@@ -13,6 +13,15 @@ class MyApp extends StatelessWidget {
1313 return MaterialApp (
1414 title: 'Flutter Demo' ,
1515 theme: ThemeData (
16+ // This is the theme of your application.
17+ //
18+ // Try running your application with "flutter run". You'll see the
19+ // application has a blue toolbar. Then, without quitting the app, try
20+ // changing the primarySwatch below to Colors.green and then invoke
21+ // "hot reload" (press "r" in the console where you ran "flutter run",
22+ // or simply save your changes to "hot reload" in a Flutter IDE).
23+ // Notice that the counter didn't reset back to zero; the application
24+ // is not restarted.
1625 primarySwatch: Colors .blue,
1726 ),
1827 home: const MyHomePage (title: 'Flutter Demo Home Page' ),
@@ -23,13 +32,24 @@ class MyApp extends StatelessWidget {
2332class MyHomePage extends StatefulWidget {
2433 const MyHomePage ({Key ? key, required this .title}) : super (key: key);
2534
35+ // This widget is the home page of your application. It is stateful, meaning
36+ // that it has a State object (defined below) that contains fields that affect
37+ // how it looks.
38+
39+ // This class is the configuration for the state. It holds the values (in this
40+ // case the title) provided by the parent (in this case the App widget) and
41+ // used by the build method of the State. Fields in a Widget subclass are
42+ // always marked "final".
43+
2644 final String title;
2745
2846 @override
2947 State <MyHomePage > createState () => _MyHomePageState ();
3048}
3149
3250class _MyHomePageState extends State <MyHomePage > {
51+ int _counter = 0 ;
52+
3353 String get _buildName {
3454 return const String .fromEnvironment ('FLUTTER_BUILD_NAME' );
3555 }
@@ -38,16 +58,58 @@ class _MyHomePageState extends State<MyHomePage> {
3858 return const String .fromEnvironment ('FLUTTER_BUILD_NUMBER' );
3959 }
4060
61+ void _incrementCounter () {
62+ setState (() {
63+ // This call to setState tells the Flutter framework that something has
64+ // changed in this State, which causes it to rerun the build method below
65+ // so that the display can reflect the updated values. If we changed
66+ // _counter without calling setState(), then the build method would not be
67+ // called again, and so nothing would appear to happen.
68+ _counter++ ;
69+ });
70+ }
71+
4172 @override
4273 Widget build (BuildContext context) {
74+ // This method is rerun every time setState is called, for instance as done
75+ // by the _incrementCounter method above.
76+ //
77+ // The Flutter framework has been optimized to make rerunning build methods
78+ // fast, so that you can just rebuild anything that needs updating rather
79+ // than having to individually change instances of widgets.
4380 return Scaffold (
4481 appBar: AppBar (
82+ // Here we take the value from the MyHomePage object that was created by
83+ // the App.build method, and use it to set our appbar title.
4584 title: Text (widget.title),
4685 ),
4786 body: Center (
87+ // Center is a layout widget. It takes a single child and positions it
88+ // in the middle of the parent.
4889 child: Column (
90+ // Column is also a layout widget. It takes a list of children and
91+ // arranges them vertically. By default, it sizes itself to fit its
92+ // children horizontally, and tries to be as tall as its parent.
93+ //
94+ // Invoke "debug painting" (press "p" in the console, choose the
95+ // "Toggle Debug Paint" action from the Flutter Inspector in Android
96+ // Studio, or the "Toggle Debug Paint" command in Visual Studio Code)
97+ // to see the wireframe for each widget.
98+ //
99+ // Column has various properties to control how it sizes itself and
100+ // how it positions its children. Here we use mainAxisAlignment to
101+ // center the children vertically; the main axis here is the vertical
102+ // axis because Columns are vertical (the cross axis would be
103+ // horizontal).
49104 mainAxisAlignment: MainAxisAlignment .center,
50105 children: < Widget > [
106+ const Text (
107+ 'You have pushed the button this many times:' ,
108+ ),
109+ Text (
110+ '$_counter ' ,
111+ style: Theme .of (context).textTheme.headlineMedium,
112+ ),
51113 Text (
52114 'FLUTTER_BUILD_NAME: $_buildName ' ,
53115 style: Theme .of (context).textTheme.bodyMedium,
@@ -59,6 +121,11 @@ class _MyHomePageState extends State<MyHomePage> {
59121 ],
60122 ),
61123 ),
124+ floatingActionButton: FloatingActionButton (
125+ onPressed: _incrementCounter,
126+ tooltip: 'Increment' ,
127+ child: const Icon (Icons .add),
128+ ), // This trailing comma makes auto-formatting nicer for build methods.
62129 );
63130 }
64131}
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