inverted_rounded_rectangle_painter.dart

import 'dart:math';

import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {
  const MyApp({Key? key}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return const MaterialApp(
      debugShowCheckedModeBanner: false,
      home: MyHomePage(),
    );
  }
}

class MyHomePage extends StatelessWidget {
  const MyHomePage({Key? key}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Center(
        child: CustomPaint(
          painter: InvertedRoundedRectanglePainter(
            color: Colors.blue,
            radius: 32.0,
          ),
          child: const SizedBox(width: 300, height: 200),
        ),
      ),
    );
  }
}

class InvertedRoundedRectanglePainter extends CustomPainter {
  InvertedRoundedRectanglePainter({
    required this.radius,
    required this.color,
  });

  final double radius;
  final Color color;

  @override
  void paint(Canvas canvas, Size size) {
    final cornerSize = Size.square(radius * 2);
    canvas.drawPath(
      Path()
        ..addArc(
          Offset(0, -radius) & cornerSize,
          pi,
          -pi / 2,
        )
        ..arcTo(
          Offset(size.width - cornerSize.width, -radius) & cornerSize,
          pi / 2,
          -pi / 2,
          false,
        )
        ..lineTo(size.width, size.height)
        ..lineTo(0, size.height),
      Paint()..color = color,
    );
  }

  @override
  bool shouldRepaint(InvertedRoundedRectanglePainter oldDelegate) =>
      oldDelegate.radius != radius || oldDelegate.color != color;
}