Flutter vertical curtain raising animation

vertical_curtain_raising_test.dart

import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {
  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return MaterialApp(home: CurtainScaffold());
  }
}

class CurtainScaffold extends StatefulWidget {
  @override
  _CurtainScaffoldState createState() => _CurtainScaffoldState();
}

class _CurtainScaffoldState extends State<CurtainScaffold> {
  double curtain = 0.0;

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      backgroundColor: Colors.black,
      body: SafeArea(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.spaceBetween,
          children: <Widget>[
            Slider(
              onChanged: (double val) {
                setState(() {
                  this.curtain = val;
                });
              },
              value: curtain,
              min: 0.0,
              max: 1.0,
            ),
            ClipRect(
              child: Align(
                alignment: Alignment.bottomCenter,
                heightFactor: curtain,
                child: Image.network(
                    'https://upload.wikimedia.org/wikipedia/commons/b/bd/Dts_news_bill_gates_wikipedia.JPG'),
              ),
            ),
          ],
        ),
      ),
    );
  }
}