
  double get minFlingDistance => double.infinity;

  CustomScrollPhysics({this.itemDimension, ScrollPhysics parent})
      : super(parent: parent);

  @override
  CustomScrollPhysics applyTo(ScrollPhysics ancestor) {
    return CustomScrollPhysics(
        itemDimension: itemDimension, parent: buildParent(ancestor));
  }

  double _getPage(ScrollPosition position) {
    return position.pixels / itemDimension;
  }

  double _getPixels(double page) {
    return page * itemDimension;
  }

  double _getTargetPixels(
      ScrollPosition position, Tolerance tolerance, double velocity) {
    double page = _getPage(position);
    if (velocity < -tolerance.velocity) {
      page -= 0.01;
    } else if (velocity > tolerance.velocity) {
      page += 0.01;
    }
    return _getPixels(page.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 an item 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(
          customSpring, position.pixels, target, velocity,
          tolerance: tolerance);
    }
    return null;
  }

  @override
  bool get allowImplicitScrolling => false;
}

/// Note: This Widget creates the ballistics model for a snap-to physics in a ListView.
/// Normally you might use the ListViewScrollView, however that widget currently
/// has a bug that prevents onTap detection of child widgets.

列表居中在屏幕上，大约是视图高度的 60%。

Answer 1

如果您一次只需要在列表中显示一个项目并且项目的呈现将适合屏幕，则使用PageView 小部件可能会给您带来您所追求的效果。

Answer 2

试试这个，

import 'dart:math';
 
import 'package:flutter/material.dart';
    
    void main() => runApp(MyApp());
    
    class MyApp extends StatelessWidget {
      @override
      Widget build(BuildContext context) {
        return MaterialApp(
          debugShowCheckedModeBanner: false,
          home: MyHomePage(),
        );
      }
    }
    
    class MyHomePage extends StatefulWidget {
      @override
      _MyHomePageState createState() => _MyHomePageState();
    }
    
    class _MyHomePageState extends State<MyHomePage> {
      final _listController = ScrollController();
    
      final List<int> _list = List.generate(12, (index) => index);
    
      ScrollPhysics _scrollPhysics;
    
      @override
      void initState() {
        super.initState();
    
        _listController.addListener(() {
          if (_listController.position.haveDimensions && _scrollPhysics == null) {
            setState(() {
              var dimension =
                  _listController.position.maxScrollExtent / (_list.length - 4);
              _scrollPhysics = CustomScrollPhysics(itemDimension: dimension);
            });
          }
        });
      }
    
      @override
      Widget build(BuildContext context) {
        return Scaffold(
          body: SafeArea(
            child: ListView.builder(
              scrollDirection: Axis.vertical,
              controller: _listController,
              physics: _scrollPhysics,
              itemCount: _list.length,
              itemBuilder: (context, index) => Container(
                height: 300,
                width: double.infinity,
                color: randomColor,
                child: FlutterLogo(),
                margin: const EdgeInsets.all(2.0),
              ),
            ),
          ),
        );
      }
    
      Color get randomColor =>
          Color((Random().nextDouble() * 0xFFFFFF).toInt() << 0).withOpacity(1.0);
    }
    
    class CustomScrollPhysics extends ScrollPhysics {
      final double itemDimension;
    
      CustomScrollPhysics({this.itemDimension, ScrollPhysics parent})
          : super(parent: parent);
    
      @override
      CustomScrollPhysics applyTo(ScrollPhysics ancestor) {
        return CustomScrollPhysics(
            itemDimension: itemDimension, parent: buildParent(ancestor));
      }
    
      double _getPage(ScrollPosition position) {
        return position.pixels / itemDimension;
      }
    
      double _getPixels(double page) {
        return page * itemDimension;
      }
    
      double _getTargetPixels(
          ScrollPosition position, Tolerance tolerance, double velocity) {
        double page = _getPage(position);
        if (velocity < -tolerance.velocity) {
          page -= 0.1;
        } else if (velocity > tolerance.velocity) {
          page += 0.1;
        }
        return _getPixels(page.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;
    }

现在在颤振中，没有预先定义摩擦的方法， 但这可以通过一些自定义方式实现。