i_am_animation.dart

import 'package:flutter/material.dart';

const Color darkBlue = Color.fromARGB(255, 18, 32, 47);

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      theme: ThemeData.dark().copyWith(
        scaffoldBackgroundColor: darkBlue,
      ),
      debugShowCheckedModeBanner: false,
      home: Scaffold(
        body: Center(
          child: MyWidget(),
        ),
      ),
    );
  }
}

class MyWidget extends StatefulWidget {
  @override
  State<MyWidget> createState() => _MyWidgetState();
}

class _MyWidgetState extends State<MyWidget> {
  var showThing = false;
  
  @override
  void initState() {
    super.initState();
    Future.delayed(const Duration(seconds: 3)).then((_) {
      setState(() {
        showThing = true;
      });
    });
  }
  
  @override
  Widget build(BuildContext context) {
    if (showThing) {
      return Text(
        'I am animation',
        style: Theme.of(context).textTheme.headline4,
      );
    }
    return Text(
      'Hello, World!',
      style: Theme.of(context).textTheme.headline4,
    );
  }
}