A basic way to animate shapes

animate_shapes.dart

// Copyright (c) 2019, the Dart project authors.  Please see the AUTHORS file
// for details. All rights reserved. Use of this source code is governed by a
// BSD-style license that can be found in the LICENSE file.

import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      debugShowCheckedModeBanner: false,
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: MyHomePage(title: 'Flutter Demo Home Page'),
    );
  }
}

class MyHomePage extends StatefulWidget {
  MyHomePage({Key key, this.title}) : super(key: key);

  final String title;

  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  double index = 0;

  void _animateShape() {
    setState(() {
      index = index == 2 ? 0 : index + 1;
    });
    print(index);
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(widget.title),
      ),
      body: Center(
        child: Stack(
          alignment: Alignment.center,
          children: <Widget>[
            AnimatedOpacity(
                duration: Duration(seconds: 1),
                opacity: index == 0 ? 1 : 0,
                child: Container(
                  height: 200,
                  width: 200,
                  decoration: BoxDecoration(
                    color: Colors.yellow,
                    borderRadius: BorderRadius.circular(20),
                  )
                )
            ),
            AnimatedOpacity(
                duration: Duration(seconds: 1),
                opacity: index == 1 ? 1 : 0,
                child: Container(
                  height: 150,
                  width: 150,
                  decoration: BoxDecoration(
                    color: Colors.green,
                    borderRadius: BorderRadius.circular(75),
                  )
                )
            ),
            AnimatedOpacity(
                duration: Duration(seconds: 1),
                opacity: index == 2 ? 1 : 0,
                child: Container(
                  height: 100,
                  width: 100,
                  decoration: BoxDecoration(
                    color: Colors.red,
                    borderRadius: BorderRadius.circular(20),
                  )
                )
            )
          ],
        ),
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: _animateShape,
        tooltip: 'Increment',
        child: Icon(Icons.switch_left),
      ),
    );
  }
}