Flutter3,实战java高并发程序设计pdf

wuchangjian2021-11-16 12:43:18编程学习

import ‘package:flutter/material.dart’;

void main() {

runApp(MyApp());

}

//StatelessWidget是没有状态的Widget,比如文本等

class MyApp extends StatelessWidget {

// This widget is the root of your application.

@override

Widget build(BuildContext context) {

//返回一个widget

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,

// This makes the visual density adapt to the platform that you run

// the app on. For desktop platforms, the controls will be smaller and

// closer together (more dense) than on mobile platforms.

visualDensity: VisualDensity.adaptivePlatformDensity,

),

home: MyHomePage(title: ‘Flutter Demo Home Page’),

);

}

}

//StatefulWidget有状态的widget,状态需要变化的,比如数字增加

//官方建议使用StatelessWidget,防止性能消耗

//MyHomePage类是标准的写法,最下面还重写了createState方法

class MyHomePage extends StatefulWidget {

//命名构造函数,调用父类

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 i

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n a Widget subclass are

// always marked “final”.

final String title;

@override

_MyHomePageState createState() => _MyHomePageState();

}

class _MyHomePageState extends State {

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++;

});

}

//build是构建界面,渲染这个界面

@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.

//读取到Scaffold

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),//这里的widget就是上面的MyHomePage

),

body: Center(//body是一个居中布局

// Center is a layout widget. It takes a single child and positions it

// in the middle of the parent.

child: Column(//按照列进行布局,就是纵向布局

// 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.

//

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