How To Change Text Dynamically On click Of Button In Flutter

import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      debugShowCheckedModeBanner: false,
        home: Scaffold(
            appBar: AppBar(
              centerTitle: true,
                backgroundColor: Colors.indigo,
                title: Text('Change Text Dynamically on Button Click')
            ),
            body: Center(
                child: ChangeText()
            )
        )
    );
  }
}

class ChangeText extends StatefulWidget {

  ChangeTextState createState() => ChangeTextState();

}

class ChangeTextState extends State {

  String textHolder = 'Old Text';

  changeText() {

    setState(() {
      textHolder = 'New Text';
    });

  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
        body: Center(
            child: Column(
              mainAxisAlignment: MainAxisAlignment.center,
             children: <Widget>[
              Container(
                  padding: EdgeInsets.fromLTRB(20, 20, 20, 20),
                  child: Text('$textHolder',
                      style: TextStyle(fontSize: 21))),

              RaisedButton(
                onPressed: () => changeText(),
                child: Text('Change Text Widget Text Dynamically'),
                textColor: Colors.white,
                color: Colors.red,
                padding: EdgeInsets.fromLTRB(10, 10, 10, 10),
              ),

            ]))
    );
  }
}