content: import 'package:flutter/material.dart';
  
  void main() => runApp(MyApp());
  
  class MyApp extends StatelessWidget {
    @override
    Widget build(BuildContext context) {
      return MaterialApp(
        title: 'Flutter Demo',
        theme: ThemeData(
          primarySwatch: Colors.blue,
        ),
        home: MyHomePage(title: 'AnimatedSwitcher'),
      );
    }
  }
  
  class MyHomePage extends StatefulWidget {
    MyHomePage({Key key, this.title}) : super(key: key);
    final String title;
  
    @override
    _MyHomePageState createState() => _MyHomePageState();
  }
  
  class _MyHomePageState extends State<MyHomePage> {
    bool isVisible = false;
  
    @override
    Widget build(BuildContext context) {
      return Scaffold(
        appBar: AppBar(
          title: Text(widget.title),
        ),
        body: AnimatedSwitcher(
          duration: const Duration(seconds: 3),
          child: isVisible
              ? Container(
                  height: 100.0,
                  width: 100.0,
                  color: Colors.red,
                )
              : const SizedBox.shrink(),
        ),
        floatingActionButton: FloatingActionButton(
          onPressed: () {
            setState(() {
              isVisible = !isVisible;
            });
          },
          child: const Icon(Icons.add),
        ),
      );
    }
  }
  