半夜睡不着觉,把心情写成代码,只好到这里水一篇bug
FlutterCandies QQ群:181398081 说起来这些东西,其实是一个怨念,从一个issue开始。NestedScrollView Issue 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,之前的解决方案。
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});
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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);
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demo地址See your widget demo, 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
因为暂时没发现有什么好的方法区分,只是先暂时放弃,如果你有好的idea,请告诉我,万分感谢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);
}
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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issue,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NotificationListener 我的Flutter Candies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);
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使用NotificationListener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 [email protected] and show your case.");
}
coordinator.updateCanDrag();
});
}
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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);
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4.完美,perfect,beautiful??
忘记考虑多个TabBarView/PageView对结果的影响
为啥会出现这种情况呢? 因为开始我是使用的从ScrollEndNotification的Context计算出来的RenderBox,注意这个是不管你是哪个TabBarView/PageView的Page发生变化的,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。
进入localToGlobal中,再进去getTransformTo 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;
}
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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 [email protected] and show your case.");
}
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2.extended_nested_scroll_view
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extended_nested_scroll_view
最后放上 Github extended_nested_scroll_view,如果你有什么不明白的地方,请告诉我。