플러터 다트(dart) - Statefull Widget

import 'package:flutter/material.dart';

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

class MyApp extends StatefulWidget {
  @override
  State<StatefulWidget> createState() {
    // TODO: implement createState
    return MyAppState();
  }
}
class MyAppState extends State<MyApp>{

  int counter = 0;
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: Scaffold(
        appBar: AppBar(),
        body: Center(
          child: Column(
            mainAxisAlignment: MainAxisAlignment.center,
            children: [
              Text("You have pushed the button this many ties"),
              Text(
                '$counter',
                style: Theme.of(context).textTheme.displaySmall,
              ),
             SizedBox(height: 20,),
              Row(
                mainAxisAlignment: MainAxisAlignment.center,
                children: [
                  FloatingActionButton(
                    child: Icon(Icons.add),
                    onPressed: (){
                      setState(() {
                        counter++;
                        print("$counter");
                      });
                    },
                  ),
                  SizedBox(width: 20,),
                  FloatingActionButton(
                    child: Icon(Icons.remove),
                    onPressed: (){
                      setState(() {
                        counter--;
                        print("$counter");
                      });
                    },
                  ),

                ],
              ),
            ],
          ),
        ),

      ),
    );
  }
}