StatefulWidget

main.dart

import 'package:flutter/material.dart';

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

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

  @override
  State<MyApp> createState() => _MyAppState();
}

class _MyAppState extends State<MyApp> {
  String text = 'Hello World';

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        appBar: AppBar(title: Text('FlutterZero')),
        body: Center(
          child: Column(
            mainAxisAlignment: MainAxisAlignment.center,
            children: [
              Text(text, style: TextStyle(fontSize: 30)),
              SizedBox(height: 20),
              ElevatedButton(
                onPressed: () => setState(() {
                  text = 'I am Flutter';
                  print(text);
                }),
                child: Text('Click'),
              )
            ],
          ),
        ),
      ),
    );
  }
}