在 PageView 中的 WheelEvent 上 Flutter Web "smooth scrolling"

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),
                    ),
                ],
              ),
            ],
          ),
        ),
      );
}

可以看到，在 dartpad 上运行或从此 video ,
使用鼠标滚轮滚动 PageView提供平庸的体验(充其量)，
这是一个已知问题 #35687 #32120 ，但我正在尝试找到解决方法
实现 PageView的平滑滚动或至少防止“口吃”。
有人可以帮助我或指出我正确的方向吗？
我不确定 PageScrollPhysics 的问题。 ;
我有一种直觉，问题可能出在 WheelEvent 上。
因为使用多点触控滚动滑动非常有效

Answer 1

问题源于一系列事件:

用户将鼠标滚轮旋转一格，

Scrollable接收 PointerSignal和 calls jumpTo方法，

_PagePosition的jumpTo方法(源自 ScrollPositionWithSingleContext )更新滚动位置并调用 goBallistic方法，

requested来自 PageScrollPhysics模拟将位置恢复到初始值，因为一个缺口偏移量太小而无法翻页，

从步骤(1)重复另一个凹口和过程。

解决问题的一种方法是在调用 goBallistic 之前执行延迟。方法。这可以在 _PagePosition 中完成class，但是 class 是私有(private)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听众:

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;
  }
}

Here is demo在 CodePen 上。