How do I pan and zoom an image?

Based on the code from the question and encouraged by Collin's answer, this is what I came up with:

import 'package:flutter/material.dart';
import 'package:vector_math/vector_math_64.dart';

class InteractiveImage extends StatefulWidget {
  InteractiveImage(this.image, {Key key}) : super(key: key);

  final Image image;

  @override
  _InteractiveImageState createState() => new _InteractiveImageState();
}

class _InteractiveImageState extends State<InteractiveImage> {
  _InteractiveImageState();

  double _scale = 1.0;
  double _previousScale = null;

  @override
  Widget build(BuildContext context) {
    setState(() => print("STATE SET\n"));
    return new GestureDetector(
      onScaleStart: (ScaleStartDetails details) {
        print(details);
        // Does this need to go into setState, too?
        // We are only saving the scale from before the zooming started
        // for later - this does not affect the rendering...
        _previousScale = _scale;
      },
      onScaleUpdate: (ScaleUpdateDetails details) {
        print(details);
        setState(() => _scale = _previousScale * details.scale);
      },
      onScaleEnd: (ScaleEndDetails details) {
        print(details);
        // See comment above
        _previousScale = null;
      },
      child: new Transform(
        transform: new Matrix4.diagonal3(new Vector3(_scale, _scale, _scale)),
        alignment: FractionalOffset.center,
        child: widget.image,
      ),
    );
  }
}

While it seems to work, it would be great to get confirmation or correction from someone inside the Flutter team.

(Also, this only allows for zooming. Panning is not implemented.)

EDIT: Edited to reflect Collin's comment and his his more detailed explanation. But note there is still an open question with regards to private setState() and private fields that don't immediately affect the rendering.

You can use InteractiveViewer which comes out of the box with Flutter.

@override
Widget build(BuildContext context) {
  return Center(
    child: InteractiveViewer(
      panEnabled: false, // Set it to false
      boundaryMargin: EdgeInsets.all(100),
      minScale: 0.5,
      maxScale: 2,
      child: Image.asset(
        'your_image_asset',
        width: 200,
        height: 200,
        fit: BoxFit.cover,
      ),
    ),
  );
}

I'm not sure what you mean by "doesn't work" but you're on the right track.

Check out https://github.com/flutter/flutter/blob/master/examples/layers/widgets/gestures.dart for a working example of a widget you can pan, pinch, and scale.