页面视图中 WheelEvent 上的 Flutter Web“平滑滚动”

Question

使用下面的代码

import 'package:flutter/material.dart';

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  const MyApp({Key key}) : super(key: key);

  @override
  Widget build(BuildContext context) => MaterialApp(
        home: const MyHomePage(),
      );
}

class MyHomePage extends StatelessWidget {
  const MyHomePage({Key key}) : super(key: key);

  @override
  Widget build(BuildContext context) => DefaultTabController(
        length: 2,
        child: Scaffold(
          appBar: AppBar(
            title: const Center(
            child: Text('use the mouse wheel to scroll')),
            bottom: TabBar(
              tabs: const [
                Center(child: Text('ScrollView')),
                Center(child: Text('PageView'))
              ],
            ),
          ),
          body: TabBarView(
            children: [
              SingleChildScrollView(
                child: Column(
                  children: [
                    for (int i = 0; i < 10; i++)
                      Container(
                        height: MediaQuery.of(context).size.height,
                        child: const Center(
                          child: FlutterLogo(size: 80),
                        ),
                      ),
                  ],
                ),
              ),
              PageView(
                scrollDirection: Axis.vertical,
                children: [
                  for (int i = 0; i < 10; ++i)
                    const Center(
                      child: FlutterLogo(size: 80),
                    ),
                ],
              ),
            ],
          ),
        ),
      );
}

你可以看到，在dartpad 上或从这个视频中运行它，

使用鼠标滚轮滚动 aPageView提供了平庸的体验（充其量），

这是一个已知问题#35687 #32120，但我正在尝试寻找解决方法

实现平滑滚动PageView或至少防止“口吃”。

有人可以帮助我或指出正确的方向吗？

我不确定问题出在PageScrollPhysics;

我有一种直觉，问题可能出在WheelEvent

因为使用多点触控滚动滑动效果很好

Answer 1

问题是由一系列事件引起的：

1. 用户将鼠标滚轮旋转一格，
2. Scrollable接收PointerSignal并调用 jumpTo方法，
3. _PagePosition的JumpTo方法（派生自ScrollPositionWithSingleContext）更新滚动位置并调用goBallistic方法，
4. 模拟请求PageScrollPhysics将位置恢复到初始值，因为一个凹口偏移产生的位置太小而无法翻页，
5. 另一个刻痕和过程从步骤（1）开始重复。

解决问题的一种方法是在调用方法之前执行延迟goBallistic。这可以在_PagePosition类中完成，但是该类是私有的，我们必须修补 Flutter SDK：

// <FlutterSDK>/packages/flutter/lib/src/widgets/page_view.dart
// ...

class _PagePosition extends ScrollPositionWithSingleContext implements PageMetrics {
  //...

  // add this code to fix issue (mostly borrowed from ScrollPositionWithSingleContext):
  Timer timer;

  @override
  void jumpTo(double value) {
    goIdle();
    if (pixels != value) {
      final double oldPixels = pixels;
      forcePixels(value);
      didStartScroll();
      didUpdateScrollPositionBy(pixels - oldPixels);
      didEndScroll();
    }
    if (timer != null) timer.cancel();
    timer = Timer(Duration(milliseconds: 200), () {
      goBallistic(0.0);
      timer = null;
    });
  }

  // ...
}

另一种方法是将jumpTo替换为animateTo。这可以在不修补 Flutter SDK 的情况下完成，但看起来更复杂，因为我们需要禁用默认PointerSignalEvent侦听器：

import 'dart:async';
import 'package:flutter/gestures.dart';
import 'package:flutter/material.dart';
import 'package:flutter/rendering.dart';

class PageViewLab extends StatefulWidget {
  @override
  _PageViewLabState createState() => _PageViewLabState();
}

class _PageViewLabState extends State<PageViewLab> {
  final sink = StreamController<double>();
  final pager = PageController();

  @override
  void initState() {
    super.initState();
    throttle(sink.stream).listen((offset) {
      pager.animateTo(
        offset,
        duration: Duration(milliseconds: 200),
        curve: Curves.ease,
      );
    });
  }

  @override
  void dispose() {
    sink.close();
    pager.dispose();
    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Mouse Wheel with PageView'),
      ),
      body: Container(
        constraints: BoxConstraints.expand(),
        child: Listener(
          onPointerSignal: _handlePointerSignal,
          child: _IgnorePointerSignal(
            child: PageView.builder(
              controller: pager,
              scrollDirection: Axis.vertical,
              itemCount: Colors.primaries.length,
              itemBuilder: (context, index) {
                return Padding(
                  padding: const EdgeInsets.all(8.0),
                  child: Container(color: Colors.primaries[index]),
                );
              },
            ),
          ),
        ),
      ),
    );
  }

  Stream<double> throttle(Stream<double> src) async* {
    double offset = pager.position.pixels;
    DateTime dt = DateTime.now();
    await for (var delta in src) {
      if (DateTime.now().difference(dt) > Duration(milliseconds: 200)) {
        offset = pager.position.pixels;
      }
      dt = DateTime.now();
      offset += delta;
      yield offset;
    }
  }

  void _handlePointerSignal(PointerSignalEvent e) {
    if (e is PointerScrollEvent && e.scrollDelta.dy != 0) {
      sink.add(e.scrollDelta.dy);
    }
  }
}

// workaround https://github.com/flutter/flutter/issues/35723
class _IgnorePointerSignal extends SingleChildRenderObjectWidget {
  _IgnorePointerSignal({Key key, Widget child}) : super(key: key, child: child);

  @override
  RenderObject createRenderObject(_) => _IgnorePointerSignalRenderObject();
}

class _IgnorePointerSignalRenderObject extends RenderProxyBox {
  @override
  bool hitTest(BoxHitTestResult result, {Offset position}) {
    final res = super.hitTest(result, position: position);
    result.path.forEach((item) {
      final target = item.target;
      if (target is RenderPointerListener) {
        target.onPointerSignal = null;
      }
    });
    return res;
  }
}

这是CodePen 上的演示。