|4| Animated Container:

Animated Container:

import 'dart:math';
import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {
  @override  Widget build(BuildContext context) {
    return MaterialApp(
      debugShowCheckedModeBanner: false,
      title: "Opacity",
      home: MyHomepage(),
    );
  }
}
class MyHomepage extends StatefulWidget {
  @override  _MyHomepageState createState() => _MyHomepageState();
}

class _MyHomepageState extends State<MyHomepage> {
  final _rng = Random();
  double _height = 100;
  double _width = 100;
  double _borderRadius = 8;
  Color _color = Colors.blue;

  @override  void initState() {
    super.initState();
  }

  @override  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(
            "Animated Container"        ),
      ),
      body: Center(
        child: Padding(
          padding: EdgeInsets.all(25.0),

          child: Column(
            children: <Widget>[

              AnimatedContainer(
                margin: EdgeInsets.all(45),
                child: FlutterLogo(),

                // Use the properties stored in the State class.                width: this._width,
                height: this._height,
                decoration: BoxDecoration(
                  color: this._color,
                  borderRadius: BorderRadius.circular(this._borderRadius),
                ),
                duration: Duration(seconds: 1),
                curve: Curves.fastOutSlowIn,
              ),
              RaisedButton.icon(
                icon: Icon(Icons.update),
                label: Text('Change Random Property',
                  style: TextStyle(
                    fontSize: 18.0,
                  ),
                ),
                onPressed: () => setState(
                      () {
                    // Generate a random width and height.                    _width = _rng.nextInt(100).toDouble() + 50;
                    _height = _rng.nextInt(100).toDouble() + 50;
                    _borderRadius = _rng.nextInt(50).toDouble();
                    // Generate a random color.                    _color = Color.fromRGBO(
                        _rng.nextInt(256), _rng.nextInt(256), _rng.nextInt(256), 1);
                  },
                ),
              ),
            ],
          ),
        ),
      ),
    );
  }
}