Simple_Flutter_Animation2.dart

import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      home: Scaffold(
        body: Center(child: AnimatedRect()),
      ),
    );
  }
}

class AnimatedRect extends StatefulWidget {
  @override
  State<StatefulWidget> createState() => _AnimatedRectState();
}

class _AnimatedRectState extends State<AnimatedRect>
    with SingleTickerProviderStateMixin {
  Animation<double> _opacity;
  AnimationController _controller;

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

    _controller =
        AnimationController(vsync: this, duration: const Duration(seconds: 2));
    _opacity = Tween<double>(begin: 0, end: 1.0).animate(_controller)
      ..addListener(() {
        setState(() {});
      })
      ..addStatusListener((status) {
        if (status == AnimationStatus.completed) {
          _controller.reverse();
        } else if (status == AnimationStatus.dismissed) {
          _controller.forward();
        }
      });
    _controller.forward();
  }

  @override
  Widget build(BuildContext context) {
    return Container(
        width: 200,
        height: 200,
        color: Colors.blue.withOpacity(_opacity.value));
  }
}