How to solve '_positions.isNotEmpty': ScrollController not attached to any scroll views?

Question

I have an object fixedTest that looks like a list of dictionaries, and i am taking data from it to create a scrollviewobject. The original ListWheelScrollView doesn't capture taps, so i'm using the custom Widget from package clickable_list_wheel_view. When I'm trying to launch the test code:

body: ClickableListWheelScrollView(
      scrollController: _scrollController,
      itemHeight: 100,
      itemCount: 100,
      onItemTapCallback: (index) {
        print("onItemTapCallback index: $index");
      },`

Error:

[ERROR:flutter/lib/ui/ui_dart_state.cc(186)] Unhandled Exception: 'package:flutter/src/widgets/scroll_controller.dart': Failed assertion: line 108 pos 12: '_positions.isNotEmpty': ScrollController not attached to any scroll views.

Code:

    launchURL(String url) async {
    if (await canLaunch(url)) {
      await launch(url, forceWebView: true);
    } else {
      throw 'Could not launch $url';
    }
  }

  @override
  Widget build (BuildContext context) {
    test = (ModalRoute.of(context).settings.arguments);
    var fixedTest = test['data'].news4today;
    final _scrollController = ScrollController();
    checkUrls() {
      for (int i = 0; i < fixedTest.length; i++) {
        if (fixedTest[i]['urlToImage'] == null ||
            !isURL(fixedTest[i]['urlToImage'])) {
          fixedTest[i]['urlToImage'] =
              'https://i.pinimg.com/originals/8a/eb/d8/8aebd875fbddd22bf3971c3a7159bdc7.png';
        }
        if (fixedTest[i]['url'] == null || !isURL(fixedTest[i]['url'])) {
          fixedTest[i]['url'] = 'https://google.com';
        }
      }
    }

    checkUrls();

    return Scaffold(
        backgroundColor: Colors.white,
        appBar: AppBar(
          title: Center(child: Text('News')),
          backgroundColor: Colors.deepPurpleAccent,
        ),
        body: ClickableListWheelScrollView(
          scrollController: _scrollController,
          itemHeight: 100,
          itemCount: 100,
          onItemTapCallback: (index) {
            print("onItemTapCallback index: $index");
          },
          child: ListWheelScrollView.useDelegate(
              itemExtent: 400,
              diameterRatio: 9,
              squeeze: 1.2,
              physics: BouncingScrollPhysics(),
              onSelectedItemChanged: (index) {
                // debugPrint(index.toString());
              },
              childDelegate: ListWheelChildBuilderDelegate(
                  childCount: 100,
                  builder: (context, index) => Container(
                        child: Stack(
                          alignment: Alignment.topCenter,
                          children: <Widget>[
                            Container(
                              height: 353.0,
                              decoration: BoxDecoration(
                                image: DecorationImage(
                                    fit: BoxFit.scaleDown,
                                    alignment: FractionalOffset.center,
                                    image: NetworkImage(
                                        fixedTest[index]['urlToImage'])),
                              ),
                            ),
                            Container(
                              margin: EdgeInsets.only(top: 285),
                              child: Padding(
                                padding: const EdgeInsets.all(10),
                                child: SizedBox(
                                  // child: new RichText(
                                  //   text: new TextSpan(text: 'Non touchable. ', children: [
                                  //     new TextSpan(
                                  //       text: 'Tap here.',
                                  //       recognizer: new TapGestureRecognizer()..onTap = () => print('Tap Here onTap'),
                                  //     )
                                  //   ]),
                                  // )


                                    child: Text(
                                  fixedTest[index]['title'],
                                  style: TextStyle(fontSize: 16),
                                  maxLines: 4,
                                  textAlign: TextAlign.center,
                                )
                                ),
                              ),
                              decoration: BoxDecoration(
                                color: Colors.purple[100],
                                boxShadow: [
                                  BoxShadow(
                                    color: Colors.grey.withOpacity(0.8),
                                    spreadRadius: 1,
                                    blurRadius: 3,
                                    offset: Offset(1, 1),
                                  ),
                                ],
                              ),
                            )
                          ],
                        ),
                      ))),
        ))

I can't understand what's the problem. I found about addListener(), but I misunderstand how to use it.

Answer 1

Use the scroll controller after you have build the widget tree

SchedulerBinding.instance.addPostFrameCallback((_) {
  if (scrollController.hasClients) {
   //do your stuff here
  }
});