Animated Container

main.dart

import 'dart:math';

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

const _duration = Duration(milliseconds: 400);

double randomBorderRadius() {
  return Random().nextDouble() * 64;
}

double randomMargin() {
  return Random().nextDouble() * 64;
}

Color randomColor() {
  return Color(0xFFFFFFFF & Random().nextInt(0xFFFFFFFF));
}

class AnimatedContainerDemo extends StatefulWidget {
  _AnimatedContainerDemoState createState() => _AnimatedContainerDemoState();
}

class _AnimatedContainerDemoState extends State<AnimatedContainerDemo> {
  Color color;
  double borderRadius;
  double margin;

  @override
  void initState() {
    super.initState();
    color = Colors.deepPurple;
    borderRadius = randomBorderRadius();
    margin = randomMargin();
  }

  void change() {
    setState(() {
      color = randomColor();
      borderRadius = randomBorderRadius();
      margin = randomMargin();
    });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Center(
        child: Column(
          children: <Widget>[
            SizedBox(
              width: 128,
              height: 128,
              child: AnimatedContainer(
                margin: EdgeInsets.all(margin),
                decoration: BoxDecoration(
                  color: color,
                  borderRadius: BorderRadius.circular(borderRadius),
                ),
                duration: _duration,
              ),
            ),
            MaterialButton(
              color: Theme.of(context).primaryColor,
              child: Text(
                'change',
                style: TextStyle(color: Colors.white),
              ),
              onPressed: () => change(),
            ),
          ],
        ),
      ),
    );
  }
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      debugShowCheckedModeBanner: false,
      home: AnimatedContainerDemo(),
    );
  }
}

Future<void> main() async {
  runApp(MyApp());
}