如何获取父级中动画值的子小部件大小

import 'package:flutter/material.dart';

class MyWidget extends StatefulWidget {
  @override
  _MyWidgetState createState() => _MyWidgetState();
}

class _MyWidgetState extends State<MyWidget> with SingleTickerProviderStateMixin {
  AnimationController _controller;
  Animation<double> _animation;

  @override
  void initState() {
    super.initState();
    _controller = AnimationController(
      duration: Duration(seconds: 2),
      vsync: this,
    );
    _animation = Tween<double>(begin: 0, end: 200).animate(_controller);
    _controller.forward();
  }

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

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Parent Animation'),
      ),
      body: Center(
        child: LayoutBuilder(
          builder: (BuildContext context, BoxConstraints constraints) {
            return Container(
              width: _animation.value,
              height: _animation.value,
              color: Colors.blue,
              child: Text(
                'Child Widget',
                style: TextStyle(color: Colors.white),
              ),
            );
          },
        ),
      ),
    );
  }
}