Flutter开发-Demo项目结构解读

群组
欢迎加入群组,闲聊工作&技术&问题等

        当我们用Flutter创建一个空的demo的时候,Flutter会为我们创建一个计数器的Demo.
demo page
当点击”+”号时,页面中的数字会递增.而这个功能的实现在lib/main.dart文件中:

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 Page'),
    );
  }
}

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

1.依赖包的导入:

import 'package:flutter/material.dart';

将package目录下的flutter环境下的material.dart导入到当前文件.

2.入口函数:

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

        和iOS中的程序一样,Flutter也有一个顶级入口函数main()函数,通过”=>”操作符是方法的简写,可以理解为前面方法调用后面方法,这个main()函数内部调用的是runApp(widget)方法,这个方法定义在Binding.dart文件中:
runApp
        其主要作用是创建并启动Flutter应用,这里传入的是MyApp的实例,MyApp继承自StatelessWidget,StatelessWidget继承自Widget,这里你可以重写main()函数,然后调用runApp()方法:

void main(List<String> args) {
  print("this is main method");
}

3.app页面结构

        在Flutter中你所能看到的一切都是Widget,甚至其内部调用的Center,Column等都是Widget:

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 Page'),
    );
  }
}

        这里有一个MaterialApp也是Widget,其功能是帮助构建Flutter应用的框架,通过这个widget你可以设置APP的名称,主题,首页,路由列表等,这里主页home设置是MyHomePage.

4.MyHomePage首页

        之前我们说过了,类的构造方法是一个与类同名的函数,MyHomePage的构造方法里面有两个参数key和title,内部定义了一个final修饰的不可变属性title,其值通过类初始化传递.首页由两部分组成:

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) {by
        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.
    );
  }
}

        一个是继承自StatefulWidget的 MyHomePage,一个是继承自State类的_MyHomePageState类.在MyHomePage中我们看到重写了StatefulWidget类中的createState()方法,这个方法内部调用的是_MyHomePageState()即_MyHomePageState的构造函数,createState()返回的也是_MyHomePageState即State.

        继续探究,_MyHomePageState类中重写了build()方法这个方法调用Scaffold实例化方法,返回Scaffold实例,Scaffold可以理解为APP的容器,它包括appBar,body,floatingActionButton等Widget,由它构建了Widget树(也可以称为视图树).

        floatingActionButton是那个飘起来的按钮,它接受了一个点击后的回调方法_incrementCounter,这个回调方法内部调用的是setState(() {}方法,当按钮被点击时这个方法被回调,然后计数器数字递增,然后 setState被调用,然后调用build方法,更新页面.

现在我们可以了解页面的刷新时机了:
1)创建state时,也就是createState(),调用build()刷新页面;
2)更新state时,也就是setState()时,也调用build()刷新页面.

5.Flutter APP生命周期

        现在总结下来,Flutter 应用的生命周期,借用网上一张图片,可以鲜明看出:
Flutter life cycle


 上一篇
Flutter开发-路由页面管理(导航) Flutter开发-路由页面管理(导航)
群组欢迎加入群组,闲聊工作&技术&问题等         路由可以理解为页面的转发器,或者叫做导航器,其内部处理跳转逻辑,它规定页面具体跳转到
2019-06-13
下一篇 
Flutter开发-Dart语言基础-3 Flutter开发-Dart语言基础-3
群组欢迎加入群组,闲聊工作&技术&问题等 接着上篇,这节继续Dart基础语法. 类        类是一个实例的抽象,通过类来创建实例,创建实
2019-06-12
  目录