flutter ScrollController attached to multiple scroll views

Question

when multiple instances of widget are rendered and the getValue method is called flutter throws the error ScrollController attached to multiple scroll views. I'm assuming this is because they all are using the same controller but I don't know a way to fix this without creating a separate widget for each time it is used. Is there a better way to fix this?

class NumScroller extends StatelessWidget{
  final int max,min;
  final double height,width;
  final TextAlign alignment;

  static ScrollController controller;

  NumScroller({this.height,this.width,this.alignment,this.min,this.max, initialOffset}){
    controller = new ScrollController(initialScrollOffset: initialOffset);
  }

  getValue() => (controller.offset~/height) + min;

  @override
  Widget build(BuildContext context) {
    return new Container(
        width: width,
        height: height,
        child: ListView.builder(itemBuilder: (context, index) {
          return new Container(height: height, child:Text((max - index).toString(),textAlign: alignment,));
          },
          itemCount: max - min+1,
          controller: controller,
          physics: PageScrollPhysics(),
          itemExtent: height,
        )
    );
  }

}

Because then you can have one controller per instance of the widget. Not one static controller used everywhere. — Rémi Rousselet
Sorry I didn't understand. Even with Stateless we can have one controller per instance of the widget — Dinesh Balasubramanian
@DineshBalasubramanian The controller should be kept. Recreating a new one every time a props change (which is what happens when using StatelessWidget) has undesired effects. — Rémi Rousselet

Rémi Rousselet Rémi Rousselet · Accepted Answer · 2018-09-24T18:24:47

Instead of using a static controller, you should have one controller per widget instance.

You cannot store your controller inside a StatelessWidget though (even if the compiler will allow it).

You need a StatefulWidget for that, or else when your widget is updated you will create a new controller again. Leading to weird behaviors.

Here the final code:

class NumScroller extends StatefulWidget{
  final int max,min;
  final double height,width;
  final TextAlign alignment;

  NumScroller({this.height,this.width,this.alignment,this.min,this.max, initialOffset});

  @override
  NumScrollerState createState() {
    return new NumScrollerState();
  }
}

class NumScrollerState extends State<NumScroller> {
  final ScrollController controller = ScrollController();

  getValue() => (controller.offset~/widget.height) + widget.min;

  @override
  Widget build(BuildContext context) {
    return new Container(
        width: widget.width,
        height: widget.height,
        child: ListView.builder(itemBuilder: (context, index) {
          return new Container(height: widget.height, child:Text((widget.max - index).toString(),textAlign: widget.alignment,));
          },
          itemCount: widget.max - widget.min+1,
          controller: controller,
          physics: PageScrollPhysics(),
          itemExtent: widget.height,
        )
    );
  }
}

flutter ScrollController attached to multiple scroll views

2 Answers