Drawing a inner shadow using custom paint along a path

inner_shadow_custom_paint.dart

import 'package:flutter/material.dart';

void main() => runApp(const MyApp());

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

  static const String _title = 'Flutter Code Sample';

  @override
  Widget build(BuildContext context) {
    const bgColor = Color.fromARGB(255, 249, 250, 251);
    return MaterialApp(
      title: _title,
      home: Scaffold(
        backgroundColor: bgColor,
        body: Center(
          child: SizedBox(
            height: 200,
            width: 200,
            child: CustomPaint(
              painter: MyCustomPainter(
                backgroundColor: bgColor,
              ),
            ),
          ),
        ),
      ),
    );
  }
}

class MyCustomPainter extends CustomPainter {
  MyCustomPainter({
    required this.backgroundColor,
  }) : shadow = Shadow(
          color: Colors.black38,
          offset: Offset(0, 2),
          blurRadius: 4,
        );

  final Color backgroundColor;

  final Shadow shadow;

  @override
  void paint(Canvas canvas, Size size) {
    canvas.save();
    final path = Path()
      ..lineTo(0.0, size.height - 50.0)
      ..lineTo(size.width, size.height)
      ..lineTo(size.width, 0.0)
      ..close();
    canvas.clipPath(path);

    canvas.saveLayer(Offset.zero & size, Paint());

    final outerRectPaint = Paint()
      ..color = Color.alphaBlend(shadow.color, backgroundColor);
    final innerRectPaint = Paint()
      ..color = shadow.color
      ..maskFilter = MaskFilter.blur(BlurStyle.normal, shadow.blurRadius / 2)
      ..blendMode = BlendMode.srcOut;

    canvas.drawPath(path, outerRectPaint);
    canvas.drawPath(path, innerRectPaint);

    canvas
      ..restore()
      ..restore();
  }

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

class DiagonalPathClipper extends CustomClipper<Path> {
  const DiagonalPathClipper();

  @override
  Path getClip(Size size) {
    final path = Path()
      ..lineTo(0.0, size.height - 50.0)
      ..lineTo(size.width, size.height)
      ..lineTo(size.width, 0.0)
      ..close();
    return path;
  }

  @override
  bool shouldReclip(CustomClipper<Path> oldClipper) {
    return true;
  }
}