程序的入口

只需要在runApp中传入Widget就可以了：

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

MyApp

1. StatelessWidget：无状态Widget
2. build方法：用于构建一个Widget
3. MaterialApp：相当于定义一个App的大体结构，将显示的内容（home属性）和全局配置（标题、主题等）分开
4. MyHomePage：页面内容Widget

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'),
    );
  }
}

MyHomePage

看几个关键点：

1. StatefulWidget：有状态的Widget，可以理解为内容可变
2. createState方法：因为是有状态的，重写这个方法，创建属于自己的State

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

  final String title;

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

_MyHomePageState

关键的地方：

1. 继承State<MyHomePage>：这个泛型的意思就是说这个State的宿主是谁，State是提供给谁用的。可以通过widget拿到MyHomePage实例，比如：widget.title
2. setState()方法：修改state方法，与React Native的setState相似
3. Scaffold Widget：Scaffold 是一个Material Design风格的框架。Scaffold已经定义好appBar、body等内容的位置，我们只需要在这里属性中传入Widget。

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.display1,
            ),
          ],
        ),
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: _incrementCounter,
        tooltip: 'Increment',
        child: Icon(Icons.add),
      ), // This trailing comma makes auto-formatting nicer for build methods.
    );
  }
}