ScrollController附着时发生的事件 [英] Event when ScrollController get attached

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).

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