main.dart

import 'package:flutter/material.dart';

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

class App extends StatefulWidget {
  App({Key key}) : super(key: key);

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

class _AppState extends State<App> {
  bool _completed = false;

  @override
  void initState() {
    super.initState();
    _asyncCallback().then((_) {
      if (mounted) {
        setState(() => _completed = true);
      }
    });
  }

  Future<void> _asyncCallback() {
    return Future.delayed(const Duration(seconds: 3));
  }

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      debugShowCheckedModeBanner: false,
      home: Scaffold(
        appBar: AppBar(title: Text('Async Callbacks')),
        body: Center(
          child: Column(
            mainAxisAlignment: MainAxisAlignment.center,
            children: [
              Text('Completed: $_completed'),
              if (!_completed) ...const [
                SizedBox(height: 10),
                CircularProgressIndicator(),
              ],
            ],
          ),
        ),
      ),
    );
  }
}