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07_Flutter使用NestedScrollView+TabBarView滚动位置共享问题修复

一.案发现场

可以看到，上图中三个列表的滑动位置共享了，滑动其中一个列表，会影响到另外两个，这显然不符合要求，先来看下布局，再说明产生这个问题的原因:

• 布局整体使用NestedScrollView，顶部banner和TabBar通过headerSliverBuilder创建，body为TabBarView，TabBarView中有三个列表，通过TabController与TabBar实现联动，同时每一个列表通过继承StatefulWidget构建并混入AutomaticKeepAliveClientMixin，重写wantKeepAlive的getter方法返回true，这样可以保证每次切换Tab的时候，ListView不会重新创建，实现懒加载。

  NestedScrollView(headerSliverBuilder: (BuildContext context, bool innerBoxIsScrolled) {return [SliverToBoxAdapter(child: Container(height: 200,color: Colors.red,alignment: Alignment.center,child: const Text("banner",style: TextStyle(color: Colors.white,fontSize: 16),),),),SliverOverlapAbsorber(handle: NestedScrollView.sliverOverlapAbsorberHandleFor(context),sliver: StickySliverToBoxAdapter(child: Container(color: Colors.white,child: TabBar(tabs: List.generate(_tabs.length, (index) {return Padding(padding: const EdgeInsets.symmetric(vertical: 15),child: Text(_tabs[index]),);}),unselectedLabelColor: const Color(0xFF7B7B7B),labelColor: const Color(0xFF5E80FF),isScrollable: false,indicatorSize: TabBarIndicatorSize.label,indicator: UnderlineTabIndicator(borderRadius: BorderRadius.circular(3),borderSide: const BorderSide(color: Color(0xFF5E80FF), width: 3),insets: const EdgeInsets.symmetric(horizontal: 3, vertical: 9)),controller: _tabController,),),),),];},body: LayoutBuilder(builder: (context, _) {return Container(padding: EdgeInsets.only(top: NestedScrollView.sliverOverlapAbsorberHandleFor(context).layoutExtent ?? 0),child: NestedTabBarView(controller: _tabController,children: List.generate(_tabs.length, (index) {return _TabInnerListView(tabName: _tabs[index],);})));})
  )class _TabInnerListViewState extends State<_TabInnerListView> with AutomaticKeepAliveClientMixin {final int length = 20;@overrideWidget build(BuildContext context) {return CustomScrollView(physics: const ClampingScrollPhysics(),slivers: [...(List.generate(length, (index) {return SliverToBoxAdapter(child: Container(height: 100,margin: const EdgeInsets.only(top: 16, left: 16, right: 16),color: Colors.orange,alignment: Alignment.center,child: Text("${widget.tabName} item $index",style: const TextStyle(color: Colors.white),),),);})),const SliverToBoxAdapter(child: SizedBox(height: 16,),)],);}@overridebool get wantKeepAlive => true;}
  

• 上述问题产生的原因，需要追踪NestedScrollView的源码，NestedScrollView整体的布局结构如下：

  

  如果没有_NestedScrollCoordinator的加持，那么外层的CustomScrollView和内层的CustomScrollView就会各划各的。_NestedScrollCoordinator处理嵌套滑动是在applyUserOffset方法中完成的:

  class _NestedScrollCoordinator implements ScrollActivityDelegate, ScrollHoldController {@overridevoid applyUserOffset(double delta) {updateUserScrollDirection(delta > 0.0 ? ScrollDirection.forward : ScrollDirection.reverse,);assert(delta != 0.0);if (_innerPositions.isEmpty) {_outerPosition!.applyFullDragUpdate(delta);} else if (delta < 0.0) {...} else {double innerDelta = delta;if (_floatHeaderSlivers) {innerDelta = _outerPosition!.applyClampedDragUpdate(delta);}if (innerDelta != 0.0) {double outerDelta = 0.0;final List<double> overscrolls = <double>[];final List<_NestedScrollPosition> innerPositions = _innerPositions.toList();for (final _NestedScrollPosition position in innerPositions) {final double overscroll = position.applyClampedDragUpdate(innerDelta);outerDelta = math.max(outerDelta, overscroll);overscrolls.add(overscroll);}if (outerDelta != 0.0) {outerDelta -= _outerPosition!.applyClampedDragUpdate(outerDelta);}for (int i = 0; i < innerPositions.length; ++i) {final double remainingDelta = overscrolls[i] - outerDelta;if (remainingDelta > 0.0) {innerPositions[i].applyFullDragUpdate(remainingDelta);}}}}}
  }
  

  可以看到在applyUserOffset中，是通过_NestedScrollPosition的applyFullDragUpdate响应滑动事件的，如果调用_outerPosition!.applyFullDragUpdate，则外层的CustomScrollView滑动。同理，内层CustomScrollView滑动，只不过applyUserOffset在处理内层滑动时，是遍历innerPositions把所有内层CustomScrollView的_NestedScrollPosition滚动相同的位移。

  _NestedScrollPosition? get _outerPosition {if (!_outerController.hasClients) {return null;}return _outerController.nestedPositions.single;
  }Iterable<_NestedScrollPosition> get _innerPositions {return _innerController.nestedPositions;
  }
  

  这也就解释了上图中，为什么滚动其中一个列表，其他列表也会跟着滑动相同的位置？。

二.解决方案

综上所述，_NestedScrollCoordinator的_innerPositions的返回结果是所有内层CustomScrollView的_NestedScrollPosition，要解决这个问题，我们只需要想办法将_NestedScrollCoordinator的_innerPositions的返回结果改变成只包含当前选中的内层CustomScrollView的_NestedScrollPosition即可，而_innerPositions的取值是来源于_innerController的nestedPositions。_innerController是一个_NestedScrollController对象，接着看_NestedScrollController的源码:

class _NestedScrollController extends ScrollController {...@overrideScrollPosition createScrollPosition(ScrollPhysics physics,ScrollContext context,ScrollPosition? oldPosition,) {return _NestedScrollPosition(coordinator: coordinator,physics: physics,context: context,initialPixels: initialScrollOffset,oldPosition: oldPosition,debugLabel: debugLabel,);}@overridevoid attach(ScrollPosition position) {assert(position is _NestedScrollPosition);super.attach(position);coordinator.updateParent();coordinator.updateCanDrag();position.addListener(_scheduleUpdateShadow);_scheduleUpdateShadow();}@overridevoid detach(ScrollPosition position) {assert(position is _NestedScrollPosition);(position as _NestedScrollPosition).setParent(null);position.removeListener(_scheduleUpdateShadow);super.detach(position);_scheduleUpdateShadow();}...Iterable<_NestedScrollPosition> get nestedPositions {// TODO(vegorov): use instance method version of castFrom when it is available.return Iterable.castFrom<ScrollPosition, _NestedScrollPosition>(positions);}
}

可以看到_NestedScrollController是私有类，并且NestedScrollView从头到尾都没有暴露任何可以修改或替换_innerController的方法给我们，因此，想在外部直接修改是不可能的。怎么办呢?

首先，内层的每一个CustomScrollView都是我们在外部人为编写的，我们可以在外部给内层的每一个CustomScrollView重新指定ScrollController，虽然暂时没什么卵用😄，但是别着急。

class _TabInnerListViewState extends State<_TabInnerListView> with AutomaticKeepAliveClientMixin {final int length = 20;late ScrollController _scrollController;@overridevoid initState() {super.initState();_scrollController = ScrollController();}@overridevoid dispose() {_scrollController.dispose();super.dispose();}@overrideWidget build(BuildContext context) {return CustomScrollView(controller: _scrollController,physics: const ClampingScrollPhysics(),slivers: [...(List.generate(length, (index) {return SliverToBoxAdapter(child: Container(height: 100,margin: const EdgeInsets.only(top: 16, left: 16, right: 16),color: Colors.orange,alignment: Alignment.center,child: Text("${widget.tabName} item $index",style: const TextStyle(color: Colors.white),),),);})),const SliverToBoxAdapter(child: SizedBox(height: 16,),)],);}@overridebool get wantKeepAlive => true;}

可以看到，此时嵌套滑动失效了，这是因为我们为内层的每一个CustomScrollView单独指定ScrollController后，CustomScrollView的滑动全部交给了这个这个ScrollController处理，跟NestedScrollView的_innerController已经没有半毛钱关系了。既然没有关系，那我们就建立关系，怎么建立:

• 创建NestedInnerScrollController类继承ScrollController

• 重写createScrollPosition方法，通过PrimaryScrollController.maybeOf(context)获取NestedScrollView的_innerController，将createScrollPosition转交给_innerController完成

• 重写attach方法，将attach转交给_innerController完成

• 重写detach方法，将detach转交给_innerController完成

• 为每一个内层的CustomScrollView指定controller为NestedInnerScrollController的实例

  class NestedInnerScrollController extends ScrollController {ScrollController? _inner;NestedInnerScrollController();@overrideScrollPosition createScrollPosition(ScrollPhysics physics, ScrollContext context, ScrollPosition? oldPosition) {ScrollPosition scrollPosition;ScrollableState? scrollableState = context as ScrollableState;if(scrollableState != null) {_inner = PrimaryScrollController.maybeOf(scrollableState.context);}if(_inner == null) {scrollPosition = super.createScrollPosition(physics, context, oldPosition);} else {scrollPosition = _inner!.createScrollPosition(physics, context, oldPosition);}return scrollPosition;}@overridevoid attach(ScrollPosition position) {super.attach(position);_inner?.attach(position);}@overridevoid detach(ScrollPosition position) {_inner?.detach(position);super.detach(position);}}class _TabInnerListViewState extends State<_TabInnerListView> with AutomaticKeepAliveClientMixin {final int length = 20;late ScrollController _scrollController;@overridevoid initState() {super.initState();_scrollController = NestedInnerScrollController();}@overridevoid dispose() {_scrollController.dispose();super.dispose();}@overrideWidget build(BuildContext context) {return CustomScrollView(controller: _scrollController,physics: const ClampingScrollPhysics(),slivers: [...(List.generate(length, (index) {return SliverToBoxAdapter(child: Container(height: 100,margin: const EdgeInsets.only(top: 16, left: 16, right: 16),color: Colors.orange,alignment: Alignment.center,child: Text("${widget.tabName} item $index",style: const TextStyle(color: Colors.white),),),);})),const SliverToBoxAdapter(child: SizedBox(height: 16,),)],);}@overridebool get wantKeepAlive => true;}
  

  

可以看到，嵌套滑动它又回来了😄。那么接下来…，就只剩下解决共享滑动了:

• 将TabBarView单独定义成StatefulWidget，这样我们就可以很方便的为每一个内层的CustomScrollView维护上面定义好的NestedInnerScrollController，同时通过TabController监听TabBar的选中状态。

  class NestedTabBarView extends StatefulWidget {final TabController? controller;final List<Widget> children;final ScrollPhysics? physics;final DragStartBehavior dragStartBehavior;final double viewportFraction;final Clip clipBehavior;const NestedTabBarView({super.key,required this.children,this.controller,this.physics,this.dragStartBehavior = DragStartBehavior.start,this.viewportFraction = 1.0,this.clipBehavior = Clip.hardEdge,});@overrideState<StatefulWidget> createState() => _NestedTabBarViewState();
  }class _NestedTabBarViewState extends State<NestedTabBarView> {List<NestedInnerScrollController> _nestedInnerControllers = [];@overridevoid initState() {super.initState();_initNestedInnerControllers();widget.controller?.addListener(_onTabChange);}@overridevoid didUpdateWidget(covariant NestedTabBarView oldWidget) {super.didUpdateWidget(oldWidget);if(oldWidget.children.length != widget.children.length) {_initNestedInnerControllers();}}@overridevoid dispose() {widget.controller?.removeListener(_onTabChange);_disposeNestedInnerControllers();super.dispose();}void _onTabChange() {}void _initNestedInnerControllers() {_disposeNestedInnerControllers();List<NestedInnerScrollController> controllers = List.generate(widget.children.length, (index) {return NestedInnerScrollController();});if(mounted) {setState(() {_nestedInnerControllers = controllers;});} else {_nestedInnerControllers = controllers;}}void _disposeNestedInnerControllers() {_nestedInnerControllers.forEach((element) {element.dispose();});}@overrideWidget build(BuildContext context) {return TabBarView(controller: widget.controller,physics: widget.physics,dragStartBehavior: widget.dragStartBehavior,viewportFraction: widget.viewportFraction,clipBehavior: widget.clipBehavior,children: widget.children);}
  }
  

• 使用InheritedWidget，将NestedInnerScrollController暴露给对应的内层CustomScrollView使用

  class _InheritedInnerScrollController extends InheritedWidget {final ScrollController controller;const _InheritedInnerScrollController({required super.child,required this.controller});@overridebool updateShouldNotify(covariant _InheritedInnerScrollController oldWidget) => controller != oldWidget.controller;}class _NestedTabBarViewState extends State<NestedTabBarView> {...@overrideWidget build(BuildContext context) {return TabBarView(controller: widget.controller,physics: widget.physics,dragStartBehavior: widget.dragStartBehavior,viewportFraction: widget.viewportFraction,clipBehavior: widget.clipBehavior,children: List<Widget>.generate(widget.children.length, (index) {return _InheritedInnerScrollController(controller: _nestedInnerControllers[index],child: widget.children[index],);}));}
  }class NestedInnerScrollController extends ScrollController {...static ScrollController of(BuildContext context) {final _InheritedInnerScrollController? target = context.dependOnInheritedWidgetOfExactType<_InheritedInnerScrollController>();assert(target != null,'NestedInnerScrollController.of must be called with a context that contains a NestedTabBarView\'s children.',);return target!.controller;}static ScrollController? maybeOf(BuildContext context) {final _InheritedInnerScrollController? target = context.dependOnInheritedWidgetOfExactType<_InheritedInnerScrollController>();return target?.controller;}}class _TabInnerListViewState extends State<_TabInnerListView> with AutomaticKeepAliveClientMixin {final int length = 20;@overrideWidget build(BuildContext context) {return CustomScrollView(controller: NestedInnerScrollController.maybeOf(context),physics: const ClampingScrollPhysics(),slivers: [...],);}@overridebool get wantKeepAlive => true;}
  

  使用的时候

  NestedScrollView(headerSliverBuilder: (BuildContext context, bool innerBoxIsScrolled) {return [...];},body: LayoutBuilder(builder: (context, _) {return Container(padding: EdgeInsets.only(top: NestedScrollView.sliverOverlapAbsorberHandleFor(context).layoutExtent ?? 0),child: NestedTabBarView(controller: _tabController,children: List.generate(_tabs.length, (index) {return _TabInnerListView(tabName: _tabs[index],);})));})
  )
  

• 监听TabBar的选中状态，然后通过NestedInnerScrollController将NestedScrollView的_innerController中所有的ScrollPosition detach，然后再attach与当前选中的NestedInnerScrollController对应的ScrollPosition。

  class NestedInnerScrollController extends ScrollController {...void attachCurrent() {if(_inner != null) {while(_inner!.positions.isNotEmpty) {_inner!.detach(_inner!.positions.first);}_inner!.attach(position);}}}class _NestedTabBarViewState extends State<NestedTabBarView> {...void _onTabChange() {int index = widget.controller!.index;if (index == widget.controller!.animation?.value) {_nestedInnerControllers[index].attachCurrent();}}...
  }
  

  

搞定。

三.完整代码

class _NestedScrollPageState extends State<NestedScrollPage> with TickerProviderStateMixin {final List<String> _tabs = ["tab1", "tab2", "tab3"];late TabController _tabController;@overridevoid initState() {super.initState();_tabController = TabController(length: _tabs.length, vsync: this);}@overridevoid dispose() {_tabController.dispose();super.dispose();}@overrideWidget build(BuildContext context) {return Scaffold(appBar: AppBar(title: const Text("nested scroll"),),body: SafeArea(child: NestedScrollView(headerSliverBuilder: (BuildContext context, bool innerBoxIsScrolled) {return [SliverToBoxAdapter(child: Container(height: 200,color: Colors.red,alignment: Alignment.center,child: const Text("banner",style: TextStyle(color: Colors.white,fontSize: 16),),),),SliverOverlapAbsorber(handle: NestedScrollView.sliverOverlapAbsorberHandleFor(context),sliver: StickySliverToBoxAdapter(child: Container(color: Colors.white,child: TabBar(tabs: List.generate(_tabs.length, (index) {return Padding(padding: const EdgeInsets.symmetric(vertical: 15),child: Text(_tabs[index]),);}),unselectedLabelColor: const Color(0xFF7B7B7B),labelColor: const Color(0xFF5E80FF),isScrollable: false,indicatorSize: TabBarIndicatorSize.label,indicator: UnderlineTabIndicator(borderRadius: BorderRadius.circular(3),borderSide: const BorderSide(color: Color(0xFF5E80FF), width: 3),insets: const EdgeInsets.symmetric(horizontal: 3, vertical: 9)),controller: _tabController,),),),),];},body: LayoutBuilder(builder: (context, _) {return Container(padding: EdgeInsets.only(top: NestedScrollView.sliverOverlapAbsorberHandleFor(context).layoutExtent ?? 0),child: NestedTabBarView(controller: _tabController,children: List.generate(_tabs.length, (index) {return _TabInnerListView(tabName: _tabs[index],);})));}))),);}}class _TabInnerListView extends StatefulWidget {final String? tabName;const _TabInnerListView({this.tabName});@overrideState<StatefulWidget> createState() => _TabInnerListViewState();}class _TabInnerListViewState extends State<_TabInnerListView> with AutomaticKeepAliveClientMixin {final int length = 20;@overrideWidget build(BuildContext context) {return CustomScrollView(controller: NestedInnerScrollController.maybeOf(context),physics: const ClampingScrollPhysics(),slivers: [...(List.generate(length, (index) {return SliverToBoxAdapter(child: Container(height: 100,margin: const EdgeInsets.only(top: 16, left: 16, right: 16),color: Colors.orange,alignment: Alignment.center,child: Text("${widget.tabName} item $index",style: const TextStyle(color: Colors.white),),),);})),const SliverToBoxAdapter(child: SizedBox(height: 16,),)],);}@overridebool get wantKeepAlive => true;}

class NestedTabBarView extends StatefulWidget {final TabController? controller;final List<Widget> children;final ScrollPhysics? physics;final DragStartBehavior dragStartBehavior;final double viewportFraction;final Clip clipBehavior;const NestedTabBarView({super.key,required this.children,this.controller,this.physics,this.dragStartBehavior = DragStartBehavior.start,this.viewportFraction = 1.0,this.clipBehavior = Clip.hardEdge,});@overrideState<StatefulWidget> createState() => _NestedTabBarViewState();
}class _NestedTabBarViewState extends State<NestedTabBarView> {List<NestedInnerScrollController> _nestedInnerControllers = [];@overridevoid initState() {super.initState();_initNestedInnerControllers();widget.controller?.addListener(_onTabChange);}@overridevoid didUpdateWidget(covariant NestedTabBarView oldWidget) {super.didUpdateWidget(oldWidget);if(oldWidget.children.length != widget.children.length) {_initNestedInnerControllers();}}@overridevoid dispose() {widget.controller?.removeListener(_onTabChange);_disposeNestedInnerControllers();super.dispose();}void _onTabChange() {int index = widget.controller!.index;if (index == widget.controller!.animation?.value) {_nestedInnerControllers[index].attachCurrent();}}void _initNestedInnerControllers() {_disposeNestedInnerControllers();List<NestedInnerScrollController> controllers = List.generate(widget.children.length, (index) {return NestedInnerScrollController();});if(mounted) {setState(() {_nestedInnerControllers = controllers;});} else {_nestedInnerControllers = controllers;}}void _disposeNestedInnerControllers() {_nestedInnerControllers.forEach((element) {element.dispose();});}@overrideWidget build(BuildContext context) {return TabBarView(controller: widget.controller,physics: widget.physics,dragStartBehavior: widget.dragStartBehavior,viewportFraction: widget.viewportFraction,clipBehavior: widget.clipBehavior,children: List<Widget>.generate(widget.children.length, (index) {return _InheritedInnerScrollController(controller: _nestedInnerControllers[index],child: widget.children[index],);}));}
}class _InheritedInnerScrollController extends InheritedWidget {final ScrollController controller;const _InheritedInnerScrollController({required super.child,required this.controller});@overridebool updateShouldNotify(covariant _InheritedInnerScrollController oldWidget) => controller != oldWidget.controller;}class NestedInnerScrollController extends ScrollController {ScrollController? _inner;NestedInnerScrollController();@overrideScrollPosition createScrollPosition(ScrollPhysics physics, ScrollContext context, ScrollPosition? oldPosition) {ScrollPosition scrollPosition;ScrollableState? scrollableState = context as ScrollableState;if(scrollableState != null) {_inner = PrimaryScrollController.maybeOf(scrollableState.context);}if(_inner == null) {scrollPosition = super.createScrollPosition(physics, context, oldPosition);} else {scrollPosition = _inner!.createScrollPosition(physics, context, oldPosition);}return scrollPosition;}@overridevoid attach(ScrollPosition position) {super.attach(position);_inner?.attach(position);}@overridevoid detach(ScrollPosition position) {_inner?.detach(position);super.detach(position);}void attachCurrent() {if(_inner != null) {while(_inner!.positions.isNotEmpty) {_inner!.detach(_inner!.positions.first);}_inner!.attach(position);}}static ScrollController of(BuildContext context) {final _InheritedInnerScrollController? target = context.dependOnInheritedWidgetOfExactType<_InheritedInnerScrollController>();assert(target != null,'NestedInnerScrollController.of must be called with a context that contains a NestedTabBarView\'s children.',);return target!.controller;}static ScrollController? maybeOf(BuildContext context) {final _InheritedInnerScrollController? target = context.dependOnInheritedWidgetOfExactType<_InheritedInnerScrollController>();return target?.controller;}}