半夜睡不着觉，把心情写成代码，只好到这里水一篇bug
NestedScrollView里面有2个Scroll Control,一个outer(header),一个是inner(body)，当inner里面有PageView/TabBarView，并且每个page被缓存(AutomaticKeepAliveClientMixin or PageStorageKey)的，滑动inner会对全部的列表都有影响
之前通过key的方式来判断哪个一个列表是当前可视区域里面激活的，让NestedScrollView滑动只对它有影响，。
其实我一开始就想知道怎么知道一个widget是不是在可视区域，日夜苦读，终于找到个可行的方案来优美的解决这个问题。
文字图代码会比较多。建议准备好瓜子水。边看边吃。。
我找到的第一个API是getOffsetToReveal
/// The optional `rect` parameter describes which area of that `target` object /// should be revealed in the viewport. If `rect` is null, the entire /// `target` [RenderObject] (as defined by its [RenderObject.paintBounds]) /// will be revealed. If `rect` is provided it has to be given in the /// coordinate system of the `target` object. /// /// The `alignment` argument describes where the target should be positioned /// after applying the returned offset. If `alignment` is 0.0, the child must /// be positioned as close to the leading edge of the viewport as possible. If /// `alignment` is 1.0, the child must be positioned as close to the trailing /// edge of the viewport as possible. If `alignment` is 0.5, the child must be /// positioned as close to the center of the viewport as possible. /// /// The target might not be a direct child of this viewport but it must be a /// descendant of the viewport and there must not be any other /// [RenderAbstractViewport] objects between the target and this object. /// /// This method assumes that the content of the viewport moves linearly, i.e. /// when the offset of the viewport is changed by x then `target` also moves /// by x within the viewport. /// /// See also: /// /// * [RevealedOffset], which describes the return value of this method. RevealedOffset getOffsetToReveal(RenderObject target, double alignment, {Rect rect});复制代码 简单说下，就是获得目标RenderOject跟Viewport的距离,下面是主要用法
RenderAbstractViewport viewport = RenderAbstractViewport.of(renderObject); /// Distance between top edge of screen and MyWidget bottom edge var offsetToRevealLeading = viewport.getOffsetToReveal(renderObject, 0.0); /// Distance between bottom edge of screen and MyWidget top edge var offsetToRevealTrailingEdge = viewport.getOffsetToReveal(renderObject, 1.0);复制代码
demo地址, demo中展示了怎么判断一个ListView里面一个Widget是否进入可视区域的
这是一个新的发现，吓的我赶快在TabBarView里面试了一下。。结果。。。
这个方法能判断出每个Tab相对于自己PageView/TabBarView可视区域的相对位置。通过判断PageView/TabBarView的position.pixels 与offsetToRevealLeading是否相等，来判断当前激活的Tab,但是当有多个PageView/TabBarView的时候。你就搞不清楚到底是哪个算是激活的，因为你需要先判断父PageView/TabBarView是否激活，然后才是子PageView/TabBarView
后来我又找到个一个API （localToGlobal）
/// Convert the given point from the local coordinate system for this box to /// the global coordinate system in logical pixels. /// /// If `ancestor` is non-null, this function converts the given point to the /// coordinate system of `ancestor` (which must be an ancestor of this render /// object) instead of to the global coordinate system. /// /// This method is implemented in terms of [getTransformTo]. Offset localToGlobal(Offset point, { RenderObject ancestor }) { return MatrixUtils.transformPoint(getTransformTo(ancestor), point); }复制代码 大概的意思是。。可以算出目标跟指定对象(ancestor)的相对位置。。结果如下
你能看出来什么吗？ 哇塞，跟我想的一样，完美，用一个图表示为
这看起来是一条路。。
现在我们回到最上面那个，想解决这个issue我们还将遇到以下问题：
1.我们需要知道什么时候TabBarView/PageView的Page改变了。
为此我再次使用了熟悉的好东西 我的当中大量使用到它
if (widget.keepOnlyOneInnerNestedScrollPositionActive) { ///get notifications and compute active one in _innerController.nestedPositions return NotificationListener ( onNotification: (ScrollNotification notification) { if (notification is ScrollEndNotification && notification.metrics is PageMetrics && notification.metrics.axis == Axis.horizontal) { final PageMetrics metrics = notification.metrics; var depth = notification.depth; final int currentPage = metrics.page.round(); var page = _pageMetricsList[depth]; //ComputeActivatedNestedPosition only when page changed if (page != currentPage) { print("Page changed ${currentPage}"); _coordinator._innerController ._computeActivatedNestedPosition(notification); } _pageMetricsList[depth] = currentPage; } return false; }, child: child);复制代码 使用监听PageMetrics，并且在Page changed时候通知去计算当前在可视区域的NestedPosition.
2.只用localToGlobal 这个玩意就足够了吗？？
答案是不够的,因为ScrollEndNotification的时机还是不足够精确,导致会出现0.4，0.9之类的误差。。
解决方法：
1.加了一个100 milliseconds的延迟来执行计算
2.最后在结算与0的相比的值的时候做了个误差计算（因为不同Page的差至少为一个屏幕的差距，所以1的误差是可以忍受的）
void _computeActivatedNestedPosition(ScrollNotification notification, { Duration delay: const Duration(milliseconds: 100)}) { ///if layout is not completed, the data will has some gap. ///need more accurate time to compute ///delay it in case. ///to do Future.delayed(delay, () { /// this is the page changed of PageView's renderBox, /// it maybe not the renderBox of [nestedPositions] /// because it maybe has more one tabbarview or pageview in NestedScrollView body final RenderBox pageChangedRenderBox = notification.context.findRenderObject(); int activeCount = 0; nestedPositions.forEach((item) { item._computeActived(pageChangedRenderBox); if (item._isActived) activeCount++; }); if (activeCount > 1) { print( "activeCount more than 1, please report to zmtzawqlp@live.com and show your case."); } coordinator.updateCanDrag(); }); }复制代码 3.你以为这样就可以搞定了吗？
错了，我们忘记考虑padding和margin.
比如我给TabBarView的每个页面的List加了个一个PaddingEdgeInsets.only(left: 190.0),，让我们看看会有什么效果。
解决方式如下： position 是List跟PageView/TabBarView的相对位置 size 是List跟PageView/TabBarView 大小的差距
通过这样的计算就能抵消padding和margin的影响，当然我这里没有再考虑transform这种东西了。。放过我吧。。
顺手送个Size的获取方式，RenderBox 有个Size属性
final Offset position = child.localToGlobal(Offset.zero, ancestor: parent); ///remove the margin/padding final Offset size = Offset(parentSize.width - child.size.width, parentSize.height - child.size.height); ///if layout is not completed, the data will has some gap. ///need more accurate time to compute ///to do bool childIsActivedInViewport = ((position.dx - size.dx).abs() < 1 && (position.dy - size.dy).abs() < 1);复制代码
4.完美，perfect，beautiful？？
忘记考虑多个TabBarView/PageView对结果的影响
但是其实上，比如Tab0切换到Tab1的时候。你应该关心的是Tab1 下面的Tab10，Tab11，Tab12，Tab13的状态，Tab0下面应该都是不激活的.
其实我们应该还要找到_NestedScrollPosition所对应的PageView/TabBarView，计算_NestedScrollPosition和PageView/TabBarView的相对位置。
所以判断_NestedScrollPosition是否为当前可视区域的激活的条件应该如下：
1.ScrollEndNotification的RenderBox和_NestedScrollPosition的RenderBox的相对位置符合
2._NestedScrollPosition对应的PageView/TabBarView的RenderBox跟_NestedScrollPosition的RenderBox的相对位置符合
打印结果也证明了这点：
5.结束了？？
没有，localToGlobal这个方法，在一种情况下会报错。
Matrix4 getTransformTo(RenderObject ancestor) { assert(attached); if (ancestor == null) { final AbstractNode rootNode = owner.rootNode; if (rootNode is RenderObject) ancestor = rootNode; } final List renderers = []; for (RenderObject renderer = this; renderer != ancestor; renderer = renderer.parent) { assert(renderer != null); // Failed to find ancestor in parent chain. renderers.add(renderer); } final Matrix4 transform = Matrix4.identity(); for (int index = renderers.length - 1; index > 0; index -= 1) renderers[index].applyPaintTransform(renderers[index - 1], transform); return transform; }复制代码 这里可能会触发
assert(renderer != null); // Failed to find ancestor in parent
分析：说明你提供的ancestor 跟_NestedScrollPosition 没有关联，这时候我们直接try catch, 设置为不激活状态就好了。。
6.应该可以睡觉了吧
可以，但是我还想说2点。
1.如果当计算之后，有超过2个的nestedPositions，请告诉我一下，看看你那个复杂的case是啥（实际上，demo里面栗子已经是很复杂的了）
int activeCount = 0; nestedPositions.forEach((item) { item._computeActived(pageChangedRenderBox); if (item._isActived) activeCount++; }); if (activeCount > 1) { print( "activeCount more than 1, please report to zmtzawqlp@live.com and show your case."); }复制代码 2.
我只考虑了NestedScrollView滚动方向是垂直而且PageView/TabBarView是水平滚动的情况.
如果你有啥子妖魔鬼怪的布局，你可以试试老的
最后放上 Github ，如果你有什么不明白的地方，请告诉我。