App_Flutter/lib/view/demo2.dart

import 'package:flutter/material.dart';

class Demo3 extends StatefulWidget {
  const Demo3({Key? key}) : super(key: key);

  @override
  State<Demo3> createState() => _Demo3State();
}

class _Demo3State extends State<Demo3> with SingleTickerProviderStateMixin {
  late AnimationController _controller;
  late Animation _size;

  @override
  void initState() {
    _controller =
        AnimationController(duration: const Duration(seconds: 10), vsync: this);
          // ..addListener(() {
          //   setState(() {});
          // });

    // _controller = AnimationController(vsync: this,duration: const Duration(seconds: 2));
    _controller.addListener(() { 
      setState(() {
        
      });
    });
    _size = Tween(begin: 100.0, end: 200.0).animate(
        CurvedAnimation(parent: _controller, curve: const Interval(0.2, 0.23)));

    _controller.forward();
    super.initState();
  }

  @override
  void dispose() {
    _controller.dispose();
    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    return Container(
      color: Colors.white,
      child: Column(
        crossAxisAlignment: CrossAxisAlignment.start,
        children: [
          Container(
            width: _size.value,
            height: _size.value,
            color: Colors.orange,
          )
        ],
      ),
    );
  }
}