Custom Debug Paint - Paint custom graphics only in when show debug paint is enabled in the inspector - by Simon Lightfoot - 27/09/2020

custom_debug_paint.dart

// MIT License
//
// Copyright (c) 2020 Simon Lightfoot
//
// Permission is hereby granted, free of charge, to any person obtaining a copy
// of this software and associated documentation files (the "Software"), to deal
// in the Software without restriction, including without limitation the rights
// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
// copies of the Software, and to permit persons to whom the Software is
// furnished to do so, subject to the following conditions:
//
// The above copyright notice and this permission notice shall be included in all
// copies or substantial portions of the Software.
//
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
// SOFTWARE.
//
import 'package:flutter/material.dart';
import 'package:flutter/rendering.dart';

typedef CustomDebugPaintFn = void Function(Canvas canvas, Size size);

class CustomDebugPaint extends SingleChildRenderObjectWidget {
  const CustomDebugPaint({
    Key key,
    this.paint,
    Widget child,
  }) : super(key: key, child: child);

  final CustomDebugPaintFn paint;

  @override
  RenderObject createRenderObject(BuildContext context) {
    return RenderCustomDebugPaint(painter: paint);
  }

  @override
  void updateRenderObject(BuildContext context, covariant RenderCustomDebugPaint renderObject) {
    super.updateRenderObject(context, renderObject);
    renderObject.painter = paint;
  }
}

class RenderCustomDebugPaint extends RenderProxyBox {
  RenderCustomDebugPaint({CustomDebugPaintFn painter, RenderBox child})
      : _painter = painter,
        super(child);

  CustomDebugPaintFn _painter;

  CustomDebugPaintFn get painter => _painter;

  set painter(CustomDebugPaintFn value) {
    if (_painter != value) {
      _painter = value;
      markNeedsPaint();
    }
  }

  @override
  void debugPaint(PaintingContext context, Offset offset) {
    super.debugPaint(context, offset);
    if (debugPaintSizeEnabled) {
      _painter?.call(context.canvas, size);
    }
  }
}

example.dart

// MIT License
//
// Copyright (c) 2020 Simon Lightfoot
//
// Permission is hereby granted, free of charge, to any person obtaining a copy
// of this software and associated documentation files (the "Software"), to deal
// in the Software without restriction, including without limitation the rights
// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
// copies of the Software, and to permit persons to whom the Software is
// furnished to do so, subject to the following conditions:
//
// The above copyright notice and this permission notice shall be included in all
// copies or substantial portions of the Software.
//
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
// SOFTWARE.
//
import 'package:flutter/material.dart';

import 'custom_debug_paint.dart';

void main() => runApp(TestApp());

class TestApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Material(
        child: CustomDebugPaint(
          paint: (Canvas canvas, Size size) {
            canvas.drawCircle(
              (Offset.zero & size).center,
              size.shortestSide / 3,
              Paint()..color = Colors.red.withOpacity(0.2),
            );
          },
          child: Center(
            child: Text('hello world'),
          ),
        ),
      ),
    );
  }
}