ScrollController附着时发生的事件

本文介绍了ScrollController附着时发生的事件的处理方法，对大家解决问题具有一定的参考价值，需要的朋友们下面随着小编来一起学习吧！ 问题描述

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).