Flutter:如何通知CustomPainter重绘？

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('CustomPainter Example')),
        body: Center(child: PainterWidget()),
      ),
    );
  }
}

class PainterWidget extends StatefulWidget {
  @override
  _PainterWidgetState createState() => _PainterWidgetState();
}

class _PainterWidgetState extends State<PainterWidget> {
  bool _redraw = false;

  void _changeState() {
    setState(() {
      _redraw = !_redraw;
    });
  }

  @override
  Widget build(BuildContext context) {
    return Column(
      mainAxisAlignment: MainAxisAlignment.center,
      children: <Widget>[
        CustomPaint(
          painter: MyCustomPainter(_redraw),
          child: Container(), // 这里可以放置一个子控件，如果需要的话
        ),
        ElevatedButton(
          onPressed: _changeState,
          child: Text('Redraw'),
        ),
      ],
    );
  }
}

class MyCustomPainter extends CustomPainter {
  final bool redraw;

  MyCustomPainter(this.redraw);

  @override
  void paint(Canvas canvas, Size size) {
    // 根据 redraw 的值来决定绘制的内容
    if (redraw) {
      // 绘制一些内容
      final paint = Paint()..color = Colors.blue;
      canvas.drawRect(Rect.fromLTWH(0, 0, size.width, size.height), paint);
    } else {
      // 绘制其他内容
      final paint = Paint()..color = Colors.red;
      canvas.drawCircle(Offset(size.width / 2, size.height / 2), size.width / 4, paint);
    }
  }

  @override
  bool shouldRepaint(covariant CustomPainter oldDelegate) {
    return true; // 总是请求重绘
  }
}