inner_shadow_painter.dart

import 'package:flutter/rendering.dart';

enum InnerShadowAlgo { sides, corners, overlap }

class InnerShadowPainter extends CustomPainter {
  const InnerShadowPainter({
    super.repaint,
    required this.boxShadow,
    this.borderRadius = BorderRadius.zero,
    this.algo = InnerShadowAlgo.sides,
  });

  final List<BoxShadow> boxShadow;
  final BorderRadius borderRadius;
  final InnerShadowAlgo algo;

  @override
  void paint(Canvas canvas, Size size) {
    for (final boxShadow in boxShadow) {
      final paint = boxShadow.toPaint();
      final rect = Offset.zero & size;
      final Size(width: w, height: h) = size;
      final rrect = RRect.fromRectAndCorners(
        rect,
        topLeft: borderRadius.topLeft,
        topRight: borderRadius.topRight,
        bottomRight: borderRadius.bottomRight,
        bottomLeft: borderRadius.bottomLeft,
      );

      final top = Rect.fromLTWH(-w, -h, w * 3, h);
      final right = Rect.fromLTWH(w, -h, w, h * 3);
      final bottom = Rect.fromLTWH(-w, h, w * 3, h);
      final left = Rect.fromLTWH(-w, -h, w, h * 3);

      final topLeft = Rect.fromLTWH(-w, -h, w, h);
      final topRight = Rect.fromLTWH(w, -h, w, h);
      final bottomRight = Rect.fromLTWH(w, h, w, h);
      final bottomLeft = Rect.fromLTWH(-w, h, w, h);

      final topLeftV = Rect.fromLTWH(-w, -h, w, h * 1.5);
      final topLeftH = Rect.fromLTWH(-w, -h, w * 1.5, h);

      final topRightV = Rect.fromLTWH(w * 0.5, -h, w * 1.5, h);
      final topRightH = Rect.fromLTWH(w, -h, w, h * 1.5);

      final bottomRightV = Rect.fromLTWH(w, h * 0.5, w, h * 1.5);
      final bottomRightH = Rect.fromLTWH(w * 0.5, h, w * 1.5, h);

      final bottomLeftV = Rect.fromLTWH(-w, h * 0.5, w, h * 1.5);
      final bottomLeftH = Rect.fromLTWH(-w, h, w * 1.5, h);

      final rectangles = switch (algo) {
        InnerShadowAlgo.sides => [left, top, right, bottom],
        InnerShadowAlgo.corners => [topLeft, topRight, bottomRight, bottomLeft],
        InnerShadowAlgo.overlap => [
          topLeftV,
          topLeftH,
          topRightV,
          topRightH,
          bottomRightV,
          bottomRightH,
          bottomLeftV,
          bottomLeftH,
        ],
      };

      canvas.save();
      canvas.clipRRect(rrect);

      for (final rect in rectangles) {
        canvas.drawRect(
          rect.shift(boxShadow.offset).inflate(boxShadow.spreadRadius),
          paint,
        );
      }

      canvas.restore();
    }
  }

  @override
  bool shouldRepaint(covariant InnerShadowPainter oldDelegate) =>
      oldDelegate.boxShadow != boxShadow ||
      oldDelegate.borderRadius != borderRadius ||
      oldDelegate.algo != algo;
}