slider_with_initial_animation.dart

import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {
  const MyApp({Key? key}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Material App',
      home: Scaffold(
        appBar: AppBar(title: const Text('Material App Bar')),
        body: const Center(child: Home()),
      ),
    );
  }
}

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

  @override
  _HomeState createState() => _HomeState();
}

class _HomeState extends State<Home> with SingleTickerProviderStateMixin {
  late final AnimationController _controller;
  @override
  void initState() {
    _controller = AnimationController.unbounded(
      vsync: this,
      duration: const Duration(seconds: 2),
      value: 0,
    )..animateTo(10);
    super.initState();
  }

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

  @override
  Widget build(BuildContext context) {
    return AnimatedBuilder(
      animation: _controller,
      builder: (context, __) {
        return Slider(
          min: 0.0,
          max: 10,
          value: _controller.value,
          onChanged: (value) => _controller.value = value,
        );
      },
    );
  }
}