How to create a custom InnerShadow widget in Flutter. Overrides the RenderProxyBox class to add a smooth inner shadow to any child widget using the canvas' paint and save/restore techniques. MIT License

inner_shadow.dart

// Custom widget for adding inner shadow
class InnerShadow extends SingleChildRenderObjectWidget {
  const InnerShadow({
    Key? key,
    this.blur = 5, // Default blur value for shadow
    this.color = Colors.black38, // Default shadow color
    this.offset = const Offset(50, 50), // Default shadow offset
    Widget? child,
  }) : super(key: key, child: child);

  final double blur; // Amount of blur for the shadow
  final Color color; // Color of the shadow
  final Offset offset; // Offset of the shadow

  @override
  RenderObject createRenderObject(BuildContext context) {
    // Create and return the custom render object
    final renderObject = _RenderInnerShadow();
    updateRenderObject(context, renderObject);
    return renderObject;
  }

  @override
  void updateRenderObject(
      BuildContext context, _RenderInnerShadow renderObject) {
    // Update the render object with the new values of blur, color, and offset
    renderObject
      ..color = color
      ..blur = blur
      ..dx = offset.dx
      ..dy = offset.dy;
  }
}

// Custom render object class for inner shadow effect
class _RenderInnerShadow extends RenderProxyBox {
  double blur = 5; // Default blur for shadow
  Color color = Color(0xFFF15923); // Default color for shadow
  double dx = 5; // Default x-axis translation for shadow
  double dy = 5; // Default y-axis translation for shadow

  @override
  void paint(PaintingContext context, Offset offset) {
    // Ensure there is a child to paint
    if (child == null) return;

    // Calculate the outer and inner bounds of the shadow
    final Rect outerRect = offset & size;
    final Rect innerRect =
        outerRect.deflate(blur); // Adjust the inner rect for shadow

    final Canvas canvas = context.canvas;

    // Draw the child widget
    canvas.saveLayer(outerRect, Paint());
    context.paintChild(child!, offset);

    // Create the paint for the shadow layer
    final Paint shadowPaint = Paint()
      ..blendMode = BlendMode.srcATop // Apply the shadow over the widget
      ..colorFilter =
          ColorFilter.mode(color, BlendMode.srcOut) // Set shadow color
      ..imageFilter =
          ImageFilter.blur(sigmaX: blur, sigmaY: blur); // Apply blur

    // Draw the shadow layer on the canvas
    canvas.saveLayer(outerRect, shadowPaint);
    context.paintChild(child!, offset);

    // Restore the canvas stack
    canvas.restore();
    canvas.restore();
  }
}

run_this_example.dart

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

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        backgroundColor: Colors.white, // White background for contrast
        body: Center(
          // Center the widget
          child: InnerShadow(
            blur: 15,
            color: Color.fromARGB(255, 70, 70, 70),
            offset: const Offset(5, 5), // Apply the shadow offset
            child: Container(
              decoration: const BoxDecoration(
                borderRadius: BorderRadius.all(Radius.circular(8)),
                color: Color(0xFFF15923), // The orange background color
              ),
              height: 100,
              width: 200,
              child: const Center(
                child: Text(
                  "Test", // Centered white text
                  style: TextStyle(
                    color: Colors.white,
                    fontSize: 24,
                    fontWeight: FontWeight.bold,
                  ),
                ),
              ),
            ),
          ),
        ),
      ),
    );
  }
}

// Custom widget for adding inner shadow
class InnerShadow extends SingleChildRenderObjectWidget {
  const InnerShadow({
    Key? key,
    this.blur = 5, // Default blur value for shadow
    this.color = Colors.black38, // Default shadow color
    this.offset = const Offset(50, 50), // Default shadow offset
    Widget? child,
  }) : super(key: key, child: child);

  final double blur; // Amount of blur for the shadow
  final Color color; // Color of the shadow
  final Offset offset; // Offset of the shadow

  @override
  RenderObject createRenderObject(BuildContext context) {
    // Create and return the custom render object
    final renderObject = _RenderInnerShadow();
    updateRenderObject(context, renderObject);
    return renderObject;
  }

  @override
  void updateRenderObject(
      BuildContext context, _RenderInnerShadow renderObject) {
    // Update the render object with the new values of blur, color, and offset
    renderObject
      ..color = color
      ..blur = blur
      ..dx = offset.dx
      ..dy = offset.dy;
  }
}

// Custom render object class for inner shadow effect
class _RenderInnerShadow extends RenderProxyBox {
  double blur = 5; // Default blur for shadow
  Color color = Color(0xFFF15923); // Default color for shadow
  double dx = 5; // Default x-axis translation for shadow
  double dy = 5; // Default y-axis translation for shadow

  @override
  void paint(PaintingContext context, Offset offset) {
    // Ensure there is a child to paint
    if (child == null) return;

    // Calculate the outer and inner bounds of the shadow
    final Rect outerRect = offset & size;
    final Rect innerRect =
        outerRect.deflate(blur); // Adjust the inner rect for shadow

    final Canvas canvas = context.canvas;

    // Draw the child widget
    canvas.saveLayer(outerRect, Paint());
    context.paintChild(child!, offset);

    // Create the paint for the shadow layer
    final Paint shadowPaint = Paint()
      ..blendMode = BlendMode.srcATop // Apply the shadow over the widget
      ..colorFilter =
          ColorFilter.mode(color, BlendMode.srcOut) // Set shadow color
      ..imageFilter =
          ImageFilter.blur(sigmaX: blur, sigmaY: blur); // Apply blur

    // Draw the shadow layer on the canvas
    canvas.saveLayer(outerRect, shadowPaint);
    context.paintChild(child!, offset);

    // Restore the canvas stack
    canvas.restore();
    canvas.restore();
  }
}