Flutter入门学习（二）第一个Flutter应用_const myapp({key? key}) : super(key: key);

Flutter 开发环境搭建好之后，创建第一个Flutter应用

使用VSCode来创建第一个Flutter应用

打开 VSCode 后，Cmd + Shift + p， 选择 Flutter: New Project 即可创建。

如下图：

如果右下角报找不到 Flutter SDK 的话需要配置一下 Flutter 的路径

如下图：

在配置环境的时候我们已经下载了 Flutter SDK，选择 Locate SDK，然后选择下载好的 Flutter 路径就好了。

创建好的项目如下：

运行的话使用 VSCode 的 运行就可以了，如下图：

Flutter入门学习，第一个Flutter应用就运行起来了，点击加号，会走VSCode里的方法，可以打断点查看。

代码分析

import 'package:flutter/material.dart';
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import并不陌生，一般就是在导库或者文件之类的

void main() {
  runApp(const MyApp());
}
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应用程序的 main 函数，mian 函数里面有一个方法，runApp()。

函数 const MyApp() 作为 runApp 的函数参数，调用runApp，继续会执行到 MyApp() 这个函数。

class MyApp extends StatelessWidget {
  const MyApp({Key? key}) : super(key: key);

  // 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: const MyHomePage(title: 'Flutter Demo Home Page'),
    );
  }
}
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从 class MyApp 我们可以看到，MyApp 是定义的一个类。原来const MyApp()是一个初始化函数，返回一个 const 类型的对象 app，然后 main 函数将这个app对象给 run 起来了，就有了我们的应用程序，通俗理解就是这样的。

const MyApp({Key? key}) : super(key: key);
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虽然不懂dart语言，但是从写法上我们可以猜，这是一个初始化函数

@override
Widget build(BuildContext context) 
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从这个override我们可以看到，这个函数应该是系统函数，复写了而已。

  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: const MyHomePage(title: 'Flutter Demo Home Page'),
    );
  }
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这个函数直接返回了一个 MaterialApp 对象，这个对象的一些属性，例如 title，theme，home都在下面进行了一些设置。这种函数式编程，代码看起来还是挺舒适的。

主要的来看下home这里

home: const MyHomePage(title: 'Flutter Demo Home Page')
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MyHomePage() 这又是一个初始化函数，MyHomePage是一个类

class MyHomePage extends StatefulWidget {
  const MyHomePage({Key? key, required 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
  State<MyHomePage> createState() => _MyHomePageState();
}
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主要看这里

  @override
  State<MyHomePage> createState() => _MyHomePageState();
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createState() 是一个函数，_MyHomePageState() 也是一个函数， _MyHomePageState() 这个函数直接作为 createState() 的函数返回值。

调用 createState() 实际上就执行到了 _MyHomePageState()

最终应用程序页面的构建落脚到了这里

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>[
            const Text(
              'You have pushed the button this many times:',
            ),
            Text(
              '$_counter',
              style: Theme.of(context).textTheme.headline4,
            ),
          ],
        ),
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: _incrementCounter,
        tooltip: 'Increment',
        child: const Icon(Icons.add),
      ), // This trailing comma makes auto-formatting nicer for build methods.
    );
  }
}
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里面定义了一个全局变量 _counter

定义了一个函数 _incrementCounter()

复写了 Widget build(BuildContext context) 方法，这个方法中返回了一个Scaffold 对象

这个 Scaffold 包含 appBar 和 body

还在 build 这个函数里添加了一个按钮 floatingActionButton

floatingActionButton: FloatingActionButton(
    onPressed: _incrementCounter,
    tooltip: 'Increment',
    child: const Icon(Icons.add),
), // This trailing comma makes auto-formatting nicer for build methods.
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按钮点击的方法函数即 _incrementCounter()

整体上代码是从上依次往下进行执行，每一处都不是多余的，而且很合理

mian 函数里运行了一个app对象，app里有一个 home 对象，即app 的主页，homePage中初始化了主页的模块，里面是一个Scaffold的对象，这个对象有appBar，有body，导航栏有内容，有一个按钮，可以点击。

总之应用程序的main函数运行了一个app，app中构建了一个主页，主页上添加了一些内容。