https://github.com/kyorohiro/hello_skyengine/tree/master/anime_rect
// following code is checked in 2016/01/13import 'package:flutter/widgets.dart';import 'package:flutter/rendering.dart';import 'dart:async';import 'dart:math';void main() {runApp(new DrawRectWidget()..anime());}class DrawRectWidget extends OneChildRenderObjectWidget {double angle = 0.0;DrawRectObject o = new DrawRectObject();RenderObject createRenderObject() {return o;}Future anime() async {while (true) {await new Future.delayed(new Duration(milliseconds: 20));o.x = 100 * cos(PI * angle / 180.0) + 100.0;o.y = 100 * sin(PI * angle / 180.0) + 100.0;angle++;o.markNeedsPaint();}}}class DrawRectObject extends RenderBox {double x = 50.0;double y = 50.0;void paint(PaintingContext context, Offset offset) {Paint p = new Paint();p.color = new Color.fromARGB(0xff, 0xff, 0xff, 0xff);Rect r = new Rect.fromLTWH(x, y, 25.0, 25.0);context.canvas.drawRect(r, p);}}