Flutter ScrollController NoSuchMethodError: invalid member on null: 'addListener'

Question

i've been trying to make a listview in flutter and get how many pixels the view had scrolled. Unfortunately, i used NotificationListener

Like this:

          NotificationListener<ScrollUpdateNotification>(
              child: ListView(
                children: [
                    //content here
                ],
              ),
              onNotification: (notification) {
                //How many pixels scrolled from pervious frame
                // print("scrollDelta: ${notification.scrollDelta}");
                setState(() {
                  this.scrollValue = notification.metrics.pixels;
                });
                //List scroll position
                // print("metrics.pixels: ${notification.metrics.pixels}");
                return true;
              },
            ),

Issue is that it gave some very poor performance with my application.

So i noticed that you could use a controller directly with the ListView and i made the following code:

ScrollController controller;
    controller.addListener(() {
      print("i'm here");
    });

    var scaffold = Scaffold(
        backgroundColor: Color(0xFFFFFFFF),
        resizeToAvoidBottomPadding: false,
        body: Stack(
          children: [
            ListView(
              controller: controller,
              children: [
                  //content here
              ],
            ),
          ],
        ),
     );

But then i get the following error :

NoSuchMethodError: invalid member on null: 'addListener'

I currently have no idea how to get how many pixel were scrolled in a listview without hindering greatly the performances.

Wilson Wilson · Accepted Answer · 2021-01-21 11:17:11Z

1

Your ScrollController is initialized as null.

Just give it a value:

ScrollController controller = ScrollController();

Make sure you dispose of it using the dispose method of your StatefulWidget.

@override
void dispose(){
   controller.dispose();
}

answered Jan 21, 2021 at 11:17

Wilson Wilson

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Ahmed Raza · Accepted Answer · 2021-01-21 11:28:16Z

0

ScrollController controller;



@override
  void initState() {
    // TODO: implement initState
    controller = new ScrollController()..addListener(_scrollListener);
    super.initState();
  }



void _scrollListener() {
   print(controller.position.extentAfter);
    if (controller.position.extentAfter < 500) {
      // you reached at page bottom
    }
  }

//////////////////////////

ListView(
              controller: controller,
              children: [
                  //content here
              ],
            ),

answered Jan 21, 2021 at 11:28

Ahmed Raza

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Blanky · Accepted Answer · 2021-01-21 12:08:11Z

0

Sorry i forgot to simply initialise it! Now it works but there is still my problem with lag whenever i add a listener.


class Search extends StatefulWidget {
  String name;
  String location;
  Search({
    this.name,
    this.location,
    Key key,
  }) : super(key: key);

  ScrollController controller = ScrollController();
  dispose() {
    controller.dispose();
  }

  @override
  _Search createState() => _Search();
}

class _Search extends State<Search> {

    widget.controller.addListener(() {
      setState(() {
        scrollValue = widget.controller.position.pixels;
      });
    });

   /// some unrelated code here

            ListView(
              controller: widget.controller,
              children: [
   /// some unrelated code here again

}

whenever i add the addListener the performances drop very harshly

answered Jan 21, 2021 at 12:08

Blanky

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1 Comment

Blanky Over a year ago

Turns out using setstate with a scrollcontroller event was the issue. I solved this issue by linking the scroll event to an AnimatedBuilder et it solved the issue of lag

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