Using Interval

main.dart

import 'package:flutter/material.dart';

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

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

class IntervalDemo extends StatefulWidget {
  @override
  _IntervalDemoState createState() => _IntervalDemoState();
}

class _IntervalDemoState extends State<IntervalDemo> with SingleTickerProviderStateMixin {
  late AnimationController _controller;
  late Animation<Color?> _colorAnimation;
  late Animation<double> _sizeAnimation;

  @override
  void initState() {
    super.initState();
    _controller = AnimationController(
      duration: const Duration(seconds: 4),
      vsync: this,
    );

    _colorAnimation = ColorTween(begin: Colors.blue, end: Colors.red).animate(
      CurvedAnimation(
        parent: _controller,
        curve: Interval(0.0, 0.5, curve: Curves.easeIn),
      ),
    ) as Animation<Color?>;

    _sizeAnimation = Tween<double>(begin: 50.0, end: 150.0).animate(
      CurvedAnimation(
        parent: _controller,
        curve: Interval(0.5, 1.0, curve: Curves.easeOut),
      ),
    );

    _controller.repeat();
  }

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

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(title: Text('Interval Demo')),
      body: Center(
        child: AnimatedBuilder(
          animation: _controller,
          builder: (context, child) {
            return Container(
              width: _sizeAnimation.value,
              height: _sizeAnimation.value,
              color: _colorAnimation.value,
            );
          },
        ),
      ),
    );
  }
}