大道至简：Flutter嵌套滑动冲突解决之路

  @override
  Widget build(BuildContext context) {
    ......
    return CustomScrollView(
      controller: _coordinator.outerController, // 负责控制外部列表的滚动
      slivers: [
        SliverToBoxAdapter(......),
        SliverAppBar(......),
        SliverToBoxAdapter(
          child: Container(
            height: innerListHeight,
            child: PageView.builder(
              onPageChanged: _onPageChange,
              controller: pageController,
              itemBuilder: (BuildContext context, int index) {
                return _buildList();
              },
              itemCount: 4,
            ),
          ),
        ),
      ],
    );
  }
  Widget _buildList() {
    return GridView.builder(
      controller: _coordinator.innerController, // 负责控制内部列表的滚动
      gridDelegate: SliverGridDelegateWithFixedCrossAxisCount(crossAxisCount: 3),
      itemBuilder: (context, index) {
        return Container(
          alignment: Alignment.center,
          child: Text("项目$index"),
        );
      },
    );
  }

class WebViewGestureRecognizer extends OneSequenceGestureRecognizer {
  bool scrollEnabled = false;

  Offset _dragDistance = Offset.zero;
  Offset _webViewDragDistance = Offset.zero;

  WebViewGestureRecognizer();

  @override
  void addPointer(PointerEvent event) {
    startTrackingPointer(event.pointer);
  }
  @override
  String get debugDescription => 'custom';

  @override
  void didStopTrackingLastPointer(int pointer) {}

  @override
  void handleEvent(PointerEvent event) {
    _dragDistance = _dragDistance + event.delta;
    if (event is PointerMoveEvent) {
      final double dy = _dragDistance.dy.abs();
      final double dx = _dragDistance.dx.abs();
      if (_webViewDragDistance.dy > 0) {
        WidgetsBinding.instance.cancelPointer(event.pointer);
        scrollEnabled = false;
        _webViewDragDistance = Offset.zero;
      } else { // isPinned标记外部Flutter列表是否滑动完，通过监听滑动距离更新
        scrollEnabled = isPinned;
      }
      if (scrollEnabled) { // Vertical drag: accept
        _webViewDragDistance = _webViewDragDistance+ event.delta; // 累计滚动距离
        resolve(GestureDisposition.accepted); // WebView internal will win
        _dragDistance = Offset.zero;
      } else if (dx > kTouchSlop && dx > dy) { // Horizontal drag: stop tracking
        stopTrackingPointer(event.pointer); // Native external will win
        _dragDistance = Offset.zero;
      }
    }
  }
}

class _RenderSingleChildViewport extends RenderBox
    with RenderObjectWithChildMixin<RenderBox> implements RenderAbstractViewport {
  ......
}

// Flutter侧逻辑
@override
void paint(PaintingContext context, Offset offset) {
  Offset offsetReset = Offset.zero;
  // 滑动距离同步过去
  scrollSyncChannel.invokeMethod('', {
    'dx': -offset.dx,
    'dy': -offset.dy,
  });
  paintOffset = offset;
  super.paint(context, offsetReset);
}

// Android侧逻辑
scrollSyncChannel.setMethodCallHandler(new MethodChannel.MethodCallHandler() {
    @Override
    public void onMethodCall(@NonNull MethodCall call, @NonNull MethodChannel.Result result) {
        @SuppressWarnings("unchecked")
        Map<String, Object> map = (Map<String, Object>) call.arguments;
        try {
            double dx = (double) map.get(DX);
            double dy = (double) map.get(DY);
            dx *= density;
            dy *= density;
            if (platformView != null) {
                platformView.syncTo(dx, dy); // 将调用WebView的scrollTo
            }
        } catch (NullPointerException ignored) {
        }
    }
});

// CustomWebView.java
@Override
protected void onDraw(Canvas canvas) {
    super.onDraw(canvas);
    if (platformViewScrollHelper != null) {
        platformViewScrollHelper.onHeightUpdate(getContentHeight());
    }
}
// PlatformViewScrollHelper.java
public void onHeightUpdate(double newSize) {
    if (Double.valueOf(newSize).equals(currentSize)) {
        return;
    }
    currentSize = newSize;
    heightSyncChannel.invokeMethod("", newSize);
}

  _RenderPlatformViewShell({
    ......
  }) : super(child: child, additionalConstraints: additionalConstraints) {
    ...... // 省略其他逻辑
    heightSyncChannel.setMethodCallHandler((call) {
      if (call.arguments is! double || realHeight == call.arguments) {
        return Future.value(0);
      }
      realHeight = call.arguments; // 更新高度
      markNeedsLayout();
      return Future.value(0);
    });
  }

  Size get size { // 每次布局的时候，Viewport会根据这个信息计算列表大小
    return Size.fromHeight(realHeight);
  }

@Override
protected void onOverScrolled(int scrollX, int scrollY, boolean clampedX, boolean clampedY) {
    super.onOverScrolled(scrollX, scrollY, clampedX, clampedY);
    if (platformViewScrollHelper != null) {
        platformViewScrollHelper.onOverScrolled(scrollX, scrollY, clampedX, clampedY);
    }
}

  void _handlePlatformViewEvent(PointerEvent event) {
    if (!attached) return;
    if (event is PointerDownEvent) {
      lastX = event.position.dx; // 每次按下的时候都做记录
      isHorizontalOverScrolled = false; // 默认WebView没有滑动越界
    }
    if (isHorizontalOverScrolled) {
      if (event is PointerMoveEvent) {
        double delta = lastX - event.position.dx; // 计算越界距离
        onOverScrolled?.call(delta); // 通知本次有效滑动
        lastX = event.position.dx;
      }
    }
  }

/// Changes the scrolling position based on a pointer signal from current
/// value to delta without animation and without checking if new value is in
/// range, taking min/max scroll extent into account.
///
/// Any active animation is canceled. If the user is currently scrolling, that
/// action is canceled.
///
/// This method dispatches the start/update/end sequence of scrolling
/// notifications.
///
/// This method is very similar to [jumpTo], but [pointerScroll] will
/// update the [ScrollDirection].
///
// TODO(YeungKC): Support trackpad scroll, https://github.com/flutter/flutter/issues/23604.
void pointerScroll(double delta);

void _onOverScrolled(double delta){
  if (delta == 0.0) { // 处理PlatformView的越界信息
    pageController.position.hold(() {
    }).cancel(); // 没有滑动时能悬停住
  } else {
    pageController.position.pointerScroll(delta);
  }
}

bool hitTest(BoxHitTestResult result, { required Offset position }) {
  if (_size!.contains(position)) {
    if (hitTestChildren(result, position: position) || hitTestSelf(position)) {
      result.add(BoxHitTestEntry(this, position));
      return true;
    }
  }
  return false;
}

bool hitTest(BoxHitTestResult result, { required Offset position }) {
  Offset newPos =  Offset(position.dx + paintOffset.dx, position.dy + paintOffset.dy);
  if (hitTestChildren(result, position: newPos) || hitTestSelf(newPos)) {
    result.add(BoxHitTestEntry(this, newPos));
    return true;
  }
  return false;
}

  Offset globalToLocal(Offset point, { RenderObject? ancestor }) {
    final Matrix4 transform = getTransformTo(ancestor);
    final double det = transform.invert();
    if (det == 0.0)
      return Offset.zero;
    final Vector3 n = Vector3(0.0, 0.0, 1.0);
    final Vector3 i = transform.perspectiveTransform(Vector3(0.0, 0.0, 0.0));
    final Vector3 d = transform.perspectiveTransform(Vector3(0.0, 0.0, 1.0)) - i;
    final Vector3 s = transform.perspectiveTransform(Vector3(point.dx, point.dy, 0.0));
    final Vector3 p = s - d * (n.dot(s) / n.dot(d));
    return Offset(p.x, p.y);
  }
  Matrix4 getTransformTo(RenderObject? ancestor) {
    ...... // 省略其他逻辑
    for (int index = renderers.length - 1; index > 0; index -= 1) {
      renderers[index].applyPaintTransform(renderers[index - 1], transform);
    }
    return transform;
  }

@override
void applyPaintTransform(covariant RenderObject child, Matrix4 transform) {
  transform.translate(-paintOffset.dx, - paintOffset.dy);
}

class EnableInterceptNotification extends Notification {
  static MethodChannel _channel =  MethodChannel('flutter_nested_scroll/global');
  static void emitToPlatform() {
    // 不需要任何参数，一般是在Move过程中告知Platform ，Platform会在每次事件开始时重置
    _channel.invokeMethod('', '');
  }
}

class NotificationScaffold extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return GestureDetector(
      // 滑动一个不会消费横滑事件的区域
      onHorizontalDragUpdate: (DragUpdateDetails details) {
        EnableInterceptNotification().dispatch(context);
      },
      child: NotificationListener(
        onNotification: (notification) {
          // 监听PageView滑动到边界的情况
          if (notification is OverscrollNotification) {
            EnableInterceptNotification().dispatch(context);
          }
          return false;
        },
        child: DemoPage(), // 真正的UI
      ),
    );
  }
}

  void _onOverScrolled(double delta){
    if (delta == 0.0) { // 处理PlatformView的越界信息
      pageController.position.hold(() {
      }).cancel();
    } else {
      pageController.position.pointerScroll(delta);
      if ((delta > 0 && pageController.page! >= 3.0) // 右侧越界滑动
          || (delta < 0 && pageController.page! <= 0.0)) { // 左侧越界滑动
        // 发送出去，由根部统一处理
        EnableInterceptNotification().dispatch(context);
      }
    }
  }

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(primarySwatch: Colors.blue,),
      home: SafeArea(
        child: Scaffold(
          body: NotificationListener(
            onNotification: (notification) {
              // 通知宿主
              if (notification is EnableInterceptNotification) {
                EnableInterceptNotification.emitToPlatform();
                return true;
              }
              return false;
            },
            child: NotificationScaffold(),
          ),
        ),
      ),
    );
  }
}