新建一个Flutter项目，目录结构如下：
    1638327485(1).png
    需要关注的几个目录和文件：

    • android

    Android应用信息配置目录，诸如权限、原生交互、打包配置等，需要在该文件夹中编辑。

    • ios

    同android，是ios的配置目录，权限配置、打包等都需要在该文件夹中执行。

    • lib

    Flutter的dart代码目录，所有的Flutter代码都必须在该目录下。

    • web

    同android和ios，是生成web应用的配置目录。

    • pubspec.yaml

    Flutter项目的配置文件，可配置引用的第三方插件、字体，本地（包内）的静态资源以及应用版本。

    在lib目录下会默认新建一个main.dart文件，该文件为项目的唯一入口，在里边执行了一个main方法：

    1. void main() {
    2.   runApp(const MyApp());
    3. }
    1. 其中MyApp是一个自定义的Widget，一般来说需要返回一个MaterialApp，来看一下demo项目的写法：
    1. import 'package:flutter/material.dart';
    3. void main() {
    4.   runApp(const MyApp());
    5. }
    7. class MyApp extends StatelessWidget {
    8.   const MyApp({Key? key}) : super(key: key);
    10.   // This widget is the root of your application.
    11.   @override
    12.   Widget build(BuildContext context) {
    13.     return MaterialApp(
    14.       title: 'Flutter Demo',
    15.       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.
    25.         primarySwatch: Colors.blue,
    26.       ),
    27.       home: const MyHomePage(title: 'Flutter Demo Home Page'),
    28.     );
    29.   }
    30. }
    32. class MyHomePage extends StatefulWidget {
    33.   const MyHomePage({Key? key, required this.title}) : super(key: key);
    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.
    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".
    44.   final String title;
    46.   @override
    47.   State<MyHomePage> createState() => _MyHomePageState();
    48. }
    50. class _MyHomePageState extends State<MyHomePage> {
    51.   int _counter = 0;
    53.   void _incrementCounter() {
    54.     setState(() {
    55.       // This call to setState tells the Flutter framework that something has
    56.       // changed in this State, which causes it to rerun the build method below
    57.       // so that the display can reflect the updated values. If we changed
    58.       // _counter without calling setState(), then the build method would not be
    59.       // called again, and so nothing would appear to happen.
    60.       _counter++;
    61.     });
    62.   }
    64.   @override
    65.   Widget build(BuildContext context) {
    66.     // This method is rerun every time setState is called, for instance as done
    67.     // by the _incrementCounter method above.
    68.     //
    69.     // The Flutter framework has been optimized to make rerunning build methods
    70.     // fast, so that you can just rebuild anything that needs updating rather
    71.     // than having to individually change instances of widgets.
    72.     return Scaffold(
    73.       appBar: AppBar(
    74.         // Here we take the value from the MyHomePage object that was created by
    75.         // the App.build method, and use it to set our appbar title.
    76.         title: Text(widget.title),
    77.       ),
    78.       body: Center(
    79.         // Center is a layout widget. It takes a single child and positions it
    80.         // in the middle of the parent.
    81.         child: Column(
    82.           // Column is also a layout widget. It takes a list of children and
    83.           // arranges them vertically. By default, it sizes itself to fit its
    84.           // children horizontally, and tries to be as tall as its parent.
    85.           //
    86.           // Invoke "debug painting" (press "p" in the console, choose the
    87.           // "Toggle Debug Paint" action from the Flutter Inspector in Android
    88.           // Studio, or the "Toggle Debug Paint" command in Visual Studio Code)
    89.           // to see the wireframe for each widget.
    90.           //
    91.           // Column has various properties to control how it sizes itself and
    92.           // how it positions its children. Here we use mainAxisAlignment to
    93.           // center the children vertically; the main axis here is the vertical
    94.           // axis because Columns are vertical (the cross axis would be
    95.           // horizontal).
    96.           mainAxisAlignment: MainAxisAlignment.center,
    97.           children: <Widget>[
    98.             const Text(
    99.               'You have pushed the button this many times:',
    100.             ),
    101.             Text(
    102.               '$_counter',
    103.               style: Theme.of(context).textTheme.headline4,
    104.             ),
    105.           ],
    106.         ),
    107.       ),
    108.       floatingActionButton: FloatingActionButton(
    109.         onPressed: _incrementCounter,
    110.         tooltip: 'Increment',
    111.         child: const Icon(Icons.add),
    112.       ), // This trailing comma makes auto-formatting nicer for build methods.
    113.     );
    114.   }
    115. }
    1. 将以上代码分为几个部分解读：
    • 第1行

    import ‘package:flutter/material.dart’;
    表示引入material包，material是一个UI包，几乎所有的页面都需要基于该包来开发。

    • 第3-5行

    执行main方法，这是程序的入口。

    • 第7-30行

    新建一个无状态的widget，继承自StatelessWidget，重写build方法，在其中返回一个MaterialApp，配置它的主页为MyHomePage()。

    • 第32-48行

    实现一个有状态的MyHomePage，继承自StatefulWidget，重写createState方法，返回一个_MyHomePageState()。

    • 第50-115行

    实现_MyHomePageState，在build中构建UI，其中54-61表示通过setState更新数值，只有通过setState方法更新值，才能刷新UI的显示。Scaffold是一个常用的脚手架，几乎所有页面都基于他来编写，类似android中的activity。