From ChatGPT

interactive_viewer_demo.dart

import 'package:flutter/material.dart';

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        appBar: AppBar(
          title: Text('InteractiveViewer Example'),
        ),
        body: Center(
          child: LayoutBuilder(
            builder: (context, constraints) => InteractiveViewerExample(size: constraints.biggest),
          ),
        ),
      ),
    );
  }
}

class InteractiveViewerExample extends StatefulWidget {
  const InteractiveViewerExample({required this.size});
  
  final Size size;
  
  @override
  _InteractiveViewerExampleState createState() => _InteractiveViewerExampleState();
}

const _childSize = 3000.0;

class _InteractiveViewerExampleState extends State<InteractiveViewerExample> {
  TransformationController _transformationController = TransformationController();
  final double _scaleFactor = 1.5;

  @override
  void initState() {
    super.initState();
    _transformationController.value = Matrix4.translationValues(
      -_childSize / 2 + widget.size.width / 2,
      -_childSize / 2 + widget.size.height / 2,
      0
    );
  }

  void _zoomIn() {
    setState(() {
      _transformationController.value = _scaleMatrix(_scaleFactor);
    });
  }

  void _zoomOut() {
    setState(() {
      _transformationController.value = _scaleMatrix(1 / _scaleFactor);
    });
  }

  Matrix4 _scaleMatrix(double scale) {
    final center = MediaQuery.of(context).size.center(Offset.zero);
    final translationToCenter = Matrix4.translationValues(center.dx, center.dy, 0);
    final scaleMatrix = Matrix4.diagonal3Values(scale, scale, scale);
    final translationBack = Matrix4.translationValues(-center.dx, -center.dy, 0);

    return translationToCenter * scaleMatrix * translationBack * _transformationController.value;
  }

  @override
  Widget build(BuildContext context) {
    return Column(
      children: [
        Expanded(
          child: InteractiveViewer(
            transformationController: _transformationController,
            boundaryMargin: EdgeInsets.all(20.0),
            minScale: 0.1,
            maxScale: 5.0,
            constrained: false,
            child: Container(
              width: _childSize,
              height: _childSize,
              color: Colors.blue,
              child: Center(
                child: Text(
                  'Pinch or zoom',
                  style: TextStyle(color: Colors.white, fontSize: 24),
                ),
              ),
            ),
          ),
        ),
        Row(
          mainAxisAlignment: MainAxisAlignment.center,
          children: [
            ElevatedButton(
              onPressed: _zoomIn,
              child: Text('Zoom In'),
            ),
            SizedBox(width: 20),
            ElevatedButton(
              onPressed: _zoomOut,
              child: Text('Zoom Out'),
            ),
          ],
        ),
      ],
    );
  }
}