接着上篇，没看上篇的小伙伴建议先看下上篇，免得断片中。。

我继续讲下第2个问题的解决方案。

当在里面放上tabview，并且tab是缓存状态的时候，会出现滚动会互相影响的问题

上篇我们说到 在我们的主角NestedScrollView当中，有2个ScrollController.

class _NestedScrollController extends ScrollController {_NestedScrollController(this.coordinator, {double initialScrollOffset = 0.0,String debugLabel,
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一个是inner，一个outer。 outer是负责headerSliverBuilder里面的滚动widgets inner是负责body里面的滚动widgets 当outer滚动到底了之后，就会看看inner里面是否有能滚动的东东，开始滚动

Tabview是在body里面，这里我们肯定需要对inner进行处理。 首先我们要明白，NestedScrollView是怎么处理outer和inner的关系的。

找到这个_NestedScrollCoordinator 的applyUserOffset方法中处理了整个NestedScrollView的滑动处理

 @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) {// dragging "up"// TODO(ianh): prioritize first getting rid of overscroll, and then the// outer view, so that the app bar will scroll out of the way asap.// Right now we ignore overscroll. This works fine on Android but looks// weird on iOS if you fling down then up. The problem is it's not at all// clear what this should do when you have multiple inner positions at// different levels of overscroll.final double innerDelta = _outerPosition.applyClampedDragUpdate(delta);if (innerDelta != 0.0) {for (_NestedScrollPosition position in _innerPositions)position.applyFullDragUpdate(innerDelta);}} else {// dragging "down" - delta is positive// prioritize the inner views, so that the inner content will move before the app bar growsdouble outerDelta = 0.0; // it will go positive if it changesfinal List<double> overscrolls = <double>[];final List<_NestedScrollPosition> innerPositions = _innerPositions.toList();for (_NestedScrollPosition position in innerPositions) {final double overscroll = position.applyClampedDragUpdate(delta);outerDelta = math.max(outerDelta, overscroll);overscrolls.add(overscroll);}if (outerDelta != 0.0)outerDelta -= _outerPosition.applyClampedDragUpdate(outerDelta);// now deal with any overscrollfor (int i = 0; i < innerPositions.length; ++i) {final double remainingDelta = overscrolls[i] - outerDelta;if (remainingDelta > 0.0)innerPositions[i].applyFullDragUpdate(remainingDelta);}}}
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Iterable<_NestedScrollPosition> get _innerPositions {return _innerController.nestedPositions;}
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看到_innerPositions是我们要关注的东西，通过debug，我发现，如果tabview的每个tab做了缓存，那么每个tab里面列表的ScrollPosition将一直缓存在这个ScrollController里面。 当tab到tabview的某个tab的时候，ScrollController将会将这ScrollPosition attach 上,如果没有缓存，将会在离开的时候detach掉。

@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.removeListener(_scheduleUpdateShadow);super.detach(position);_scheduleUpdateShadow();}
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真相只有一个。。是的。。可以说。。造成缓存tabview的各个tab里面的列表互相影响的原因，是因为歪果仁说： as design（我就是这样设计的，不服吗）.

按照我的思想啊，我滚动的时候。当然只想影响当前显示的这个列表啊。这不科学啊。。

找到原因找到原理，一切就都好解决了。现在的关键点在于，我怎么能知道显示对应的是哪个列表的？！

这个问题问了很多人。。也查找了好久都没找到好的方式去获取当前 激活的 列表对应的 ScrollPosition。。终于我只能想到一个 workaround。暂时解决这个问题。

提供一个容器，把inner里面的滚动列表包裹起来，并且设置它的tab 的唯一key

//pack your inner scrollables which are in  NestedScrollView body
//so that it can find the active scrollable
//compare with NestedScrollViewInnerScrollPositionKeyBuilder
class NestedScrollViewInnerScrollPositionKeyWidget extends StatefulWidget {final Key scrollPositionKey;final Widget child;NestedScrollViewInnerScrollPositionKeyWidget(this.scrollPositionKey, this.child);@override_NestedScrollViewInnerScrollPositionKeyWidgetState createState() =>_NestedScrollViewInnerScrollPositionKeyWidgetState();
}class _NestedScrollViewInnerScrollPositionKeyWidgetStateextends State<NestedScrollViewInnerScrollPositionKeyWidget> {@overrideWidget build(BuildContext context) {return widget.child;}//  @override
//  void didChangeDependencies() {//    // TODO: implement didChangeDependencies
//    //print("didChangeDependencies"+widget.scrollPositionKey.toString());
//    super.didChangeDependencies();
//  }
//
//  @override
//  void didUpdateWidget(NestedScrollViewInnerScrollPositionKeyWidget oldWidget) {//    // TODO: implement didUpdateWidget
//    //print("didUpdateWidget"+widget.scrollPositionKey.toString()+oldWidget.scrollPositionKey.toString());
//    super.didUpdateWidget(oldWidget);
//  }
}
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然后在刚才attach方法中通过先祖NestedScrollViewInnerScrollPositionKeyWidget

@overridevoid attach(ScrollPosition position) {assert(position is _NestedScrollPosition);super.attach(position);attachScrollPositionKey(position as _NestedScrollPosition);coordinator.updateParent();coordinator.updateCanDrag();position.addListener(_scheduleUpdateShadow);_scheduleUpdateShadow();}@overridevoid detach(ScrollPosition position) {assert(position is _NestedScrollPosition);position.removeListener(_scheduleUpdateShadow);super.detach(position);detachScrollPositionKey(position as _NestedScrollPosition);_scheduleUpdateShadow();}void attachScrollPositionKey(_NestedScrollPosition position) {if (position != null && scrollPositionKeyMap != null) {var key = position.setScrollPositionKey();if (key != null) {if (!scrollPositionKeyMap.containsKey(key)) {scrollPositionKeyMap[key] = position;} else if (scrollPositionKeyMap[key] != position) {//in demo ,when tab to tab03, the tab02 key will be tab00 at first//then it become tab02.//this is not a good solutionposition.clearScrollPositionKey();Future.delayed(Duration(milliseconds: 500), () {attachScrollPositionKey(position);});}}}}void detachScrollPositionKey(_NestedScrollPosition position) {if (position != null &&scrollPositionKeyMap != null &&position.key != null &&scrollPositionKeyMap.containsKey(position.key)) {scrollPositionKeyMap.remove(position.key);position.clearScrollPositionKey();}}复制代码

获取先祖NestedScrollViewInnerScrollPositionKeyWidget方法

Key setScrollPositionKey() {//if (haveDimensions) {final type = _typeOf<NestedScrollViewInnerScrollPositionKeyWidget>();NestedScrollViewInnerScrollPositionKeyWidget keyWidget =(this.context as ScrollableState)?.context?.ancestorWidgetOfExactType(type);_key = keyWidget?.scrollPositionKey;return _key;}
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找到这个_NestedScrollCoordinator 的applyUserOffset方法中我们现在要替换掉 _innerPositions为_currentInnerPositions

Iterable<_NestedScrollPosition> get _innerPositions {//return _currentPositions;return _innerController.nestedPositions;}Iterable<_NestedScrollPosition> get _currentInnerPositions {return _innerController.getCurrentNestedPositions(innerScrollPositionKeyBuilder);}复制代码

getCurrentNestedPositions里面的代码

  Iterable<_NestedScrollPosition> getCurrentNestedPositions(NestedScrollViewInnerScrollPositionKeyBuilderinnerScrollPositionKeyBuilder) {if (innerScrollPositionKeyBuilder != null &&scrollPositionKeyMap.length > 1) {var key = innerScrollPositionKeyBuilder();if (scrollPositionKeyMap.containsKey(key)) {return <_NestedScrollPosition>[scrollPositionKeyMap[key]];} else {return nestedPositions;}}return nestedPositions;}
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SampeCode

extended.NestedScrollView(headerSliverBuilder: (c, f) {return _buildSliverHeader(primaryTabBar);},//pinnedHeaderSliverHeightBuilder: () {return pinnedHeaderHeight;},innerScrollPositionKeyBuilder: () {var index = "Tab";if (primaryTC.index == 0) {index +=(primaryTC.index.toString() + secondaryTC.index.toString());} else {index += primaryTC.index.toString();}return Key(index);},
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这里由你自己协定tab key。。我这里是一级tab+二级tab的index。。比如 Tab00代表一级tab第一个下面的二级tab的第一个。

定义tab里面的列表的时候如下，比如第一个tab下面的二级tab的第一个列表，那么它的key 为Tab00.

return extended.NestedScrollViewInnerScrollPositionKeyWidget(Key("Tab00"),// myRefresh.RefreshIndicator(// child:ListView.builder(itemBuilder: (c, i) {return Container(//decoration: BoxDecoration(border: Border.all(color: Colors.orange,width: 1.0)),alignment: Alignment.center,height: 60.0,child: Text(widget.tabKey.toString() + ": List$i"),);},itemCount: 100)//,//onRefresh: onRefresh,// ));
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最后放上 Github extended_nested_scroll_view，如果你有更好的方式解决这个问题或者有什么不明白的地方，都请告诉我，由衷感谢。