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分享使用Android Studio配置安装好flutterSDK创建的第一个移动应用demo示例
一、示例代码
import 'package:flutter/material.dart';
void main() {
runApp(const MyApp());
}
class MyApp extends StatelessWidget {
const MyApp({super.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 THIS: Try running your application with "flutter run". You'll see
// the application has a purple toolbar. Then, without quitting the app,
// try changing the seedColor in the colorScheme below to Colors.green
// and then invoke "hot reload" (save your changes or press the "hot
// reload" button in a Flutter-supported IDE, or press "r" if you used
// the command line to start the app).
//
// Notice that the counter didn't reset back to zero; the application
// state is not lost during the reload. To reset the state, use hot
// restart instead.
//
// This works for code too, not just values: Most code changes can be
// tested with just a hot reload.
colorScheme: ColorScheme.fromSeed(seedColor: Colors.deepPurple),
useMaterial3: true,
),
home: const MyHomePage(title: 'Flutter Demo Home Page'),
);
}
}
class MyHomePage extends StatefulWidget {
const MyHomePage({super.key, required this.title});
// 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();
}
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(
// TRY THIS: Try changing the color here to a specific color (to
// Colors.amber, perhaps?) and trigger a hot reload to see the AppBar
// change color while the other colors stay the same.
backgroundColor: Theme.of(context).colorScheme.inversePrimary,
// 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.
//
// 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).
//
// TRY THIS: Invoke "debug painting" (choose the "Toggle Debug Paint"
// action in the IDE, or press "p" in the console), to see the
// wireframe for each widget.
mainAxisAlignment: MainAxisAlignment.center,
children: <Widget>[
const Text(
'You have pushed the button this many times:',
),
Text(
'$_counter',
style: Theme.of(context).textTheme.headlineMedium,
),
],
),
),
floatingActionButton: FloatingActionButton(
onPressed: _incrementCounter,
tooltip: 'Increment',
child: const Icon(Icons.add),
), // This trailing comma makes auto-formatting nicer for build methods.
);
}
}
二、解释说明
下面是你的 Flutter 应用程序的代码解释。这个简单的应用程序展示了如何使用 Flutter 框架创建一个计数器应用。
### 1. 引入依赖
```dart
import 'package:flutter/material.dart';
```
这行代码引入了 Flutter 的 Material 设计库，为我们提供构建用户界面所需的基本构件。
### 2. 主函数
```dart
void main() {
runApp(const MyApp());
}
```
这是应用程序的入口。`runApp` 函数接受一个根组件（在这里是 `MyApp`）并开始运行应用程序。
### 3. 主应用程序组件
```dart
class MyApp extends StatelessWidget {
const MyApp({super.key});
```
`MyApp` 是一个无状态组件（`StatelessWidget`），它是整个应用程序的根组件。
### 4. 应用程序主题
```dart
@override
Widget build(BuildContext context) {
return MaterialApp(
title: 'Flutter Demo',
theme: ThemeData(
colorScheme: ColorScheme.fromSeed(seedColor: Colors.deepPurple),
useMaterial3: true,
),
home: const MyHomePage(title: 'Flutter Demo Home Page'),
);
}
```
- `MaterialApp` 是 Flutter 中的一个容器，包含了应用程序的主题和路由等信息。
- `ThemeData` 定义了应用程序的主题颜色和风格。
- `ColorScheme.fromSeed` 使用深紫色作为应用程序的种子颜色，生成对应的调色板。
- `home` 属性设置了默认显示的组件，这里是 `MyHomePage`。
### 5. 主页面组件
```dart
class MyHomePage extends StatefulWidget {
const MyHomePage({super.key, required this.title});
final String title;
```
`MyHomePage` 是一个有状态组件（`StatefulWidget`），表示主要的用户界面元素，初始化时需要传入一个标题。
### 6. 组件状态
```dart
class _MyHomePageState extends State<MyHomePage> {
int _counter = 0;
void _incrementCounter() {
setState(() {
_counter++;
});
}
```
- `_MyHomePageState` 类保存了 `MyHomePage` 的状态，这里有一个 `_counter` 变量来记录点击次数。
- `_incrementCounter` 方法用于增加 `_counter` 的值，并通过 `setState` 通知 Flutter 重新构建 UI。
### 7. 构建方法
```dart
@override
Widget build(BuildContext context) {
return Scaffold(
appBar: AppBar(
backgroundColor: Theme.of(context).colorScheme.inversePrimary,
title: Text(widget.title),
),
body: Center(
child: Column(
mainAxisAlignment: MainAxisAlignment.center,
children: <Widget>[
const Text('You have pushed the button this many times:'),
Text('$_counter', style: Theme.of(context).textTheme.headlineMedium),
],
),
),
floatingActionButton: FloatingActionButton(
onPressed: _incrementCounter,
tooltip: 'Increment',
child: const Icon(Icons.add),
),
);
}
```
- `build` 方法定义了如何构建 UI。
- `Scaffold` 是一个设计良好的 Material 设计应用程序布局的结构，包括一个应用栏、主要内容和浮动操作按钮。
- `AppBar` 显示应用程序的标题，可以自定义背景颜色。
- `Center` 和 `Column` 组件用于将计数器文本居中显示。
- `FloatingActionButton` 是一个圆形按钮，点击后会调用 `_incrementCounter` 方法增加计数。
### 总结
这个 Flutter 应用程序实现了一个基本的计数器功能，通过点击浮动按钮来增加计数并实时更新显示。在用户界面（UI）上，它展示了 Material 风格的元素，包括应用栏、居中的文本和浮动按钮。
...全文
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