Dart Flutter：如何从范围模型中运行动画？

如何在范围模型中作为ScopedModeldescendant的窗口小部件中运行动画 . 我有一个红色按钮，点击时显示一个正方形 . 3秒后，方块会消失，这很快就会发生 . 我想做的是让广场在几秒钟内消失 . 我尝试了AnimationController，但它需要一个'vsync：'参数，我看到它通常在initState（）中作为'vsync：this'完成，并在其他地方使用时出错 .

import 'package:flutter/material.dart';
import 'dart:async';
import 'package:scoped_model/scoped_model.dart';

void main() {
  runApp(new MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return new MaterialApp(
      title: 'scoped model Opacity',
      home: new ScopedModel<MyScopedModel>(
        model: new MyScopedModel(),
        child: new MyPage(),
      ),
    );
  }
}

class MyScopedModel extends Model {
  double myOpacity = 0.0;
  Timer myTimer;

  AnimationController myAnimationController;

  void myShowSquare() {
    myOpacity = 1.0;
    myTimer = new Timer(new Duration(seconds: 3), _removeSquare);
    notifyListeners();
  }

  void _removeSquare() {
    myOpacity = 0.0;
    myTimer.cancel();
    notifyListeners();
  }
}

class MyPage extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return new Container(
        color: Colors.grey,
        child: new ScopedModelDescendant<MyScopedModel>(
          builder: (context, child, model) => new Column(
                crossAxisAlignment: CrossAxisAlignment.center,
                children: <Widget>[
                  new GestureDetector(
                    onTap: model.myShowSquare,
                    child: new Stack(
                      alignment: Alignment.center,
                      children: <Widget>[
                        new Container(
                          constraints: new BoxConstraints.expand(
                              width: 50.0, height: 50.0),
                          color: Colors.deepOrange,
                        ),
                        new Text(
                          '+',
                          style: new TextStyle(color: Colors.black),
                        ),
                      ],
                    ),
                  ),
                  new Opacity(
                    opacity: model.myOpacity,
                    child: new Container(
                      constraints:
                          new BoxConstraints.expand(width: 50.0, 
                                height: 50.0),
                      color: Colors.amber,
                    ),
                  )
                ],
              ),
        ));
  }
}