flutter自绘组件，实现图片展示在屏幕正中间，图片宽高自适应，图片可拖动，放大缩小，写出示例代码。

以下是一个Flutter自绘组件的示例代码，可以实现图片展示在屏幕正中间，图片宽高自适应，图片可拖动，放大缩小的功能：

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

class ImageEditor extends StatefulWidget {
  final ImageProvider imageProvider;

  ImageEditor({Key key, this.imageProvider}) : super(key: key);

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

class _ImageEditorState extends State<ImageEditor> {
  double _scale = 1.0;
  double _previousScale = 1.0;
  Offset _offset = Offset.zero;
  Offset _previousOffset = Offset.zero;
  Size _imageSize;

  @override
  Widget build(BuildContext context) {
    return GestureDetector(
      onScaleStart: _handleScaleStart,
      onScaleUpdate: _handleScaleUpdate,
      onScaleEnd: _handleScaleEnd,
      child: CustomPaint(
        painter: _ImagePainter(
          imageProvider: widget.imageProvider,
          scale: _scale,
          offset: _offset,
          imageSize: _imageSize,
        ),
        child: SizedBox.expand(),
      ),
    );
  }

  void _handleScaleStart(ScaleStartDetails details) {
    _previousScale = _scale;
    _previousOffset = _offset;
  }

  void _handleScaleUpdate(ScaleUpdateDetails details) {
    setState(() {
      _scale = _previousScale * details.scale;
      _offset = _previousOffset +
          (details.focalPoint - details.localFocalPoint) / _scale;
    });
  }

  void _handleScaleEnd(ScaleEndDetails details) {
    _previousScale = 1.0;
    _previousOffset = _offset;
  }

  @override
  void initState() {
    super.initState();
    Image image = Image(image: widget.imageProvider);
    image.image.resolve(ImageConfiguration()).addListener(
      ImageStreamListener((info, _) {
        setState(() {
          _imageSize = Size(
            info.image.width.toDouble(),
            info.image.height.toDouble(),
          );
        });
      }),
    );
  }
}

class _ImagePainter extends CustomPainter {
  final ImageProvider imageProvider;
  final double scale;
  final Offset offset;
  final Size imageSize;

  _ImagePainter({
    this.imageProvider,
    this.scale,
    this.offset,
    this.imageSize,
  });

  @override
  void paint(Canvas canvas, Size size) async {
    Paint paint = Paint();
    paint.isAntiAlias = true;
    paint.filterQuality = FilterQuality.high;

    Rect rect = Offset.zero & size;
    canvas.clipRect(rect);

    if (imageSize != null) {
      double width = imageSize.width * scale;
      double height = imageSize.height * scale;
      double x = offset.dx + (size.width - width) / 2;
      double y = offset.dy + (size.height - height) / 2;

      Rect imageRect = Rect.fromLTWH(x, y, width, height);
      ui.Image image = await imageProvider.resolve(
        ImageConfiguration(
          size: Size(imageRect.width, imageRect.height),
        ),
      );
      canvas.drawImage(image, imageRect.topLeft, paint);
    }
  }

  @override
  bool shouldRepaint(_ImagePainter oldDelegate) {
    return oldDelegate.scale != scale ||
        oldDelegate.offset != offset ||
        oldDelegate.imageSize != imageSize;
  }
}