inset_shadow.dart

import 'dart:ui';
import 'package:flutter/material.dart';
import 'package:flutter/rendering.dart';

///Inset Shadow
class InsetShadow extends SingleChildRenderObjectWidget {
  const InsetShadow({
    Key key,
    this.shadows = const <Shadow>[],
    Widget child,
  }) : super(key: key, child: child);

  final List<Shadow> shadows;

  @override
  RenderObject createRenderObject(BuildContext context) {
    final renderObject = _RenderInnerShadow();
    updateRenderObject(context, renderObject);
    return renderObject;
  }

  @override
  void updateRenderObject(
      BuildContext context, _RenderInnerShadow renderObject) {
    renderObject.shadows = shadows;
  }
}

class _RenderInnerShadow extends RenderProxyBox {
  List<Shadow> shadows;

  @override
  void paint(PaintingContext context, Offset offset) {
    if (child == null) return;
    final bounds = offset & size;
    
    context.canvas.saveLayer(bounds, Paint());
    context.paintChild(child, offset);

    for (final shadow in shadows) {
      final shadowRect = bounds.inflate(shadow.blurSigma);
      final shadowPaint = Paint()
        ..blendMode = BlendMode.srcATop
        ..colorFilter = ColorFilter.mode(shadow.color, BlendMode.srcOut)
        ..imageFilter = ImageFilter.blur(
            sigmaX: shadow.blurSigma, sigmaY: shadow.blurSigma);
      context.canvas
        ..saveLayer(shadowRect, shadowPaint)
        ..translate(shadow.offset.dx, shadow.offset.dy);
      context.paintChild(child, offset);
      context.canvas.restore();
    }

    context.canvas.restore();
  }
}