paint onClick

main.dart

import 'package:flutter/material.dart';

const Color darkBlue = Color.fromARGB(255, 18, 32, 47);

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      theme: ThemeData.dark().copyWith(
        scaffoldBackgroundColor: darkBlue,
      ),
      debugShowCheckedModeBanner: false,
      home: Scaffold(
        body: Center(
          child: MYWidget(),
        ),
      ),
    );
  }
}


class MyPainter extends CustomPainter {
  @override
  void paint(Canvas canvas, Size size) {
    final paint = Paint()..color = Colors.yellow;

    var path = Path();

    final startPoint = Offset(0, size.height);
    final endPoint = Offset(size.width, size.height);

    path.quadraticBezierTo(
        startPoint.dx, startPoint.dy, endPoint.dx, endPoint.dy);

    path.close();
    canvas.drawPath(path, paint);
  }

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

class MYWidget extends StatefulWidget {
  MYWidget({Key? key}) : super(key: key);

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

class _MYWidgetState extends State<MYWidget> {
  bool _showPaint = false;

  @override
  Widget build(BuildContext context) {
    return Column(
      children: [
        CustomPaint(
          foregroundPainter: _showPaint ? MyPainter() : null,
          child: SizedBox(
            height: 100,
            width: 100,
          ),
        ),
        ElevatedButton(
          onPressed: () {
            setState(() {
              _showPaint = true;
            });
          },
          child: Text(
            "show",
          ),
        ),
      ],
    );
  }
}