import 'dart:math' as math show pi;

import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Container(
      color: Colors.white,
      child: CustomPaint(painter: TestPainter()),
    );
  }
}

class TestPainter extends CustomPainter {
  @override
  void paint(Canvas canvas, Size size) {
    canvas.drawArc(
      Rect.fromCircle(center: size.center(Offset.zero), radius: 100),
      0,
      1.5 * math.pi,
      true,
      Paint()
        ..color = Colors.blue.withAlpha(127)
        ..style = PaintingStyle.fill,
    );
  }

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