如何在flutter中从CustomPainter中获取png

import 'dart:ui' as ui;
import 'package:flutter/material.dart';
import 'package:flutter/rendering.dart';
import 'package:image/image.dart' as img;
import 'package:image_gallery_saver/image_gallery_saver.dart';

class MyCustomPainter extends CustomPainter {
  @override
  void paint(Canvas canvas, Size size) {
    // 在canvas上进行绘制操作，绘制自定义图形
    // ...
  }

  @override
  bool shouldRepaint(CustomPainter oldDelegate) {
    return false;
  }
}

class MyWidget extends StatefulWidget {
  @override
  _MyWidgetState createState() => _MyWidgetState();
}

class _MyWidgetState extends State<MyWidget> {
  GlobalKey globalKey = GlobalKey();

  Future<ui.Image> capturePng() async {
    RenderRepaintBoundary boundary =
        globalKey.currentContext.findRenderObject() as RenderRepaintBoundary;
    ui.Image image = await boundary.toImage(pixelRatio: 3.0);
    return image;
  }

  void savePng(ui.Image image) async {
    img.Image imgData = img.Image.fromBytes(
      image.width,
      image.height,
      await image.toByteData(format: ui.ImageByteFormat.png),
    );
    final result = await ImageGallerySaver.saveImage(
      img.encodePng(imgData),
    );
    print(result);
  }

  @override
  Widget build(BuildContext context) {
    return RepaintBoundary(
      key: globalKey,
      child: CustomPaint(
        painter: MyCustomPainter(),
      ),
    );
  }

  @override
  void initState() {
    super.initState();
    WidgetsBinding.instance.addPostFrameCallback((_) async {
      ui.Image image = await capturePng();
      savePng(image);
    });
  }
}