How to use SizeChangedLayoutNotifier? - flutter

Question

I got a Wrap inside my flexibleSpace in my SliverAppBar. Now when the size of my Wrap changes (dynamically) I need to get a notification, so I can change the height of my flexibleSpace in my SliverAppBar accordingly.
I read the docs to the SizeChangedLayoutNotifier but I don't really understand how to use it. I wrapped my Wrap as a child in a SizeChangedLayoutNotifier but for me it is very unclear how to catch the notification with the NotificationListener<SizeChangedLayoutNotification>. I couldn't find any code example.
I would really appreciate your help :) Thanks!

Answer 1

I finally figured it out and will post the solution here for others, having the same problem.
I put my Wrap inside like this:
new NotificationListener<SizeChangedLayoutNotification>(
onNotification: gotNotification,
child: SizeChangedLayoutNotifier(
key: _filterBarChangeKey,
child: Wrap( // my WRAP stuff)
)
);
and having my callback like this:
bool gotNotification(SizeChangedLayoutNotification notification) {
// change height here
_filterBarChangeKey = GlobalKey();
}
I also found another solution from here not using SizeChangedLayoutNotification at all to solve my problem. For me this was even better. I just wrapped my Widget inside an MeaserSize widget which provides an onSizeChanged callback.
typedef void OnWidgetSizeChange(Size size);
class MeasureSize extends StatefulWidget {
final Widget child;
final OnWidgetSizeChange onChange;
const MeasureSize({
Key key,
#required this.onChange,
#required this.child,
}) : super(key: key);
#override
_MeasureSizeState createState() => _MeasureSizeState();
}
class _MeasureSizeState extends State<MeasureSize> {
var widgetKey = GlobalKey();
#override
Widget build(BuildContext context) {
WidgetsBinding.instance
.addPostFrameCallback((_) => widget.onChange(widgetKey.currentContext.size));
return Container(
key: widgetKey,
child: widget.child,
);
}
}