Main.Dart

import 'package:flutter/material.dart';

void main() => runApp(new MyApp());

class MyApp extends StatelessWidget {
  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return new MaterialApp(
      title: 'AnimatedSize',
      theme: new ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: new MyHomePage(),
    );
  }
}

class MyHomePage extends StatefulWidget {
  @override
  _MyHomePageState createState() => new _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> with TickerProviderStateMixin {
  double _height = 80.0;
  double _width = 80.0;
  var _color = Colors.blue;
  bool _resized = false;

  @override
  Widget build(BuildContext context) {
    return new Scaffold(
      body: new Center(
        child: new Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            new AnimatedSize(
              duration: null,
              vsync: null,
              child: new Container(
                width: _width,
                height: _height,
                color: _color,
              ),
            ),
          ],
        ),
      ),
    );
  }
}