AnimatedWidget的子类，使用Rect类型的动画器让子组件在两个Rect对象之间进行过渡动画。

相关组件

DecoratedBox

RelativePositionedTransition基本使用

  1.    <br />【child】 : 孩子组件   【Widget】<br />【size】 : 左和上的偏移量   【Size】<br />【rect】 : 动画   【Animation<Rect>】<br />   PositionedTransition组件只能在Stack内起作用<br />![141.gif](https://cdn.nlark.com/yuque/0/2020/gif/326147/1589457387238-c1723fcd-8d97-4b1c-aae2-caa325741edb.gif#align=left&display=inline&height=226&margin=%5Bobject%20Object%5D&name=141.gif&originHeight=226&originWidth=804&size=318559&status=done&style=none&width=804)
import 'package:flutter/material.dart';
class CustomRelativePositionedTransition extends StatefulWidget {
  @override
  _CustomRelativePositionedTransitionState createState() =>
      _CustomRelativePositionedTransitionState();
}

class _CustomRelativePositionedTransitionState
    extends State<CustomRelativePositionedTransition>
    with SingleTickerProviderStateMixin {
  AnimationController _ctrl;

  @override
  void initState() {
    _ctrl = AnimationController(vsync: this, duration: Duration(seconds: 2));
    _ctrl.forward();
    super.initState();
  }

  @override
  void dispose() {
    _ctrl.dispose();
    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    return GestureDetector(
        onTap: () {
          setState(() {
            _ctrl.reset();
            _ctrl.forward();
          });
        },
        child: Container(
          color: Colors.grey.withAlpha(33),
          width: 200,
          height: 100,
          child: Stack(
            children: <Widget>[
              RelativePositionedTransition(
                size: Size(200, 100),
                rect: RectTween(
                  begin: Rect.fromLTRB(0, 0, 50, 50),
                  end: Rect.fromLTRB(0, 0, 50, 50).translate(100, 50),
                ).animate(_ctrl),
                child: Icon(
                  Icons.android,
                  color: Colors.green,
                  size: 50,
                ),
              )
            ],
          ),
        ));
  }
}