1. import 'package:flutter/material.dart';
    3. void main() {
    4.   runApp(MyApp());
    5. }
    7. class MyApp extends StatelessWidget {
    8.   // This widget is the root of your application.
    9.   @override
    10.   Widget build(BuildContext context) {
    11.     return MaterialApp(
    12.       title: 'Flutter Demo',
    13.       theme: ThemeData(
    14.         primarySwatch: Colors.blue,
    15.       ),
    16.       home: MyHomePage(title: 'Flutter Demo Home Page'),
    17.     );
    18.   }
    19. }
    21. class MyHomePage extends StatefulWidget {
    22.   MyHomePage({Key key, this.title}) : super(key: key);
    23.   final String title;
    25.   @override
    26.   _MyHomePageState createState() => _MyHomePageState();
    27. }
    29. class _MyHomePageState extends State<MyHomePage> {
    30.   int _counter = 0;
    32.   void _incrementCounter() {
    33.     setState(() {
    34.       _counter++;
    35.     });
    36.   }
    38.   @override
    39.   Widget build(BuildContext context) {
    40.     return Scaffold(
    41.       appBar: AppBar(
    42.         title: Text(widget.title),
    43.       ),
    44.       body: Center(
    45.         child: Column(
    46.           mainAxisAlignment: MainAxisAlignment.center,
    47.           children: <Widget>[
    48.             Text(
    49.               'You have pushed the button this many times:',
    50.             ),
    51.             Text(
    52.               '$_counter',
    53.               style: Theme.of(context).textTheme.headline4,
    54.             ),
    55.           ],
    56.         ),
    57.       ),
    58.       floatingActionButton: FloatingActionButton(
    59.         onPressed: _incrementCounter,
    60.         tooltip: 'Increment',
    61.         child: Icon(Icons.add),
    62.       ), // This trailing comma makes auto-formatting nicer for build methods.
    63.     );
    64.   }
    65. }