A single child scroll view widget that allows control over the number of pointers that enable scrolling

n_pointer_single_child_scroll_view.dart

import 'package:flutter/gestures.dart';
import 'package:flutter/material.dart';

///copied and modified from [stackoverflow](https://stackoverflow.com/questions/76155321/how-can-i-recognize-two-fingers-on-the-screen-in-flutter/76156071?noredirect=1#comment134304616_76156071)
class NPointerSingleChildScrollView extends StatefulWidget {
  final int numberOfPointers;

  final Axis scrollDirection;
  final bool reverse;
  final EdgeInsetsGeometry? padding;
  final ScrollController? controller;
  final bool? primary;
  final ScrollPhysics? physics;
  final Widget? child;
  final DragStartBehavior dragStartBehavior;
  final Clip clipBehavior;
  final String? restorationId;
  final ScrollViewKeyboardDismissBehavior keyboardDismissBehavior;

  const NPointerSingleChildScrollView({
    this.scrollDirection = Axis.vertical,
    this.reverse = false,
    this.padding,
    this.primary,
    this.physics,
    this.controller,
    this.child,
    this.dragStartBehavior = DragStartBehavior.start,
    this.clipBehavior = Clip.hardEdge,
    this.restorationId,
    this.keyboardDismissBehavior = ScrollViewKeyboardDismissBehavior.manual,
    super.key,
    required this.numberOfPointers,
  });

  @override
  State<NPointerSingleChildScrollView> createState() =>
      _NPointerSingleChildScrollViewState();
}

class _NPointerSingleChildScrollViewState
    extends State<NPointerSingleChildScrollView> {
  @override
  Widget build(BuildContext context) {
    // Need logic to make it true when two or more fingers on the screen
    Set<int> touchPositions = {};
    void savePointerPosition(int index) {
      setState(() {
        touchPositions.add(index);
      });
    }

    void clearPointerPosition(int index) {
      setState(() {
        touchPositions.remove(index);
      });
    }

    return Listener(
      onPointerDown: (opm) {
        savePointerPosition(opm.pointer);
      },
      onPointerMove: (opm) {
        savePointerPosition(opm.pointer);
      },
      onPointerCancel: (opc) {
        clearPointerPosition(opc.pointer);
      },
      onPointerUp: (opc) {
        clearPointerPosition(opc.pointer);
      },
      child: SingleChildScrollView(
        physics: touchPositions.length != widget.numberOfPointers
            ? const NeverScrollableScrollPhysics()
            : widget.physics,
        scrollDirection: widget.scrollDirection,
        reverse: widget.reverse,
        padding: widget.padding,
        primary: widget.primary,
        controller: widget.controller,
        dragStartBehavior: widget.dragStartBehavior,
        clipBehavior: widget.clipBehavior,
        restorationId: widget.restorationId,
        keyboardDismissBehavior: widget.keyboardDismissBehavior,
        child: widget.child,
      ),
    );
  }
}