mehul.dart

import 'package:flutter/material.dart';

main ()
{
  runApp (HelloWorld ());
}
class HelloWorld extends StatefulWidget
{
  @override
  _HelloWorldState createState () => _HelloWorldState ();
}

class _HelloWorldState extends State <HelloWorld>
{
  var message = "Hello World";

  changeMyState ()
  {
    setState (()
    {
      this.message = "Goodbye World";
    });
  }
  @override
  Widget build (BuildContext context)
  {
    return MaterialApp(
      home:Scaffold(
        appBar:AppBar(
          title:Text ("Hello World App")
        ),
        body:Center (
        child:GestureDetector(
          onTap: changeMyState,
          child:Text("$message")
        ),
        ),
      )

    );
  }
}