Detect widget visibility?

[Lee Crocker]

I have a stateful widget that displays incoming data from a stream. I subscribe in initState() and close in dispose() as usual, and all works fine. When the widget is off-screen, or otherwise not visible, the state continues to update--which works just fine--but I'd rather it didn't, because acquiring the data for its update is somewhat expensive, and I'd rather suspend that when it's not needed and resume when the widget becomes visible again. I've tried using "if (mounted)" in the updater, but that continues to run when the widget is off-screen. Is there any way to detect when the widget is physically visible? 

I suppose I could simply keep track of which page I'm on and only update when those pages containing the widget are active, but that wouldn't handle the case of the widget being scrolled away or covered by a popup.

[EthicCoders Apps]

To keep track of the scrolling of the page (and its offset), in one of our project we used: ScrollController...  (This was useful for us to either show or hide the floating button...) 

Below is the code and see if this helps your case to activate or deactivate the subscription:

 

class _HomeViewState extends State<HomeView> {
  ScrollController _scrollController = new ScrollController();

....

.......

@override
void initState() {
  // TODO: implement initState
  super.initState();
  ..

  initScrollController();

}

initScrollController() {
  _scrollController.addListener((){
    if(_scrollController.offset > 135.0) {
      ....
      setState(() {
        makeShareVisible(true, generateShareText(celebrations));
      });
    }else {
      ....
      setState(() {
        makeShareVisible(false, generateShareText(celebrations));
      });
    }
  });
}

On Fri, Dec 14, 2018 at 2:29 AM Lee Crocker <l...@ansync.com> wrote:

I have a stateful widget that displays incoming data from a stream. I subscribe in initState() and close in dispose() as usual, and all works fine. When the widget is off-screen, or otherwise not visible, the state continues to update--which works just fine--but I'd rather it didn't, because acquiring the data for its update is somewhat expensive, and I'd rather suspend that when it's not needed and resume when the widget becomes visible again. I've tried using "if (mounted)" in the updater, but that continues to run when the widget is off-screen. Is there any way to detect when the widget is physically visible? 

I suppose I could simply keep track of which page I'm on and only update when those pages containing the widget are active, but that wouldn't handle the case of the widget being scrolled away or covered by a popup.

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[Olaide Nojeem Ekeolere]

Hi Lee,

  Why not use a StreamBuilder instead of subscribing to the stream yourself? Anyway you can make your widget implement RouteAware like below:

final RouteObserver<PageRoute> routeObserver = new RouteObserver<PageRoute>();

class MyClass extends StatefulWidget{
  createState() => _MyClassState();
}
class _MyClassState extends State<MyClass> with RouteAware{
  @override
  void didChangeDependencies() {
    super.didChangeDependencies();
    routeObserver.subscribe(this, ModalRoute.of(context));
  }

  @override
  void didPopNext() {
    // Covering route was popped off the navigator.
    print('Popped A route');
  }

  @override
  void didPushNext() {
    // Route was pushed onto navigator and is now topmost route.
    print('Pushed A Route');
  }

  @override
  Widget build(BuildContext context) {
    // TODO: implement build
    return null;
  }

}

On Thu, Dec 13, 2018 at 9:59 PM Lee Crocker <l...@ansync.com> wrote:

I have a stateful widget that displays incoming data from a stream. I subscribe in initState() and close in dispose() as usual, and all works fine. When the widget is off-screen, or otherwise not visible, the state continues to update--which works just fine--but I'd rather it didn't, because acquiring the data for its update is somewhat expensive, and I'd rather suspend that when it's not needed and resume when the widget becomes visible again. I've tried using "if (mounted)" in the updater, but that continues to run when the widget is off-screen. Is there any way to detect when the widget is physically visible? 

I suppose I could simply keep track of which page I'm on and only update when those pages containing the widget are active, but that wouldn't handle the case of the widget being scrolled away or covered by a popup.

[James Lin]

You might be interested in a VisibilityDetector widget we wrote:

https://github.com/google/flutter.widgets/tree/master/lib/src/visibility_detector

It's not yet available via pub (but we plan to add it there soon).

If you use it, feel free to let me know if you have any questions or if you run into any issues.

- James

On Thu, Dec 13, 2018 at 12:59 PM Lee Crocker <l...@ansync.com> wrote:

I have a stateful widget that displays incoming data from a stream. I subscribe in initState() and close in dispose() as usual, and all works fine. When the widget is off-screen, or otherwise not visible, the state continues to update--which works just fine--but I'd rather it didn't, because acquiring the data for its update is somewhat expensive, and I'd rather suspend that when it's not needed and resume when the widget becomes visible again. I've tried using "if (mounted)" in the updater, but that continues to run when the widget is off-screen. Is there any way to detect when the widget is physically visible? 

I suppose I could simply keep track of which page I'm on and only update when those pages containing the widget are active, but that wouldn't handle the case of the widget being scrolled away or covered by a popup.

[Lee Crocker]

Thanks, all, for the suggestions. The hard part for me now seems to be trapping the Android back button. That pops a page, but does not go through the Navigator, and so doesn't notify the observers, unless I'm missing something.