making logic in flutter

 import 'package:flutter/cupertino.dart';

import 'package:flutter/material.dart';
// Import for Android features.

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

//This is main homepage or we can say main body
class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    Color color = Theme.of(context).primaryColor;

    return MaterialApp(
      title: 'Welcome to flutter',
      home: MyHomePage(),
    );
  }
}

class MyHomePage extends StatefulWidget {
  @override
  State<MyHomePage> createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  String myText = "hello world";

  void changeState() {
    setState(() {
      if (myText.startsWith("h")) {
        myText = "Welcome to flultter";
      } else {
        myText = "hello world";
      }
    });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
        appBar: AppBar(
          title: Center(
              child: Text(
            'Welcome to flutter',
            style: TextStyle(fontWeight: FontWeight.bold),
          )),
        ),
        body: Center(
          child: Container(
            child: Column(
              mainAxisAlignment: MainAxisAlignment.center,
              children: [
                Text(myText),
                FloatingActionButton(
                    child: Icon(Icons.add),
                    onPressed: () {
                      changeState();
                    })
              ],
            ),
          ),
        ));
  }
}