Flutter 学习（二）：创建第一个Flutter项目 & StatelessWidget & StatefulWidget
Flutter 学习（二）：创建第一个Flutter项目 & StatelessWidget & StatefulWidget
- 1.创建项目
- 2.项目展示
- 3.分析
- 4.为什么要将build方法放在State中，而不是放在StatefulWidget中
1.创建项目
当我们环境配置好了之后，运行创建项目命令：
$ flutter create myapp
$ cd myapp
& flutter run //运行项目
2.项目展示
当项目创建完成后，在lib中main.dart为项目的初始入口，官方代码如下：
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 {
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: [
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.
);
}
}
3.分析
3.1 导入包
import 'package:flutter/material.dart';
此行代码作用是导入Material UI组件库，以便我们使用。Flutter为提供了一些封装好的不同风格的Widget，方便我们使用。如Material是一种标准的移动端和web端的视觉设计语言。cupertino iOS风格的UI库, 还有一些基础组件
3.2 应用入口
void main() => runApp(MyApp());
- 与C/C++、Java类似，Flutter 应用中main函数为应用程序的入口，main函数中调用了，runApp 方法，它的功能是启动Flutter应用，它接受一个Widget参数，在本示例中它是MyApp类的一个实例，该参数代表Flutter应用。
- main函数使用了(=>)符号，这是Dart中单行函数或方法的简写。
3.3 应用结构
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'),
);
}
}
-
MyApp类继承于StatelessWidget，说明了一个重要的信息，类本身也是Widget。在Flutter中很多UI布局，甚至包括对齐(alignment)、填充(padding)和布局(layout)都是Widget.一个完整的界面就是由各种Widget组合而成.
-
Flutter在构建页面时，会调用组件的build方法，widget的主要工作是提供一个build()方法来描述如何构建UI界面（通常是通过组合、拼装其它基础widget）
-
MaterialApp是Material库中提供的框架Widget，通过它可以设置应用的名称、主题、语言、首页及路由列表等。
-
home是MaterialApp的属性，用来设置界面主体，也是一个Widget
3.4 界面主体
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();
}
MyHomePage是界面主体，继承于StatefulWidget类，这便是它是一个有状态，可以由外部动态改变。现在我们讲下StatelessWidget, StatefulWidget两者的不同。
- StatelessWidget是不可变的，是静态的，被负值后就不能在改变了。
- StatefulWidget是可变的，是动态的，可以拥有状态，这些状态在生命周期类都是可以被修改的。StatefulWidget有StatefulWidget类和State类组成
- StatefulWidget本身是不可变的，但是它持有State类，允许State类在widget生命周期中操作状态发生变化。
- State类持有状态，可以操作状态。
3.5 UI构建
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++;
});
}
@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: [
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.
);
}
}
- Scaffold 是Material库中提供的页面导航路由控制器，它包含导航栏和Body以及FloatingActionButton。
- appbar：提供导航修改设置，是Material库提供的导航设置Widget
- body: Scaffold的属性也是一个Widget，显示的是出导航外的主体。
4.为什么要将build方法放在State中，而不是放在StatefulWidget中
？
为什么build()方法在State（而不是StatefulWidget）中 ？这主要是为了开发的灵活性。如果将build()方法在StatefulWidget中则会有两个问题：
-
状态访问不便
试想一下，如果我们的Stateful widget 有很多状态，而每次状态改变都要调用build方法，由于状态是保存在State中的，如果将build方法放在StatefulWidget中，那么构建时读取状态将会很不方便，试想一下，如果真的将build方法放在StatefulWidget中的话，由于构建用户界面过程需要依赖State，所以build方法将必须加一个State参数，大概是下面这样：
Widget build(BuildContext context, State state){ //state.counter ... }这样的话就只能将State的所有状态声明为公开的状态，这样才能在State类外部访问状态，但将状态设置为公开后，状态将不再具有私密性，这样依赖，对状态的修改将会变的不可控。将build()方法放在State中的话，构建过程则可以直接访问状态，这样会很方便
-
继承StatefulWidget不便
例如，Flutter中有一个动画widget的基类AnimatedWidget，它继承自StatefulWidget类。AnimatedWidget中引入了一个抽象方法build(BuildContext context)，继承自AnimatedWidget的动画widget都要实现这个build方法。现在设想一下，如果StatefulWidget 类中已经有了一个build方法，正如上面所述，此时build方法需要接收一个state对象，这就意味着AnimatedWidget必须将自己的State对象(记为_animatedWidgetState)提供给其子类，因为子类需要在其build方法中调用父类的build方法，代码可能如下：
class MyAnimationWidget extends AnimatedWidget{ @override Widget build(BuildContext context, State state){ //由于子类要用到AnimatedWidget的状态对象_animatedWidgetState， //所以AnimatedWidget必须通过某种方式将其状态对象_animatedWidgetState //暴露给其子类 super.build(context, _animatedWidgetState) } }这样很显然是不合理的，因为
- AnimatedWidget的状态对象是AnimatedWidget内部实现细节，不应该暴露给外部。
- 如果要将父类状态暴露给子类，那么必须得有一种传递机制，而做这一套传递机制是无意义的，因为父子类之间状态的传递和子类本身逻辑是无关的。
参考文档
1.Flutter中文网。
2.Flutter实战。