custom_image_scroller.dart

class CustomImageScroller extends StatefulWidget {
  final String asset;
  CustomImageScroller({Key key, this.asset}) : super(key: key);

  @override
  _CustomImageScrollerState createState() => _CustomImageScrollerState();
}

class _CustomImageScrollerState extends State<CustomImageScroller> with TickerProviderStateMixin {
  AnimationController xc;
  AnimationController yc;
  ValueNotifier<ui.Image> imageNotifier;
  double dx = 0.0;
  double dy = 0.0;

  @override
  void initState() {
    super.initState();
    xc = AnimationController.unbounded(vsync: this);
    yc = AnimationController.unbounded(vsync: this);
    imageNotifier = ValueNotifier(null);
    Future(() => rootBundle.load(widget.asset).then(_decodeAssetData));
  }

  @override
  Widget build(BuildContext context) {
    print('_CustomImageScrollerState.build');
    return GestureDetector(
      onPanUpdate: (d) {
        _handlePanUpdate(xc, dx, d.delta.dx);
        _handlePanUpdate(yc, dy, d.delta.dy);
      },
      onPanEnd: (d) {
        _handlePanEnd(xc, dx, d.velocity.pixelsPerSecond.dx);
        _handlePanEnd(yc, dy, d.velocity.pixelsPerSecond.dy);
      },
      child: CustomPaint(
        painter: ImagePainter(xc, yc, imageNotifier),
        child: SizedBox.expand(),
      ),
    );
  }

  void _decodeAssetData(data) async {
    print('_decodeAssetData');
    var image = await decodeImageFromList(data.buffer.asUint8List());
    imageNotifier.value = image;
    RenderBox rb = context.findRenderObject();
    dx = rb.size.width - image.width;
    dy = rb.size.height - image.height;
    if (dx > 0) xc.value = dx / 2;
    if (dy > 0) yc.value = dy / 2;
  }

  void _handlePanUpdate(AnimationController controller, double sizeDifference, double delta) {
    if (sizeDifference < 0) {
      controller.value += delta;
    }
  }

  final sd = SpringDescription.withDampingRatio(
    mass: 0.5,
    stiffness: 300.0,
    ratio: 0.85,
  );
  void _handlePanEnd(AnimationController controller, double sizeDifference, double velocity) {
    if (sizeDifference < 0) {
      var simulation = (sizeDifference <= controller.value && controller.value <= 0)?
        ClampedSimulation(FrictionSimulation(0.001, controller.value, velocity), xMin: sizeDifference, xMax: 0) :
        SpringSimulation(sd, controller.value, controller.value.clamp(sizeDifference, 0.0), 0);
      controller.animateWith(simulation);
    }
  }
}

class ImagePainter extends CustomPainter {
  final AnimationController xc, yc;
  final ValueNotifier<ui.Image> imageNotifier;

  ImagePainter(this.xc, this.yc, this.imageNotifier) : super(repaint: Listenable.merge([xc, yc, imageNotifier]));

  @override
  void paint(ui.Canvas canvas, ui.Size size) {
    // print('$xc $yc $imageNotifier');
    canvas.clipRect(Offset.zero & size);
    if (imageNotifier.value != null) {
      canvas.drawImage(imageNotifier.value, Offset(xc.value, yc.value), Paint());
    }
  }

  @override
  bool shouldRepaint(CustomPainter oldDelegate) => true;
}