Inner shadow effect in flutter

I've taken the answer by @AlexandrPriezzhev and improved it to use the standard Shadow class (including the semantics of its offset field), added support for multiple shadows, and shaved off a saveLayer() call which should make it a bit more efficient:

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

class InnerShadow extends SingleChildRenderObjectWidget {
  const InnerShadow({
    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 {
  late 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();
  }
}

decoration: BoxDecoration(
        boxShadow: [
          const BoxShadow(
            color: your_shadow_color,
          ),
          const BoxShadow(
            color: your_bg_color,
            spreadRadius: -12.0,
            blurRadius: 12.0,
          ),
        ],
      ),

Here is what I do:

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

class InnerShadow extends SingleChildRenderObjectWidget {
  const InnerShadow({
    Key key,
    this.blur = 10,
    this.color = Colors.black38,
    this.offset = const Offset(10, 10),
    Widget child,
  }) : super(key: key, child: child);

  final double blur;
  final Color color;
  final Offset offset;

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

  @override
  void updateRenderObject(
      BuildContext context, _RenderInnerShadow renderObject) {
    renderObject
      ..color = color
      ..blur = blur
      ..dx = offset.dx
      ..dy = offset.dy;
  }
}

class _RenderInnerShadow extends RenderProxyBox {
  double blur;
  Color color;
  double dx;
  double dy;

  @override
  void paint(PaintingContext context, Offset offset) {
    if (child == null) return;

    final Rect rectOuter = offset & size;
    final Rect rectInner = Rect.fromLTWH(
      offset.dx,
      offset.dy,
      size.width - dx,
      size.height - dy,
    );
    final Canvas canvas = context.canvas..saveLayer(rectOuter, Paint());
    context.paintChild(child, offset);
    final Paint shadowPaint = Paint()
      ..blendMode = BlendMode.srcATop
      ..imageFilter = ImageFilter.blur(sigmaX: blur, sigmaY: blur)
      ..colorFilter = ColorFilter.mode(color, BlendMode.srcOut);

    canvas
      ..saveLayer(rectOuter, shadowPaint)
      ..saveLayer(rectInner, Paint())
      ..translate(dx, dy);
    context.paintChild(child, offset);
    context.canvas..restore()..restore()..restore();
  }
}

then just use it somewhere:

InnerShadow(
  blur: 5,
  color: const Color(0xFF477C70),
  offset: const Offset(5, 5),
  child: Container(
    decoration: const BoxDecoration(
      borderRadius: BorderRadius.all(Radius.circular(8)),
      color: Color(0xFFE9EFEC),
    ),
    height: 100,
  ),
)

The result: