custom_painter_shadow.dart

import 'package:flutter/material.dart';

final 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 MyWidget extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Center(
        child: CustomPaint(
      painter: MyPainter(),
      size: Size(150, 150),
    ));
  }
}

class MyPainter extends CustomPainter {
  @override
  void paint(Canvas canvas, Size size) {
    Offset center = size.center(Offset.zero);
    Paint circlePainter = Paint()
      ..color = Colors.white
      ..style = PaintingStyle.stroke
      ..strokeWidth = 12;

    Paint shadowPainter = Paint()
      ..color = Colors.white
      ..maskFilter = MaskFilter.blur(BlurStyle.normal, 100);

    canvas.drawCircle(center, 50, circlePainter);
    canvas.drawCircle(center, 50, shadowPainter);
  }

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