如何更改以编程方式创建的animatedPositioned小部件的状态？

import 'package:flutter/material.dart';

class MyAnimatedPositionedWidget extends StatefulWidget {
  @override
  _MyAnimatedPositionedWidgetState createState() =>
      _MyAnimatedPositionedWidgetState();
}

class _MyAnimatedPositionedWidgetState extends State<MyAnimatedPositionedWidget>
    with SingleTickerProviderStateMixin {
  AnimationController _controller;
  Animation<EdgeInsets> _animation;

  @override
  void initState() {
    super.initState();
    _controller = AnimationController(
      duration: Duration(seconds: 1),
      vsync: this,
    );

    final tween = EdgeInsetsTween(
      begin: EdgeInsets.all(0),
      end: EdgeInsets.all(100),
    );

    _animation = tween.animate(CurvedAnimation(
      parent: _controller,
      curve: Curves.easeInOut,
    ));
  }

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

  void _startAnimation() {
    _controller.forward();
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('AnimatedPositioned Example'),
      ),
      body: Center(
        child: AnimatedBuilder(
          animation: _animation,
          builder: (BuildContext context, Widget child) {
            return Container(
              width: 200,
              height: 200,
              color: Colors.blue,
              child: AnimatedPositioned(
                duration: Duration(seconds: 1),
                left: _animation.value.left,
                top: _animation.value.top,
                right: _animation.value.right,
                bottom: _animation.value.bottom,
                child: Container(
                  color: Colors.red,
                ),
              ),
            );
          },
        ),
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: _startAnimation,
        child: Icon(Icons.play_arrow),
      ),
    );
  }
}

void main() {
  runApp(MaterialApp(
    home: MyAnimatedPositionedWidget(),
  ));
}