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,
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
@override
void 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 grows
double outerDelta = 0.0; // it will go positive if it changes
final List overscrolls = [];
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 overscroll
for (int i = 0; i < innerPositions.length; ++i) {
final double remainingDelta = overscrolls[i] - outerDelta;
if (remainingDelta > 0.0)
innerPositions[i].applyFullDragUpdate(remainingDelta);
}
}
}
Iterable<_NestedScrollPosition> get _innerPositions {
return _innerController.nestedPositions;
}
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。
@override
void attach(ScrollPosition position) {
assert(position is _NestedScrollPosition);
super.attach(position);
coordinator.updateParent();
coordinator.updateCanDrag();
position.addListener(_scheduleUpdateShadow);
_scheduleUpdateShadow();
}
@override
void detach(ScrollPosition position) {
assert(position is _NestedScrollPosition);
position.removeListener(_scheduleUpdateShadow);
super.detach(position);
_scheduleUpdateShadow();
}
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 _NestedScrollViewInnerScrollPositionKeyWidgetState
extends State {
@override
Widget 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);
// }
}
然后在刚才attach方法中通过先祖NestedScrollViewInnerScrollPositionKeyWidget
@override
void attach(ScrollPosition position) {
assert(position is _NestedScrollPosition);
super.attach(position);
attachScrollPositionKey(position as _NestedScrollPosition);
coordinator.updateParent();
coordinator.updateCanDrag();
position.addListener(_scheduleUpdateShadow);
_scheduleUpdateShadow();
}
@override
void 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 solution
position.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();
}
}
获取先祖NestedScrollViewInnerScrollPositionKeyWidget方法
Key setScrollPositionKey() {
//if (haveDimensions) {
final type = _typeOf();
NestedScrollViewInnerScrollPositionKeyWidget keyWidget =
(this.context as ScrollableState)
?.context
?.ancestorWidgetOfExactType(type);
_key = keyWidget?.scrollPositionKey;
return _key;
}
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);
}
getCurrentNestedPositions里面的代码
Iterable<_NestedScrollPosition> getCurrentNestedPositions(
NestedScrollViewInnerScrollPositionKeyBuilder
innerScrollPositionKeyBuilder) {
if (innerScrollPositionKeyBuilder != null &&
scrollPositionKeyMap.length > 1) {
var key = innerScrollPositionKeyBuilder();
if (scrollPositionKeyMap.containsKey(key)) {
return <_NestedScrollPosition>[scrollPositionKeyMap[key]];
} else {
return nestedPositions;
}
}
return nestedPositions;
}
一 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);
},
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,
// )
);
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。