main.dart

import 'package:flutter/material.dart';
import 'package:flutter/cupertino.dart';
import 'package:flutter/gestures.dart';
import 'package:flutter/rendering.dart';

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        body: Center(
          child: MyWidget(
            showingIndex: 3,
            builder: (context, index) {
              return Text(
                '$index',
                style: shinyTextStyle(index),
              );
            },
          ),
        ),
      ),
    );
  }

  TextStyle shinyTextStyle(int index) {
    if (index < 10) {
      return TextStyle(color: Colors.black);
    } else {
      return TextStyle(color: Colors.indigo);
    }
  }
}

class MyWidget extends LeafRenderObjectWidget {
  const MyWidget({
    this.showingIndex = 0,
    required this.builder,
    Key? key,
  }) : super(key: key);

  final int showingIndex;

  final IndexedWidgetBuilder builder;

  @override
  RenderMyWidget createRenderObject(BuildContext context) => RenderMyWidget(showingIndex, builder);

  @override
  void updateRenderObject(BuildContext context, RenderMyWidget renderObject) {
    renderObject
      ..showingIndex = showingIndex
      ..builder = builder;
  }
}

class RenderMyWidget extends RenderBox {
  RenderMyWidget(int showingIndex, IndexedWidgetBuilder builder)
      : _showingIndex = showingIndex,
        _builder = builder;

  int get showingIndex => _showingIndex;
  int _showingIndex;

  set showingIndex(int showingIndex) {
    if (_showingIndex == showingIndex) return;
    _showingIndex = showingIndex;
    markNeedsPaint();
  }

  IndexedWidgetBuilder get builder => _builder;
  IndexedWidgetBuilder _builder;

  set builder(IndexedWidgetBuilder value) {
    if (_builder == value) return;
    _builder = value;
  }

  @override
  void performLayout() {
    size = Size(constraints.maxWidth, constraints.maxHeight);
  }

  TextStyle hardcodedTextStyle = TextStyle(
    color: Colors.black,
    fontSize: 100,
  );

  @override
  void paint(PaintingContext context, Offset offset) {
    TextPainter textPainter = new TextPainter(
      text: TextSpan(text: '$showingIndex', style: hardcodedTextStyle),
      textDirection: TextDirection.rtl,
    );
    textPainter.layout();
    textPainter.paint(context.canvas, offset);

    // Todo: How can I paint this widget from my builder
    // Widget showingWidget = builder(buildContext, showingIndex);
    // context.paintChild(showingWidget, offset);
  }

}