RenderObjectWidget Demo

render_object_widget.dart

import 'package:flutter/material.dart';

void main() => runApp(
    RedSquare(
      center: Offset(210, 300),
      width: 100,
      height: 200,
    )
);

class RedSquare extends LeafRenderObjectWidget {
  Offset center;
  double width;
  double height;

  RedSquare({this.center, this.width, this.height});

  @override
  RenderObject createRenderObject(BuildContext context) {
    return RedSquareRenderObject(
      rect: Rect.fromCenter(
        center: center,
        width: width,
        height: height
      )
    );
  }

//  @override
//  void updateRenderObject(BuildContext context, RedSquareRenderObject renderObject) {
//    renderObject
//      ..rect = Rect.fromCenter(
//          center: center,
//          width: width,
//          height: height
//      );
//  }
}

class RedSquareRenderObject extends RenderBox {
  Rect _rect;

  RedSquareRenderObject({Rect rect}) {
    this._rect = rect;
  }

//  set rect(Rect rect) {
//    if (_rect == rect) {
//      return;
//    }
//    _rect = rect;
//    markNeedsPaint();
//  }

  @override
  void paint(PaintingContext context, Offset offset) {
    Paint p = Paint()
      ..color = Colors.red;
    context.canvas.drawRect(_rect , p);
  }

  @override
  bool get sizedByParent => true;

  @override
  void performResize() {
    size = constraints.smallest;
  }
}