$ cd ~/Desktop/flutter_in_action/
$ flutter create counter_app
$ cd counter_app
$ flutter run

counter_app % tree -L 2
.
├── README.md
├── android
├── ios
├── lib
│   └── main.dart
├── pubspec.lock
├── pubspec.yaml
├── test
│   └── widget_test.dart
└── web
└── windows

counter_app % tree -L 2
.
├── README.md
├── android
├── ios
├── lib
│   └── main.dart
├── pubspec.lock
├── pubspec.yaml
├── test
│   └── widget_test.dart
└── web
└── windows

import 'package:flutter/material.dart';

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        // This is the theme of your application.
        //
        // Try running your application with "flutter run". You'll see the
        // application has a blue toolbar. Then, without quitting the app, try
        // changing the primarySwatch below to Colors.green and then invoke
        // "hot reload" (press "r" in the console where you ran "flutter run",
        // or simply save your changes to "hot reload" in a Flutter IDE).
        // Notice that the counter didn't reset back to zero; the application
        // is not restarted.
        primarySwatch: Colors.blue,
      ),
      home: MyHomePage(title: 'Flutter Demo Home'),
    );
  }
}

class MyHomePage extends StatefulWidget {
  MyHomePage({Key key, this.title}) : super(key: key);

  // This widget is the home page of your application. It is stateful, meaning
  // that it has a State object (defined below) that contains fields that affect
  // how it looks.

  // This class is the configuration for the state. It holds the values (in this
  // case the title) provided by the parent (in this case the App widget) and
  // used by the build method of the State. Fields in a Widget subclass are
  // always marked "final".

  final String title;

  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  int _counter = 0;

  void _incrementCounter() {
    setState(() {
      // This call to setState tells the Flutter framework that something has
      // changed in this State, which causes it to rerun the build method below
      // so that the display can reflect the updated values. If we changed
      // _counter without calling setState(), then the build method would not be
      // called again, and so nothing would appear to happen.
      _counter++;
    });
  }

  @override
  Widget build(BuildContext context) {
    // This method is rerun every time setState is called, for instance as done
    // by the _incrementCounter method above.
    //
    // The Flutter framework has been optimized to make rerunning build methods
    // fast, so that you can just rebuild anything that needs updating rather
    // than having to individually change instances of widgets.
    return Scaffold(
      appBar: AppBar(
        // Here we take the value from the MyHomePage object that was created by
        // the App.build method, and use it to set our appbar title.
        title: Text(widget.title),
      ),
      body: Center(
        // Center is a layout widget. It takes a single child and positions it
        // in the middle of the parent.
        child: Column(
          // Column is also a layout widget. It takes a list of children and
          // arranges them vertically. By default, it sizes itself to fit its
          // children horizontally, and tries to be as tall as its parent.
          //
          // Invoke "debug painting" (press "p" in the console, choose the
          // "Toggle Debug Paint" action from the Flutter Inspector in Android
          // Studio, or the "Toggle Debug Paint" command in Visual Studio Code)
          // to see the wireframe for each widget.
          //
          // Column has various properties to control how it sizes itself and
          // how it positions its children. Here we use mainAxisAlignment to
          // center the children vertically; the main axis here is the vertical
          // axis because Columns are vertical (the cross axis would be
          // horizontal).
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            Text(
              'You have pushed the button this many times:',
            ),
            Text(
              '$_counter',
              style: Theme.of(context).textTheme.headline4,
            ),
          ],
        ),
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: _incrementCounter,
        tooltip: 'Increment',
        child: Icon(Icons.add),
      ), // This trailing comma makes auto-formatting nicer for build methods.
    );
  }
}

import 'package:flutter/material.dart';

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: MyHomePage(title: 'Flutter Demo Home'),
    );
  }
}

class MyHomePage extends StatefulWidget {
  MyHomePage({Key key, this.title}) : super(key: key);

  final String title;

  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  int _counter = 0;

  void _incrementCounter() {
    setState(() {
      _counter++;
    });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(widget.title),
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            Text(
              'You have pushed the button this many times:',
            ),
            Text(
              '$_counter',
              style: Theme.of(context).textTheme.headline4,
            ),
          ],
        ),
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: _incrementCounter,
        tooltip: 'Increment',
        child: Icon(Icons.add),
      ), 
    );
  }
}

import 'package:flutter/material.dart';

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: MyHomePage(title: 'Flutter Demo Home'),
    );
  }
}

class MyHomePage extends StatefulWidget {
  MyHomePage({Key key, this.title}) : super(key: key);

  final String title;

  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  int _counter = 0;

  void _incrementCounter() {
    setState(() {
      _counter++;
    });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(widget.title),
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            Text(
              'You have pushed the button this many times:',
            ),
            Text(
              '$_counter',
              style: Theme.of(context).textTheme.headline4,
            ),
          ],
        ),
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: _incrementCounter,
        tooltip: 'Increment',
        child: Icon(Icons.add),
      ), 
    );
  }
}

class MyApp extends StatelessWidget {                     

  @override                                               
  Widget build(BuildContext context) {                    
    return MaterialApp(                                   
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: MyHomePage(title: 'Flutter Demo Home Page'),  
    );
  }
}

class MyApp extends StatelessWidget {                     

  @override                                               
  Widget build(BuildContext context) {                    
    return MaterialApp(                                   
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: MyHomePage(title: 'Flutter Demo Home Page'),  
    );
  }
}

Widget build(BuildContext context) {
    return Button(
        child: new Text("Submit"),
    );
}

Widget build(BuildContext context) {
    return Button(
        child: const Text("Submit"),
    );
}

Scaffold(
  appBar: AppBar(),
  body: Container(
    child: Column(
      children: [
        Row(
          Text(),
          Icon(),
        ),
        Text(),
      ],
    ),
  ),
);

class SubmitButton extends StatelessWidget {
  final String buttonText;  
  SubmitButton(this.buttonText);

  Widget build(context) {
    return Button(
      child: const Text(buttonText);
    );
  }
}

class MyHomePage extends StatefulWidget { 
  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  @override
  Widget build(BuildContext context) {
    // ..
  }
}

void _incrementCounter() {
  setState(() {
    _counter++;
  });
}

void _incrementCounter() {
  setState(() {
    _counter++;
  });
}

class FirstNameTextState extends State<FirstNameText> {
    String name;

    FirstNameTextState(this.name);

    @override
    initState() {
        super.initState();
        name = name.toUpperCase();
    }

    Widget build(BuildContext context) {
        return Text(name);
    }
}

Human
  .rideHorse()

  Cowboy
    .chaseOutlaws()
    .fightAliens()

  Rancher
    .herdCattle()

Alien
  .flySpaceship()
  .invadeEarth()

HorseRiding
  .rideHorse()

Cowboy
  HorseRidingInstance.rideHorse()
  .chaseOutlaws()
  .fightAliens()

Rancher
  HorseRidingInstance.rideHorse()
  .herdCattle()

Alien
  HorseRidingInstance.rideHorse()
  .flySpaceship()
  .invadeEarth()

class PanicButton extends StatelessWidget {
  final Widget display;
  final VoidCallback onPressed;

  PanicButton({this.display, this.onPressed});

  Widget build(BuildContext context) {
    return RaisedButton(
      color: Colors.red,

      child: display,

      onPressed: onPressed,
    );
  }
}

body: Column(
  children: const<Widget> [
    PanicButton(
      Text("Tombol"), 
      _updateData(),
    ),
  ],
)

class PanicButton extends StatelessWidget {
  final Widget display;
  final VoidCallback onPressed;

  PanicButton({this.display, this.onPressed});

  Widget build(BuildContext context) {
    return RaisedButton(
      color: Colors.red,

      child: display,

      onPressed: onPressed,
    );
  }
}

body: Column(
  children: [
    PanicButton(
      display: Text("Tombol"), 
      onPressed: _updateData(),
    ),
  ],
)

class PanicButton extends StatelessWidget {
  final Widget display;
  final VoidCallback onPressed;

  PanicButton({this.display, this.onPressed});

  Widget build(BuildContext context) {
    return RaisedButton(
      color: Colors.red,

      child: display,

      onPressed: onPressed,
    );
  }
}

body: Column(
  children: [
    PanicButton(
      display: Icon(
        Icons.favorite,
        color: Colors.pink,
        size: 24.0,
        semanticLabel: 'Text to announce in accessibility modes',
      ),
      onPressed: _updateData(),
    ),
  ],
)