RenderObjectWidget Demo

render_object_widget.dart

import 'package:flutter/material.dart';

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

  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
  LeafRenderObjectElement createElement() {
    print('createElement');
    return super.createElement();
  }

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

  @override
  void updateRenderObject(BuildContext context, RedSquareRenderObject renderObject) {
    print('updateRenderObject');
    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;
  }
}