一旦我获得有关最大滚动范围的信息,我将尝试显示一个小部件。如果将 ScrollController 的实例分配给可滚动小部件的 controller 属性,则可以找到该数字。
I'm trying to display a widget once I have info about the max scroll extent. I can find that number if I assign an instance of ScrollController to the controller property of a scrollable widget.
我的问题是 ScrollController 在构建期间会附加到可滚动的小部件,所以我不能使用第一次构建之前的最大滚动范围数。因此,我想做的是在第一个版本中显示一个空的 Container ,然后将该空的 Container 切换为我真正想要的小部件。像这样:
My problem is that the ScrollController gets attached to the scrollable widget during the build, so I can not use the max scroll extent number before the first build. Thus what I was trying to do is display an empty Container in the first build and then switch that empty Container with the widget I actually want. Something like this:
_scrollController.positions.length == 0 ? new Container() : new Align( alignment: Alignment.center, child: new Container( width: constraints.maxWidth, height: 50.0, color: Colors.black, ) )现在这不起作用当然是因为 _scrollController.positions.length 开头将为0,并且当此值更改时,我无处调用 setState ( 所以我的问题:是否有一个地方可以在 ScrollController 通知我附加到可滚动的小部件?还是有更好的方法呢?
Now this does not work of course because _scrollController.positions.length will be 0 at the beginning and nowhere do I call setState when this value changes (when the controller gets attached). So my question: Is there a place where I can get notified whenever the ScrollController gets attached to a scrollable widget? Or is there a better approach for this?
推荐答案如果滚动条是 widget.child 。
@override Widget build(BuildContext context) { return new NotificationListener<ScrollNotification>( onNotification: _handleScrollNotification, child: widget.child, ); } bool _handleScrollNotification(ScrollNotification notification) { if (notification is ScrollUpdateNotification || notification is OverscrollNotification) { widget.child.update(notification.metrics); } return false; } @override void initState() { super.initState(); WidgetsBinding.instance.addPostFrameCallback((_) => afterFirstLayout(context)); } void afterFirstLayout(BuildContext context) { applyInitialScrollPosition(); } void applyInitialScrollPosition() { ScrollController scrollControler = widget.child.controller; ScrollPosition position = scrollControler.position; ScrollNotification notification = ScrollUpdateNotification( metrics: FixedScrollMetrics( minScrollExtent: position.minScrollExtent, maxScrollExtent: position.maxScrollExtent, pixels: position.pixels, viewportDimension: position.viewportDimension, axisDirection: position.axisDirection), context: null, scrollDelta: 0.0); _handleScrollNotification(notification); }孩子必须扩展ChangeNotifier 并具有 update 方法:
void update(ScrollMetrics metrics) { assert(metrics != null); _lastMetrics = metrics; // Save the metrics. notifyListeners(); }所有这一切仅在为可滚动对象( widget.child)。
All this only works if a scroll controller has explicitly been defined for the scrollable (widget.child).
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ScrollController附着时发生的事件
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