给 React Native 开发者的 Flutter 指南

本文面向希望基于现有的 React Native 的知识结构使用 Flutter 开发移动端应用的开发者。如果你已经对 RN 的框架有所了解，那么你可以通过这个文档入门 Flutter 开发。

This document is for React Native (RN) developers looking to apply their existing RN knowledge to build mobile apps with Flutter. If you understand the fundamentals of the RN framework then you can use this document as a way to get started learning Flutter development.

本文可以当做查询手册使用，里面涉及到的问题基本上可以满足需求。

This document can be used as a cookbook by jumping around and finding questions that are most relevant to your needs.

针对 JavaScript 开发者的 Dart 介绍

Introduction to Dart for JavaScript Developers

和 React Native 一样，Flutter 使用 reactive 风格的视图。然而，RN 需要被转译为本地对应的 widget，而 Flutter 是直接编译成本地原生代码。 Flutter 可以控制屏幕上的每一个像素，如此可以避免由于使用 JavaScript Bridge 导致的性能问题。

Like React Native, Flutter uses reactive-style views. However, while RN transpiles to native widgets, Flutter compiles all the way to native code. Flutter controls each pixel on the screen, which avoids performance problems caused by the need for a JavaScript bridge.

Dart 学习起来非常简单而且有如下特性：

Dart is an easy language to learn and offers the following features:

• 它针对 web 服务和移动应用开发提供了一种开源的，可扩展的编程语言。

  Provides an open-source, scalable programming language for building web, server, and mobile apps.

• 它提供了一种面向对象的单继承语言，使用 C 语言风格的语法并且可通过 AOT 编译为本地代码。

  Provides an object-oriented, single inheritance language that uses a C-style syntax that is AOT-compiled into native.

• 可转译为 JavaScript 代码。

  Transcompiles optionally into JavaScript.

• 支持接口和抽象类。

  Supports interfaces and abstract classes.

下面的几个例子解释了 JavaScript 和 Dart 的区别。

A few examples of the differences between JavaScript and Dart are described below.

入口函数

Entry point

JavaScript 并没有预定义的入口函数。

JavaScript doesn’t have a pre-defined entry function—you define the entry point.

// JavaScript
function startHere() {
  // Can be used as entry point
  //这里可以当做入口函数
}

在 Dart 里，每个应用程序必须有一个最顶级的 main() 函数，该函数作为应用程序的入口函数。

In Dart, every app must have a top-level main() function that serves as the entry point to the app.

// Dart
main() {
}

可以在这里查看效果 DartPad。

Try it out in DartPad.

在控制台打印输出

Printing to the console

在 Dart 中如果需要在控制台进行输出，调用 print()。

To print to the console in Dart, use print().

// JavaScript
console.log('Hello world!');

// Dart
print('Hello world!');

可以在这里查看效果 DartPad。

Try it out in DartPad.

变量

Variables

Dart 是类型安全的，它结合静态类型检查和运行时检查来保证变量的值总是和变量的静态类型相匹配。虽然类型是语法要求，有些类型标注也并不是必须要填的，因为 Dart 使用类型推断。

Dart is type safe—it uses a combination of static type checking and runtime checks to ensure that a variable’s value always matches the variable’s static type. Although types are mandatory, some type annotations are optional because Dart performs type inference.

创建变量并赋值

Creating and assigning variables

在 JavaScript 中，变量是无法指定类型的。

In JavaScript, variables cannot be typed.

在 Dart 中，变量要么被显式定义类型，要么系统会自动判断变量的类型。

In Dart, variables must either be explicitly typed or the type system must infer the proper type automatically.

// JavaScript
var name = 'JavaScript';

// Dart
String name = 'dart'; // Explicitly typed as a string.

String name = 'dart'; // 显式声明为字符串。

var otherName = 'Dart'; // Inferred string.

var otherName = 'Dart'; // 推断为字符串。

// Both are acceptable in Dart.

// 两种定义方式在 Dart 中都可以。

可以在这里查看效果 DartPad。

Try it out in DartPad.

如果想了解更多相关信息，请转向该页面 Dart’s Type System。

For more information, see Dart’s Type System.

默认值

Default value

在 JavaScript 中，未初始化的变量是 ‘undefined’。

In JavaScript, uninitialized variables are undefined.

在 Dart 中，未初始化的变量会有一个初始值 null。因为数字在 Dart 是对象，甚至未初始化的数字类型的变量也会是 null。

In Dart, uninitialized variables have an initial value of null. Because numbers are objects in Dart, even uninitialized variables with numeric types have the value null.

// JavaScript
var name; // == undefined

// Dart
var name; // == null
int x; // == null

可以在这里查看效果 DartPad。

Try it out in DartPad.

如果想了解更多详细内容，请查看这个文档 variables。

For more information, see the documentation on variables.

检查 null 或者零值。

Checking for null or zero

在 JavaScript 中，1 或者任何非空对象都相当于 true。

In JavaScript, values of 1 or any non-null objects are treated as true.

// JavaScript
var myNull = null;
if (!myNull) {
  console.log('null is treated as false');
}
var zero = 0;
if (!zero) {
  console.log('0 is treated as false');
}

在 Dart 中，只有布尔类型值 true 才是 true。

In Dart, only the boolean value true is treated as true.

// Dart
var myNull = null;
if (myNull == null) {
  print('use "== null" to check null');
}
var zero = 0;
if (zero == 0) {
  print('use "== 0" to check zero');
}

可以在这里查看效果 DartPad。

Try it out in DartPad.

函数

Functions

Dart 和 JavaScript 中的函数很相似。最大的区别是声明格式。

Dart and JavaScript functions are generally similar. The primary difference is the declaration.

// JavaScript
function fn() {
  return true;
}

// Dart
fn() {
  return true;
}
// can also be written as
bool fn() {
  return true;
}

可以在这里查看效果 DartPad。

Try it out in DartPad.

如果想了解更多相关信息，请转向该页面 functions。

For more information, see the documentation on functions.

异步编程

Asynchronous programming

Futures

和 JavaScript 类似，Dart 支持单线程。在 JavaScript 中， Promise 对象代表异步操作的完成或者失败。

Like JavaScript, Dart supports single-threaded execution. In JavaScript, the Promise object represents the eventual completion (or failure) of an asynchronous operation and its resulting value.

Dart 使用 Future 对象来实现该机制。

Dart uses Future objects to handle this.

// JavaScript
class Example {
  _getIPAddress() {
    const url = 'https://httpbin.org/ip';
    return fetch(url)
      .then(response => response.json())
      .then(responseJson => {
        const ip = responseJson.origin;
        return ip;
      });
  }
}

function main() {
  const example = new Example();
  example
    ._getIPAddress()
    .then(ip => console.log(ip))
    .catch(error => console.error(error));
}

main();

// Dart
import 'dart:convert';
import 'package:http/http.dart' as http;

class Example {
  Future<String> _getIPAddress() {
    final url = 'https://httpbin.org/ip';
    return http.get(url).then((response) {
      String ip = jsonDecode(response.body)['origin'];
      return ip;
    });
  }
}

main() {
  final example = new Example();
  example
      ._getIPAddress()
      .then((ip) => print(ip))
      .catchError((error) => print(error));
}

可以在这里查看效果 DartPad。

如果想了解更多相关信息，请参考 Futures 的相关文档。

For more information, see the documentation on Futures.

async 和 await

async and await

async 函数声明定义了一个异步函数。

The async function declaration defines an asynchronous function.

在 JavaScript 中， async 函数返回一个 Promise。await 操作符用于等待 Promise。

In JavaScript, the async function returns a Promise. The await operator is used to wait for a Promise.

// JavaScript
class Example {
  async function _getIPAddress() {
    const url = 'https://httpbin.org/ip';
    const response = await fetch(url);
    const json = await response.json();
    const data = await json.origin;
    return data;
  }
}

async function main() {
  const example = new Example();
  try {
    const ip = await example._getIPAddress();
    console.log(ip);
  } catch (error) {
    console.error(error);
  }
}

main();

在 Dart 中，async 函数返回一个 Future，而函数体会在未来执行。await 操作符用于等待 Future。

In Dart, an async function returns a Future, and the body of the function is scheduled for execution later. The await operator is used to wait for a Future.

// Dart
import 'dart:convert';
import 'package:http/http.dart' as http;

class Example {
  Future<String> _getIPAddress() async {
    final url = 'https://httpbin.org/ip';
    final response = await http.get(url);
    String ip = jsonDecode(response.body)['origin'];
    return ip;
  }
}

main() async {
  final example = new Example();
  try {
    final ip = await example._getIPAddress();
    print(ip);
  } catch (error) {
    print(error);
  }
}

可以在这里查看效果 DartPad。

如果想了解更多相关信息，请参考 async 和 await 的相关文档。

For more information, see the documentation for async and await.

基本知识

The basics

如何创建一个 Flutter 应用？

How do I create a Flutter app?

如果要使用 React Native 创建应用，你需要在命令行里运行 create-react-native-app。

To create an app using React Native, you would run create-react-native-app from the command line.

$ create-react-native-app <projectname>

要在 Flutter 中创建应用，完成下面其中一项即可：

To create an app in Flutter, do one of the following:

• 使用带有 Flutter 和 Dart 插件的 IDE。

  Use an IDE with the Flutter and Dart plugins installed.

• 在命令行中运行命令 flutter create。不过要提前确认 Flutter SDK 已经在系统环境变量 PATH 中定义。

  Use the flutter create command from the command line. Make sure that the Flutter SDK is in your PATH.

$ flutter create <projectname>

如果想要了解更多内容，详见 开始使用 Flutter，在该页面会手把手教你创建一个点击按钮进行计数的应用。创建一个 Flutter 项目就可以构建 Android 和 iOS 设备上运行应用所需的所有文件。

For more information, see Getting Started, which walks you through creating a button-click counter app. Creating a Flutter project builds all the files that you need to run a sample app on both Android and iOS devices.

我如何运行应用呢？

How do I run my app?

在 React Native, 你可以在项目文件夹中运行 npm run 或者 yarn run。

In React Native, you would run npm run or yarn run from the project directory.

你可以通过如下几个途径运行 Flutter 应用程序：

You can run Flutter apps in a couple of ways:

• 在带有 Flutter 和 Dart 插件的 IDE 中使用 “run” 选项。

  Use the “run” option in an IDE with the Flutter and Dart plugins.

• 在项目根目录运行 flutter run。

  Use flutter run from the project’s root directory.

你的应用程序会在已连接的设备、iOS 模拟器或者 Android 模拟器上运行。

Your app runs on a connected device, the iOS simulator, or the Android emulator.

如果想了解更多相关信息，可以参考 Flutter 的相关文档：开始使用 Flutter。

For more information, see the Flutter Getting Started documentation.

如何导入 widget

How do I import widgets?

在 React Native 中，你需要导入每一个所需的组件。

In React Native, you need to import each required component.

//React Native
import React from 'react';
import { StyleSheet, Text, View } from 'react-native';

在 Flutter 中，如果要使用 Material Design 库里的 widget，导入 material.dart 包。如果要使用 iOS 风格的 widget，导入 Cupertino 库。如果要使用更加基本的 widget，导入 Widget 库。或者，你可以实现自己的 widget 库并导入。

In Flutter, to use widgets from the Material Design library, import the material.dart package. To use iOS style widgets, import the Cupertino library. To use a more basic widget set, import the Widgets library. Or, you can write your own widget library and import that.

import 'package:flutter/material.dart';
import 'package:flutter/cupertino.dart';
import 'package:flutter/widgets.dart';
import 'package:flutter/my_widgets.dart';

无论你导入哪个库，Dart 仅仅引用你应用中用到的 widget。

Whichever widget package you import, Dart pulls in only the widgets that are used in your app.

如果想了解更多相关信息，可以参考 核心 Widget 目录。

For more information, see the Flutter Widgets Catalog.

在 Flutter 里有没有类似 React Native 中 “Hello world!” 应用程序？

What is the equivalent of the React Native “Hello world!” app in Flutter?

在 React Native，HelloWorldApp 继承自 React.Component 并且通过返回 view 对象实现了 render 方法。

In React Native, the HelloWorldApp class extends React.Component and implements the render method by returning a view component.

// React Native
import React from 'react';
import { StyleSheet, Text, View } from 'react-native';

export default class App extends React.Component {
  render() {
    return (
      <View style={styles.container}>
        <Text>Hello world!</Text>
      </View>
    );
  }
}

const styles = StyleSheet.create({
  container: {
    flex: 1,
    backgroundColor: '#fff',
    alignItems: 'center',
    justifyContent: 'center'
  }
});

在 Flutter 中，你可以使用核心 widget 库中的 Center 和 Text widget 创建对应的 “Hello world!” 应用程序。Center widget 是 widget 树中的根，而且只有 Text 一个子 widget。

In Flutter, you can create an identical “Hello world!” app using the Center and Text widgets from the core widget library. The Center widget becomes the root of the widget tree and has one child, the Text widget.

// Flutter
import 'package:flutter/material.dart';

void main() {
  runApp(
    Center(
      child: Text(
        'Hello, world!',
        textDirection: TextDirection.ltr,
      ),
    ),
  );
}

下面的图片展示了 Android 和 iOS 中的基本 Flutter “Hello world!” 应用程序的界面。

The following images show the Android and iOS UI for the basic Flutter “Hello world!” app.

现在大家已经明白了最基本的 Flutter 应用，接下来会告诉大家如何利用 Flutter 丰富的 widget 库来创建主流的华丽的应用程序。

Now that you’ve seen the most basic Flutter app, the next section shows how to take advantage of Flutter’s rich widget libraries to create a modern, polished app.

我如何使用 widget 并且把它们封装起来组成一个 widget 树？

How do I use widgets and nest them to form a widget tree?

在 Flutter 中，几乎任何元素都是 widget。

In Flutter, almost everything is a widget.

widget 是构建应用软件用户界面的基本元素。你可以将 widget 按照一定的层次组合，成为 widget 树。每个 widget 内嵌在父 widget 中，并且继承了父 widget 的属性。甚至应用程序本身就是一个 widget。并没有一个独立的应用程序对象。反而 root widget 充当了这个角色。

Widgets are the basic building blocks of an app’s user interface. You compose widgets into a hierarchy, called a widget tree. Each widget nests inside a parent widget and inherits properties from its parent. Even the application object itself is a widget. There is no separate “application” object. Instead, the root widget serves this role.

一个 widget 可以定义：

A widget can define:

• 一个结构化的元素 - 类似按钮或者菜单

  A structural element—like a button or menu

• 一个风格化的元素 - 类似字体或者颜色方案

  A stylistic element—like a font or color scheme

• 布局元素 - 类似填充区或者对齐元素

  An aspect of layout—like padding or alignment

下面的示例展示了使用 Material 库里 widget 实现的 “Hello world!” 应用程序。在这个示例中，该 widget 树是包含在 MaterialApp root widget 里的。

The following example shows the “Hello world!” app using widgets from the Material library. In this example, the widget tree is nested inside the MaterialApp root widget.

// Flutter
import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {
  @override
 widget build(BuildContext context) {
    return MaterialApp(
      title: 'Welcome to Flutter',
      home: Scaffold(
        appBar: AppBar(
          title: Text('Welcome to Flutter'),
        ),
        body: Center(
          child: Text('Hello world'),
        ),
      ),
    );
  }
}

下面的图片为大家展示了通过 Material Design widget 所实现的 “Hello world!” 应用。你可以获得比 “Hello world!” 应用更多的功能。

The following images show “Hello world!” built from Material Design widgets. You get more functionality for free than in the basic “Hello world!” app.

当编写应用代码的时候，你将用到下述两种 widget ：

When writing an app, you’ll use two types of widgets:

无状态 widget (StatelessWidget) 就像它的名字一样，是一个没有状态的 widget。无状态 widget 一旦创建，就不会改变。而有状态 widget (StatefulWidget) 会基于接收到的数据或者用户输入的数据动态改变状态。

StatelessWidget or StatefulWidget. A StatelessWidget is just what it sounds like—a widget with no state. A StatelessWidget is created once, and never changes its appearance. A StatefulWidget dynamically changes state based on data received, or user input.

无状态 widget 和有状态 widget 之间的主要区别是有状态 widget 包含一个 State 对象，会缓存状态数据，并且 widget 树的重构也会携带该数据，因此状态不会丢失。

The important difference between stateless and stateful widgets is that StatefulWidgets have a State object that stores state data and carries it over across tree rebuilds, so it’s not lost.

在简单的或者基本的应用程序中，封装 widget 非常简单，但是随着代码量的增加并且应用程序的功能变得更加复杂，你应该将层级复杂的 widget 封装到函数中或者稍小一些的类。创建独立的函数和 widget 可以让你更好地复用应用中组件。

In simple or basic apps it’s easy to nest widgets, but as the code base gets larger and the app becomes complex, you should break deeply nested widgets into functions that return the widget or smaller classes. Creating separate functions and widgets allows you to reuse the components within the app.

如何创建可复用的组件？

How do I create reusable components?

在 React Native 中，你可以定义一个类来创建一个可复用的组件然后使用 props 方法来设置或者返回属性或者所选元素的值。在下面的示例中，CustomCard 类在父类中被定义和调用。

In React Native, you would define a class to create a reusable component and then use props methods to set or return properties and values of the selected elements. In the example below, the CustomCard class is defined and then used inside a parent class.

// React Native
class CustomCard extends React.Component {
  render() {
    return (
      <View>
        <Text> Card {this.props.index} </Text>
        <Button
          title="Press"
          onPress={() => this.props.onPress(this.props.index)}
        />
      </View>
    );
  }
}

// Usage
<CustomCard onPress={this.onPress} index={item.key} />

在 Flutter 中，定义一个类来创建一个自定义 widget 然后复用这个 widget。你可以定义并且调用函数来返回一个可复用的 widget，正如下面示例中 build 函数所示的那样。

In Flutter, define a class to create a custom widget and then reuse the widget. You can also define and call a function that returns a reusable widget as shown in the build function in the following example.

// Flutter
class CustomCard extends StatelessWidget {
  CustomCard({@required this.index, @required 
     this.onPress});

  final index;
  final Function onPress;

  @override
 widget build(BuildContext context) {
    return Card(
      child: Column(
        children: <Widget>[
          Text('Card $index'),
          FlatButton(
            child: const Text('Press'),
            onPressed: this.onPress,
          ),
        ],
      )
    );
  }
}
    ...
// Usage
CustomCard(
  index: index,
  onPress: () { 
    print('Card $index');
  },
)
    ...

在之前的示例，CustomCard 类的构造函数使用 Dart 的曲括号 { } 来表示可选参数 optional parameters。

In the previous example, the constructor for the CustomCard class uses Dart’s curly brace syntax { } to indicate named optional parameters.

如果将这些参数设定为必填参数，要么从构造函数中删掉曲括号，或者在构造函数中加上 @required。

To require these fields, either remove the curly braces from the constructor, or add @required to the constructor.

下面的截图展示了可复用的 CustomCard 类的示例。

The following screenshots show an example of the reusable CustomCard class.

项目结构和资源

Project structure and resources

该从哪开始写代码呢？

Where do I start writing the code?

从 main.dart 文件开始。当你创建 Flutter 应用的时候会自动生成这个文件。

Start with the main.dart file. It’s autogenerated when you create a Flutter app.

// Dart
void main(){
 print('Hello, this is the main function.');
}

在 Flutter 中，入口文件是 ’projectname’/lib/main.dart 而程序执行是从 main 函数开始的。

In Flutter, the entry point file is ’projectname’/lib/main.dart and execution starts from the main function.

Flutter 应用程序中的文件是如何组织的？

How are files structured in a Flutter app?

当你创建一个新的 Flutter 工程的时候，它会创建如下所示的文件夹结构。你可以自定义这个结构，不过这是整个开发的起点。

When you create a new Flutter project, it builds the following directory structure. You can customize it later, but this is where you start.

┬
└ projectname
  ┬
  ├ android      - Contains Android-specific files.
  ├ build        - Stores iOS and Android build files.
  ├ ios          - Contains iOS-specific files.
  ├ lib          - Contains externally accessible Dart source files.
    ┬
    └ src        - Contains additional source files.
    └ main.dart  - The Flutter entry point and the start of a new app.
                   This is generated automatically when you create a Flutter
                    project.
                   It's where you start writing your Dart code.
  ├ test         - Contains automated test files.
  └ pubspec.yaml - Contains the metadata for the Flutter app.
                   This is equivalent to the package.json file in React Native.

┬
└ projectname
  ┬
  ├ android      - 包含 Android 相关文件。
  ├ build        - 存储 iOS 和 Android 构建文件。
  ├ ios          - 包含 iOS 相关文件。
  ├ lib          - 包含外部可访问 Dart 源文件。
    ┬
    └ src        - 包含附加源文件。
    └ main.dart  - Flutter 程序入口和新应用程序的起点。当你创建 Flutter 工程的时候会自动生成这些文件。你从这里开始写 Dart 代码
  ├ test         - 包含自动测试文件。
  └ pubspec.yaml - 包含 Flutter 应用程序的元数据。这个文件相当于 React Native 里的 package.json 文件。

我该把资源文件放到哪并且如何调用呢？

Where do I put my resources and assets and how do I use them?

一个 Flutter 资源就是打包到你应用程序里的一个文件并且在程序运行的时候可以访问。 Flutter 应用程序可以包含下述几种资源类型：

A Flutter resource or asset is a file that is bundled and deployed with your app and is accessible at runtime. Flutter apps can include the following asset types:

• 静态数据比如 JSON 文件

  Static data such as JSON files

• 配置文件

  Configuration files

• 图标和图片 (JPEG, PNG, GIF, Animated GIF, WebP, Animated WebP, BMP, and WBMP)

  Icons and images (JPEG, PNG, GIF, Animated GIF, WebP, Animated WebP, BMP, and WBMP)

Flutter 使用 pubspec.yaml 文件来确定应用程序中的资源。该文件在工程的根目录。

Flutter uses the pubspec.yaml file, located at the root of your project, to identify assets required by an app.

flutter:
  assets:
    - assets/my_icon.png
    - assets/background.png

assets 确定了需要包含在应用程序中的文件。每个资源都会在 pubspec.yaml 中定义所存储的相对路径。资源定义的顺序没有特殊要求。实际的文件夹（在这里指 assets ）也没影响。但是，由于资源可以放置于程序的任何目录，所以放在 assets 文件夹是比较好的。

The assets subsection specifies files that should be included with the app. Each asset is identified by an explicit path relative to the pubspec.yaml file, where the asset file is located. The order in which the assets are declared does not matter. The actual directory used (assets in this case) does not matter. However, while assets can be placed in any app directory, it’s a best practice to place them in the assets directory.

在构建期间，Flutter 会将资源放到一个称为 asset bundle 的归档文件中，应用程序可以在运行时访问该文件。当一个资源在 pubspec.yaml 中被声明时，构建进程会查询和这个文件相关的子文件夹路径。这些文件也会被包含在 asset bundle 中。当你为应用程序选择和屏幕显示分辨率相关的图片时，Flutter 会使用 asset variants。

During a build, Flutter places assets into a special archive called the asset bundle, which apps read from at runtime. When an asset’s path is specified in the assets section of pubspec.yaml, the build process looks for any files with the same name in adjacent subdirectories. These files are also included in the asset bundle along with the specified asset. Flutter uses asset variants when choosing resolution-appropriate images for your app.

在 React Native，你可以在源码文件夹中通过添加文件来增加一个静态图片并且在代码中引用它。

In React Native, you would add a static image by placing the image file in a source code directory and referencing it.

<Image source={require('./my-icon.png')} />

在 Flutter 中，如果要增加静态图片的话就在 widget 的 build 方法中使用 AssetImage 类。

In Flutter, add a static image to your app using the AssetImage class in a widget’s build method.

image: AssetImage('assets/background.png'),

如果想了解更多相关信息，请参考文档 在 Flutter 中添加资源和图片。

For more information, see Adding Assets and Images in Flutter.

如何在网络中加载图片？

How do I load images over a network?

在 React Native，你可以在 Image 的 source 属性中设置 uri 和所需的尺寸。

In React Native, you would specify the uri in the source prop of the Image component and also provide the size if needed.

在 Flutter 中，使用 Image.network 构造函数来实现通过地址加载图片的操作。

In Flutter, use the Image.network constructor to include an image from a URL.

// Flutter
body: Image.network(
          'https://flutter.io/images/owl.jpg',

我如何安装依赖包和包插件？

How do I install packages and package plugins?

Flutter 支持使用开发者向 Flutter 和 Dart 生态系统贡献的代码包。这样可以使大量开发者快速构建应用程序而无需重复造车轮。而平台相关的代码包就被称为包插件。

Flutter supports using shared packages contributed by other developers to the Flutter and Dart ecosystems. This allows you to quickly build your app without having to develop everything from scratch. Packages that contain platform-specific code are known as package plugins.

在 React Native 中，你可以在命令行中运行 yarn add {package-name} 或者 npm install --save {package-name} 来安装代码包。

In React Native, you would use yarn add {package-name} or npm install --save {package-name} to install packages from the command line.

在 Flutter 中，安装代码包需要按照如下的步骤：

In Flutter, install a package using the following instructions:

1. 在 pubspec.yaml 的 dependencies 区域添加包名和版本。下面的例子向大家展示了如何将 google_sign_in 的 Dart package 添加到 pubspec.yaml 中。一定要检查一下 YAML 文件中的空格。因为 空格很重要!

   Add the package name and version to the pubspec.yaml dependencies section. The example below shows how to add the google_sign_in Dart package to the pubspec.yaml file. Check your spaces when working in the YAML file because white space matters!

dependencies:
  flutter:
    sdk: flutter
  google_sign_in: ^3.0.3

1. 在命令行中输入 flutter packages get 来安装代码包。如果使用 IDE，它自己会运行 flutter packages get，或者它会提示你是不是要运行该命令。

   Install the package from the command line by using flutter pub get. If using an IDE, it often runs flutter pub get for you, or it might prompt you to do so.

2. 向下面代码一样在程序中引用代码包：

   Import the package into your app code as shown below:

import 'package:flutter/cupertino.dart';

如果想了解更多相关信息，请参考 在 Flutter 里使用 Packages 和 Flutter Packages 的开发和提交。

For more information, see Using Packages and Developing Packages & Plugins.

你可以找到很多 Flutter 开发者分享的代码包，就在 [Flutter packages 的 pub.dev.

You can find many packages shared by Flutter developers in the Flutter packages section of the pub.dev.

Flutter widgets

在 Flutter 中，你可以基于 widget 打造你自己的 UI，通过 widget 当前的设置和状态会呈现相应的页面效果。

In Flutter, you build your UI out of widgets that describe what their view should look like given their current configuration and state.

widget 常常通过很多小的，单一功能的 widget 组成，通过这样的封装往往能够实现很棒的效果。比如， Container widget 包含多种 widget，分别负责布局、绘图、位置变化和尺寸变化。准确的说，Container widget 包括 LimitedBox, ConstrainedBox, Align, Padding, DecoratedBox，和 Transform widget。与其继承 Container 来实现自定义效果，不如直接修改这些 widget 来实现效果。

Widgets are often composed of many small, single-purpose widgets that are nested to produce powerful effects. For example, the Container widget consists of several widgets responsible for layout, painting, positioning, and sizing. Specifically, the Container widget includes the LimitedBox, ConstrainedBox, Align, Padding, DecoratedBox, and Transform widgets. Rather than subclassing Container to produce a customized effect, you can compose these and other simple widgets in new and unique ways.

Center widget 是另一个用于控制布局的示例。如果要居中一个 widget，就把它封装到 Center widget 中，然后使用布局 widget 来进行对齐行、列和网格。这些布局 widget 并不可见。而他们的作用就是控制其它 widget 的布局。如果想搞清楚为什么一个 widget 会有这样的效果，有效的方法是研究它临近的 widget。

The Center widget is another example of how you can control the layout. To center a widget, wrap it in a Center widget and then use layout widgets for alignment, row, columns, and grids. These layout widgets do not have a visual representation of their own. Instead, their sole purpose is to control some aspect of another widget’s layout. To understand why a widget renders in a certain way, it’s often helpful to inspect the neighboring widgets.

如果想了解更多相关信息，请参考 技术概览。

For more information, see the Flutter Technical Overview.

如果想了解更多关于 Widgets 包中的核心 widget，请参考 Flutter Basic Widgets、 核心 Widget 目录 或是 Flutter Widget 目录。

For more information about the core widgets from the Widgets package, see Flutter Basic Widgets, the Flutter Widget Catalog, or the Flutter Widget Index.

视图

Views

与 View 等价容器的是什么？

What is the equivalent of the View container?

在 React Native 中， View 是支持 Flexbox 布局、风格化、触摸事件处理和访问性控制的容器。

In React Native, View is a container that supports layout with Flexbox, style, touch handling, and accessibility controls.

在 Flutter 中，你可以使用 Widgets 库中的核心布局 widget，比如 Container、 Column、 Row 和 Center。

In Flutter, you can use the core layout widgets in the Widgets library, such as Container, Column, Row, and Center.

如果想了解更多相关信息，请参考 Layout Widgets 目录。

For more information, see the Layout Widgets catalog.

和 FlatList 或者 SectionList 相对应的是什么？

What is the equivalent of FlatList or SectionList?

List 是一个可以滚动的纵向排列的组件列表。

A List is a scrollable list of components arranged vertically.

在 React Native 中，FlatList 或者 SectionList 用于渲染简单的或者分组的列表。

In React Native, FlatList or SectionList are used to render simple or sectioned lists.

// React Native
<FlatList
  data={[ ... ]}
  renderItem={({ item }) => <Text>{item.key}</Text>}
/>

ListView 是 Flutter 最常用的滑动 widget。默认构造函数需要一个数据列表的参数。 ListView 非常适合用于少量子 widget 的列表。如果列表的元素比较多，可以使用 ListView.builder，它会按需构建子项并且只创建可见的子项。

ListView is Flutter’s most commonly used scrolling widget. The default constructor takes an explicit list of children. ListView is most appropriate for a small number of widgets. For a large or infinite list, use ListView.builder, which builds its children on demand and only builds those children that are visible.

// Flutter
var data = [ ... ];
ListView.builder(
  itemCount: data.length,
  itemBuilder: (context, int index) {
    return Text(
      data[index],
    );
  },
)

如果要了解如何实现无限滑动列表，请参考 Write Your First Flutter App, Part 1 codelab。

To learn how to implement an infinite scrolling list, see the Write Your First Flutter App, Part 1 codelab.

如何使用 Canvas 绘图？

How do I use a Canvas to draw or paint?

在 React Native 中，canvas 组件是不可见的，所以需要使用类似 react-native-canvas 这样的组件。

In React Native, canvas components aren’t present so third party libraries like react-native-canvas are used.

// React Native
handleCanvas = canvas => {
  const ctx = canvas.getContext('2d');
  ctx.fillStyle = 'skyblue';
  ctx.beginPath();
  ctx.arc(75, 75, 50, 0, 2 * Math.PI);
  ctx.fillRect(150, 100, 300, 300);
  ctx.stroke();
};

render() {
  return (
    <View>
      <Canvas ref={this.handleCanvas} />
    </View>
  );
}

在 Flutter 中，你可以使用 CustomPaint 和 CustomPainter 进行绘图。

In Flutter, you can use the CustomPaint and CustomPainter classes to draw to the canvas.

下面的示例代码展示了如何使用 CustomPaint 进行绘图。它实现了抽象类 CustomPainter，然后将它赋值给 CustomPainter 的 painter 属性。CustomPainter 子类必须实现 paint 和 shouldRepaint 方法。

The following example shows how to draw during the paint phase using the CustomPaint widget. It implements the abstract class, CustomPainter, and passes it to CustomPaint’s painter property. CustomPaint subclasses must implement the paint and shouldRepaint methods.

// Flutter
class MyCanvasPainter extends CustomPainter {

  @override
  void paint(Canvas canvas, Size size) {
    Paint paint = Paint();
    paint.color = Colors.amber;
    canvas.drawCircle(Offset(100.0, 200.0), 40.0, paint);
    Paint paintRect = Paint();
    paintRect.color = Colors.lightBlue;
    Rect rect = Rect.fromPoints(Offset(150.0, 300.0), Offset(300.0, 400.0));
    canvas.drawRect(rect, paintRect);
  }

  bool shouldRepaint(MyCanvasPainter oldDelegate) => false;
  bool shouldRebuildSemantics(MyCanvasPainter oldDelegate) => false;
}
class _MyCanvasState extends State<MyCanvas> {

  @override
 widget build(BuildContext context) {
    return Scaffold(
      body: CustomPaint(
        painter: MyCanvasPainter(),
      ),
    );
  }
}

布局

Layouts

如何使用 widget 来定义布局属性？

How do I use widgets to define layout properties?

在 React Native 中，大多数布局需要通过向指定的组件传递属性参数进行设置。比如，你可以使用 View 的 style 来设置 flexbox 属性。如果要整理一列的组件，你可以使用如下的属性设置：flexDirection: “column”。

In React Native, most of the layout can be done with the props that are passed to a specific component. For example, you could use the style prop on the View component in order to specify the flexbox properties. To arrange your components in a column, you would specify a prop such as: flexDirection: “column”.

// React Native
<View
  style={{
    flex: 1,
    flexDirection: 'column',
    justifyContent: 'space-between',
    alignItems: 'center'
  }}
>

在 Flutter 中，布局主要是由专门的 widget 定义的，它们同控制类 widget 和样式属性一起发挥功能。

In Flutter, the layout is primarily defined by widgets specifically designed to provide layout, combined with control widgets and their style properties.

比如，Column 和 Row widget 接受一个数组的子元素并且分别按照纵向和横向进行排列。

For example, the Column and Row widgets take an array of children and align them vertically and horizontally respectively. A Container widget takes a combination of layout and styling properties, and a Center widget centers its child widgets.

// Flutter
Center(
  child: Column(
    children: <Widget>[
      Container(
        color: Colors.red,
        width: 100.0,
        height: 100.0,
      ),
      Container(
        color: Colors.blue,
        width: 100.0,
        height: 100.0,
      ),
      Container(
        color: Colors.green,
        width: 100.0,
        height: 100.0,
      ),
    ],
  ),
)

Flutter 在核心 widget 库中提供多种不同的布局 widget。比如 Padding、 Align 和 Stack。

Flutter provides a variety of layout widgets in its core widget library. For example, Padding, Align, and Stack.

要得到完整的 widget 列表，请参考 Layout Widgets。

For a complete list, see Layout Widgets.

如何为 widget 分层？

How do I layer widgets?

在 React Native 中，组件可以通过 absolute 划分层次。

In React Native, components can be layered using absolute positioning.

在 Flutter 中使用 Stack widget 将子 widget 进行分层。该 widget 可以将整体或者部分的子 widget 进行分层。

Flutter uses the Stack widget to arrange children widgets in layers. The widgets can entirely or partially overlap the base widget.

Stack widget 将子 widget 根据容器的边界进行布局。如果你仅仅想把子 widget 重叠摆放的话，这个 widget 非常合适。

The Stack widget positions its children relative to the edges of its box. This class is useful if you simply want to overlap several children widgets.

// Flutter
Stack(
  alignment: const Alignment(0.6, 0.6),
  children: <Widget>[
    CircleAvatar(
      backgroundImage: NetworkImage(
        'https://avatars3.githubusercontent.com/u/14101776?v=4'),
    ),
    Container(
      decoration: BoxDecoration(
          color: Colors.black45,
      ),
      child: Text('Flutter'),
    ),
  ],
)

上面的示例代码使用 Stack 将一个 Container （将 Text 显示在一个半透明的黑色背景上）覆盖在一个 CircleAvatar 上。Stack 使用对齐属性和 Alignment 坐标微调文本。

The previous example uses Stack to overlay a Container (that displays its Text on a translucent black background) on top of a CircleAvatar. The Stack offsets the text using the alignment property and Alignment coordinates.

如果想了解更多相关信息，请参考 Stack 类文档。

For more information, see the Stack class documentation.

风格化

Styling

如何设置组件的风格？

How do I style my components?

在 React Native 中，内联风格化和 stylesheets.create 可以用于设置组件的风格。

In React Native, inline styling and stylesheets.create are used to style components.

// React Native
<View style={styles.container}>
  <Text style={{ fontSize: 32, color: 'cyan', fontWeight: '600' }}>
    This is a sample text
  </Text>
</View>

const styles = StyleSheet.create({
  container: {
    flex: 1,
    backgroundColor: '#fff',
    alignItems: 'center',
    justifyContent: 'center'
  }
});

在 Flutter 中， Text widget 可以接受 TextStyle 作为它的风格化属性。如果你想在不同的场合使用相同的文本风格，你可以创建一个 TextStyle 类，并且在多个 Text widget 中使用它。

In Flutter, a Text widget can take a TextStyle class for its style property. If you want to use the same text style in multiple places, you can create a TextStyle class and use it for multiple Text widgets.

// Flutter
var textStyle = TextStyle(fontSize: 32.0, color: Colors.cyan, fontWeight:
   FontWeight.w600);
	...
Center(
  child: Column(
    children: <Widget>[
      Text(
        'Sample text',
        style: textStyle,
      ),
      Padding(
        padding: EdgeInsets.all(20.0),
        child: Icon(Icons.lightbulb_outline,
          size: 48.0, color: Colors.redAccent)
      ),
    ],
  ),
)

我如何使用 Icons 和 Colors 呢？

How do I use Icons and Colors?

React Native 并不包含默认图标，所以需要使用第三方库。

React Native doesn’t include support for icons so third party libraries are used.

在 Flutter 中，引用 Material 库的时候就同时引入了 Material icons 和 colors。

In Flutter, importing the Material library also pulls in the rich set of Material icons and colors.

Icon(Icons.lightbulb_outline, color: Colors.redAccent)

When using the Icons class, make sure to set uses-material-design: true in the project’s pubspec.yaml file. This ensures that the MaterialIcons font, which displays the icons, is included in your app.

name: my_awesome_application
flutter: uses-material-design: true

当使用 Icons 类时，确保在项目的 pubspec.yaml 文件中设置 uses-material-design: true。这样保证 MaterialIcons 相关字体被包含在你的应用中。

name: my_awesome_application
flutter: uses-material-design: true

Flutter 的 Cupertino (iOS-style) 包为 iOS 设计语言提供高分辨率的 widget。要使用 CupertinoIcons 字体，在项目的 pubspec.yaml 文件中添加 cupertino_icons 的依赖即可。

Flutter’s Cupertino (iOS-style) package provides high fidelity widgets for the current iOS design language. To use the CupertinoIcons font, add a dependency for cupertino_icons in your project’s pubspec.yaml file.

name: my_awesome_application
dependencies:
  cupertino_icons: ^0.1.0

要在全局范围内自定义组件的颜色和风格，使用 ThemeData 为不同的主题指定默认颜色。在 MaterialApp 的主题属性中设置 ThemeData 对象。Colors 类提供 Material Design color palette 中所提供的颜色配置。

To globally customize the colors and styles of components, use ThemeData to specify default colors for various aspects of the theme. Set the theme property in MaterialApp to the ThemeData object. The Colors class provides colors from the Material Design color palette.

下面的示例代码将主色调设置为 blue 然后文本颜色设置为 red。

The following example sets the primary swatch to blue and the text selection to red.

class SampleApp extends StatelessWidget {
  @override
 widget build(BuildContext context) {
    return MaterialApp(
      title: 'Sample App',
      theme: ThemeData(
        primarySwatch: Colors.blue,
        textSelectionColor: Colors.red
      ),
      home: SampleAppPage(),
    );
  }
}

如何增加风格化主题？

How do I add style themes?

在 React Native，常用主题都定义在 stylesheets 中。

In React Native, common themes are defined for components in stylesheets and then used in components.

在 Flutter 中，为所有组件创建统一风格可以在 ThemeData 类中定义，并将它赋值给 MaterialApp 的主题属性。

In Flutter, create uniform styling for almost everything by defining the styling in the ThemeData class and passing it to the theme property in the MaterialApp widget.

  @override
 widget build(BuildContext context) {
    return MaterialApp(
      theme: ThemeData(
        primaryColor: Colors.cyan,
        brightness: Brightness.dark,
      ),
      home: StylingPage(),
    );
  }

Theme 可以在不使用 MaterialApp widget 的情况下使用。Theme 接受一个 ThemeData 参数，并且将 ThemeData 应用于它的全部子 widget。

A Theme can be applied even without using the MaterialApp widget. The Theme widget takes a ThemeData in its data parameter and applies the ThemeData to all of its children widgets.

 @override
 widget build(BuildContext context) {
    return Theme(
      data: ThemeData(
        primaryColor: Colors.cyan,
        brightness: brightness,
      ),
      child: Scaffold(
         backgroundColor: Theme.of(context).primaryColor,
              ...
              ...
      ),
    );
  }

状态管理

State management

当 widget 被创建或者在 widget 的生命周期中有信息发生改变时所产生的信息叫做状态。要在 Flutter 中管理应用程序的状态，使用 StatefulWidget 和 State 对象。

State is information that can be read synchronously when a widget is built or information that might change during the lifetime of a widget. To manage app state in Flutter, use a StatefulWidget paired with a State object.

欲知更多关于 Flutter 的状态管理相关的内容，请参访 状态管理文档 页面。

For more information on ways to approach managing state in Flutter, see State management.

The StatelessWidget

StatelessWidget widget

StatelessWidget 在 Flutter 中是一个不需要状态改变的 widget，它没有内部的状态。

A StatelessWidget in Flutter is a widget that doesn’t require a state change—it has no internal state to manage.

当你展现给用户的界面并不依赖其它任何配置信息并且使用 BuildContext 来解析 widget，则需要使用无状态 widget。

Stateless widgets are useful when the part of the user interface you are describing does not depend on anything other than the configuration information in the object itself and the BuildContext in which the widget is inflated.

AboutDialog、 CircleAvatar 和 Text 是 StatelessWidget 的子类，并且是很典型的无状态 widget。

AboutDialog, CircleAvatar, and Text are examples of stateless widgets which subclass StatelessWidget.

// Flutter
import 'package:flutter/material.dart';

void main() => runApp(MyStatelessWidget(text: 'StatelessWidget Example to show immutable data'));

class MyStatelessWidget extends StatelessWidget {
  final String text;
  MyStatelessWidget({Key key, this.text}) : super(key: key);

  @override
 widget build(BuildContext context) {
    return Center(
      child: Text(
        text,
        textDirection: TextDirection.ltr,
      ),
    );
  }
}

在上面的例子中，你用到了 MyStatelessWidget 类的构造函数来传递 text。并且它被标记为 final。该类继承了 StatelessWidget，它包含不可变的数据。

In the previous example, you used the constructor of the MyStatelessWidget class to pass the text, which is marked as final. This class extends StatelessWidget—it contains immutable data.

无状态 widget 的 build 方法通常只有在三种情况下会被调用：

The build method of a stateless widget is typically called in only three situations:

• 当 widget 被插入到 widget 树中

  When the widget is inserted into a tree

• 当 widget 的父 widget 改变了配置

  When the widget’s parent changes its configuration

• 当所依赖的 InheritedWidget 发生了改变

  When an InheritedWidget it depends on, changes

The StatefulWidget

StatefulWidget widget

StatefulWidget 是携带状态变化的 widget。通过调用 setState 方法可以管理 StatefulWidget 的状态。当调用 setState 的时候，程序会通知 Flutter 框架有状态发生了改变，然后会重新运行 build 方法来更新应用的状态。

A StatefulWidget is a widget that changes state. Use the setState method to manage the state changes for a StatefulWidget. A call to setState tells the Flutter framework that something has changed in a state, which causes an app to rerun the build method so that the app can reflect the change.

状态是在 widget 被创建期间可以被同步读取的信息，并且在 widget 的生命周期中会发生改变。实现该 widget 的时候要注意保证党状态发生改变的时候程序能够获得相应的提醒。当 widget 能够动态改变的时候，请使用 StatefulWidget。比如，某个 widget 会随着用户填写表单或者移动滑块的时候发生改变。亦或者随着数据源更新的时候发生改变。

State is information that can be read synchronously when a widget is built and might change during the lifetime of the widget. It’s the responsibility of the widget implementer to ensure that the state is promptly notified when the state changes. Use StatefulWidget when a widget can change dynamically. For example, the state of the widget changes by typing into a form, or moving a slider. Or, it can change over time—perhaps a data feed updates the UI.

Checkbox、 Radio、 Slider、 InkWell、 Form、和 TextField 都是有状态的 widget，是 StatefulWidget 的子类。

Checkbox, Radio, Slider, InkWell, Form, and TextField are examples of stateful widgets, that subclass StatefulWidget.

下面的示例代码声明了一个 StatefulWidget，需要实现 createState() 方法。该方法创建一个对象来管理 widget 的状态，也就是 _MyStatefulWidgetState。

The following example declares a StatefulWidget which requires a createState() method. This method creates the state object that manages the widget’s state, _MyStatefulWidgetState.

class MyStatefulWidget extends StatefulWidget {
  MyStatefulWidget({Key key, this.title}) : super(key: key);
  final String title;

  @override
  _MyStatefulWidgetState createState() => _MyStatefulWidgetState();
}

下面的状态类，_MyStatefulWidgetState，实现了 build() 方法。当状态发生改变的时候，比如说用户点击了开关按钮，这时 setState 就会被调用，并且将新的开关状态传进来。这就会使整体框架重构这个 widget。

The following state class, _MyStatefulWidgetState, implements the build() method for the widget. When the state changes, for example, when the user toggles the button, setState is called with the new toggle value. This causes the framework to rebuild this widget in the UI.

class _MyStatefulWidgetState extends State<MyStatefulWidget> {
  bool showtext=true;
  bool toggleState=true;
  Timer t2;

  void toggleBlinkState(){
    setState((){
      toggleState=!toggleState;
    });
    var twenty = const Duration(milliseconds: 1000);
    if(toggleState==false) {
      t2 = Timer.periodic(twenty, (Timer t) {
        toggleShowText();
      });
    } else {
      t2.cancel();
    }
  }

  void toggleShowText(){
    setState((){
      showtext=!showtext;
    });
  }

  @override
 widget build(BuildContext context) {
    return Scaffold(
      body: Center(
        child: Column(
          children: <Widget>[
            (showtext
              ?(Text('This execution will be done before you can blink.'))
              :(Container())
            ),
            Padding(
              padding: EdgeInsets.only(top: 70.0),
              child: RaisedButton(
                onPressed: toggleBlinkState,
                child: (toggleState
                  ?( Text('Blink'))
                  :(Text('Stop Blinking'))
                )
              )
            )
          ],
        ),
      ),
    );
  }
}

StatefulWidget 和 StatelessWidget 的最佳实践是什么？

What are the StatefulWidget and StatelessWidget best practices?

下面有一些设计原则供大家参考。

Here are a few things to consider when designing your widget.

1. Determine whether a widget should be a StatefulWidget or a StatelessWidget

1. 确定一个 widget 应该是 StatefulWidget 还是 StatelessWidget

在 Flutter 中， widget 要么是有状态的，要么是无状态的。这取决于 widget 是否依赖状态的改变。

In Flutter, widgets are either Stateful or Stateless—depending on whether they depend on a state change.

• 如果一个 widget 发生了改变，而它所处的用户界面或者数据中断了 UI，那么该 widget 就是有状态的。

  If a widget changes—the user interacts with it or a data feed interrupts the UI, then it’s Stateful.

• 如果一个 widget 是 final 类型或者 immutable 类型的，那么该 widget 是无状态的。

  If a widget is final or immutable, then it’s Stateless.

2. 确定哪个对象来控制 widget 的状态（针对 StatefulWidget ）。

2. Determine which object manages the widget’s state (for a StatefulWidget)

在 Flutter 中，有三种途径来管理状态：

In Flutter, there are three primary ways to manage state:

• widget 管理它的自身状态

  The widget manages its own state

• 由其父 widget 管理 widget 状态

  The parent widget manages the widget’s state

• 通过混搭的方式

  A mix-and-match approach

当决定了使用哪个途径后，要考虑下述的几个原则：

When deciding which approach to use, consider the following principles:

• 如果状态信息是用户数据，比如 checkbox 是被勾选还是未被勾选，或者滑块的位置，那么父 widget 会很好的处理当前 widget 的状态。

  If the state in question is user data, for example the checked or unchecked mode of a checkbox, or the position of a slider, then the state is best managed by the parent widget.

• 如果状态是和外观效果相关的，比如动画，那么 widget 自己会处理状态的变化。

  If the state in question is aesthetic, for example an animation, then the widget itself best manages the state.

• 如果无法确定，那么父 widget 会处理子 widget 的状态。

  When in doubt, let the parent widget manage the child widget’s state.‘

3. 继承 StatefulWidget 和状态

3. Subclass StatefulWidget and State

MyStatefulWidget 类管理它自身的状态 - 它继承自 StatefulWidget，重写了 createState() 方法。该方法创建了 State 对象，同时框架会调用 createState() 方法来构建 widget。在这个例子中，createState() 方法创建了一个 _MyStatefulWidgetState 实例。下面的最佳实践中也实现了类似的方法。

The MyStatefulWidget class manages its own state—it extends StatefulWidget, it overrides the createState() method to create the State object, and the framework calls createState() to build the widget. In this example, createState() creates an instance of _MyStatefulWidgetState, which is implemented in the next best practice.

class MyStatefulWidget extends StatefulWidget {
  MyStatefulWidget({Key key, this.title}) : super(key: key);
  final String title;

  @override
  _MyStatefulWidgetState createState() => _MyStatefulWidgetState();
}

class _MyStatefulWidgetState extends State<MyStatefulWidget> {

  @override
 widget build(BuildContext context) {
    ...
  }
}

4. 将 StatefulWidget 添加到 widget 树中

4. Add the StatefulWidget into the widget tree

将你自定义的 StatefulWidget 通过应用程序的 build 方法添加到 widget 树中。

Add your custom StatefulWidget to the widget tree in the app’s build method.

class MyStatelessWidget extends StatelessWidget {
  // This widget is the root of your application.

  @override
 widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: MyStatefulWidget(title: 'State Change Demo'),
    );
  }
}

Props

Props

在 React Native 中，大多数组件都可以在创建的时候通过不同的参数或者属性来自定义，叫做 props。这些参数可以在子组件中通过 this.props 进行调用。

In React Native, most components can be customized when they are created with different parameters or properties, called props. These parameters can be used in a child component using this.props.

// React Native
class CustomCard extends React.Component {
  render() {
    return (
      <View>
        <Text> Card {this.props.index} </Text>
        <Button
          title='Press'
          onPress={() => this.props.onPress(this.props.index)}
        />
      </View>
    );
  }
}
class App extends React.Component {

  onPress = index => {
    console.log('Card ', index);
  };

  render() {
    return (
      <View>
        <FlatList
          data={[ ... ]}
          renderItem={({ item }) => (
            <CustomCard onPress={this.onPress} index={item.key} />
          )}
        />
      </View>
    );
  }
}

在 Flutter 中，你可以将构造函数中的参数值赋值给标记为 final 的本地变量或者函数。

In Flutter, you assign a local variable or function marked final with the property received in the parameterized constructor.

// Flutter
class CustomCard extends StatelessWidget {

  CustomCard({@required this.index, @required this.onPress});
  final index;
  final Function onPress;

  @override
 widget build(BuildContext context) {
  return Card(
    child: Column(
      children: <Widget>[
        Text('Card $index'),
        FlatButton(
          child: const Text('Press'),
          onPressed: this.onPress,
        ),
      ],
    ));
  }
}
    ...
//Usage
CustomCard(
  index: index,
  onPress: () {
    print('Card $index');
  },
)

本地存储

Local storage

如果你不需要在本地存储太多数据同时也不需要存储结构化数据，那么你可以使用 shared_preferences，通过它来读写一些原始数据类型键值对，数据类型包括 boolean, float, ints, longs 和 string。

If you don’t need to store a lot of data and it doesn’t require structure, you can use shared_preferences which allows you to read and write persistent key-value pairs of primitive data types: booleans, floats, ints, longs, and strings.

如何存储在应用程序中全局有效的键值对？

How do I store persistent key-value pairs that are global to the app?

在 React Native，可以使用 AsyncStorage 中的 setItem 和 getItem 函数来存储和读取应用程序中的全局数据。

In React Native, you use the setItem and getItem functions of the AsyncStorage component to store and retrieve data that is persistent and global to the app.

// React Native
await AsyncStorage.setItem( 'counterkey', json.stringify(++this.state.counter));
AsyncStorage.getItem('counterkey').then(value => {
  if (value != null) {
    this.setState({ counter: value });
  }
});

在 Flutter 中，使用 shared_preferences 插件来存储和访问应用程序内全局有效的键值对数据。 shared_preferences 插件封装了 iOS 中的 NSUserDefaults 和 Android 中的 SharedPreferences 来实现简单数据的持续存储。如果要使用该插件，可以在 pubspec.yaml 中添加依赖 shared_preferences，然后在 Dart 文件中引用包即可。

In Flutter, use the shared_preferences plugin to store and retrieve key-value data that is persistent and global to the app. The shared_preferences plugin wraps NSUserDefaults on iOS and SharedPreferences on Android, providing a persistent store for simple data. To use the plugin, add shared_preferences as a dependency in the pubspec.yaml file then import the package in your Dart file.

dependencies:
  flutter:
    sdk: flutter
  shared_preferences: ^0.4.3

// Dart
import 'package:shared_preferences/shared_preferences.dart';

要实现持久数据存储，使用 SharedPreferences 类提供的 setter 方法即可。Setter 方法适用于多种原始类型数据，比如 setInt, setBool, 和 setString。要读取数据，使用 SharedPreferences 类中相应的 getter 方法。对于每一个 setter 方法都有对应的 getter 方法。比如，getInt, getBool, 和 getString。

To implement persistent data, use the setter methods provided by the SharedPreferences class. Setter methods are available for various primitive types, such as setInt, setBool, and setString. To read data, use the appropriate getter method provided by the SharedPreferences class. For each setter there is a corresponding getter method, for example, getInt, getBool, and getString.

SharedPreferences prefs = await SharedPreferences.getInstance();
_counter = prefs.getInt('counter');
prefs.setInt('counter', ++_counter);
setState(() {
  _counter = _counter;
});

路径

Routing

大多数应用都会包含多个页面来显示不同类型的数据。比如，你有一个页面展示商品列表，用户可以通过点击其中的任意一个商品在另外一个页面查看该商品的详细信息。

Most apps contain several screens for displaying different types of information. For example, you might have a product screen that displays images where users could tap on a product image to get more information about the product on a new screen.

在 Android 中，新的页面是 Activity。在 iOS 中，新的页面是 ViewController。在 Flutter 中，页面就是 widget ！如果在 Flutter 中要切换页面，使用 Navigator widget 即可。

In Android, new screens are new Activities. In iOS, new screens are new ViewControllers. In Flutter, screens are just Widgets! And to navigate to new screens in Flutter, use the Navigator widget.

如何在页面之间进行切换？

How do I navigate between screens?

在 React Native，有三种主要的导航 widget ：StackNavigator, TabNavigator 和 DrawerNavigator。每个都提供了配置和定义页面的方法。

In React Native, there are three main navigators: StackNavigator, TabNavigator, and DrawerNavigator. Each provides a way to configure and define the screens.

// React Native
const MyApp = TabNavigator(
  { Home: { screen: HomeScreen }, Notifications: { screen: tabNavScreen } },
  { tabBarOptions: { activeTintColor: '#e91e63' } }
);
const SimpleApp = StackNavigator({
  Home: { screen: MyApp },
  stackScreen: { screen: StackScreen }
});
export default (MyApp1 = DrawerNavigator({
  Home: {
    screen: SimpleApp
  },
  Screen2: {
    screen: drawerScreen
  }
}));

在 Flutter 中，有两种主要的 widget 实现页面之间的切换：

In Flutter, there are two main widgets used to navigate between screens:

• Route 是应用程序页面的一个抽象类。

  A Route is an abstraction for an app screen or page.

• Navigator 是管理页面路径的 widget。

  A Navigator is a widget that manages routes.

Navigator 以堆栈的方式管理子 widget。它的堆栈里存储的是 Route 对象，并且提供方法管理整个堆栈，比如 Navigator.push 和 Navigator.pop。路径列表需要在 MaterialApp 中指定。或者在页面切换的时候进行构建，比如 hero 动画。下面的例子在 MaterialApp widget 中指定了页面切换路径。

A Navigator is defined as a widget that manages a set of child widgets with a stack discipline. The navigator manages a stack of Route objects and provides methods for managing the stack, like Navigator.push and Navigator.pop. A list of routes might be specified in the MaterialApp widget, or they might be built on the fly, for example, in hero animations. The following example specifies named routes in the MaterialApp widget.

// Flutter
class NavigationApp extends StatelessWidget {
  // This widget is the root of your application.
  @override
 widget build(BuildContext context) {
    return MaterialApp(
            ...
      routes: <String, WidgetBuilder>{
        '/a': (BuildContext context) => usualNavscreen(),
        '/b': (BuildContext context) => drawerNavscreen(),
      }
            ...
  );
  }
}

要切换到一个已命名的路径，Navigator 中的 of 方法被用于指定 BuildContext ( 该对象可以定位到 widget 树中的一个具体的 widget )。路径的名称传递到 pushNamed 函数来切换至指定的路径。

To navigate to a named route, the of method of the Navigator widget is used to specify the BuildContext (a handle to the location of a widget in the widget tree). The name of the route is passed to the pushNamed function to navigate to the specified route.

Navigator.of(context).pushNamed('/a');

你可以使用 Navigator 中的 push 方法添加 route 到 navigator 的历史队列中，其中包含 context 并且可以切换到指定页面。在下面的例子中，MaterialPageRoute 是一个模式化路径，可以将整个页面通过平台自适应切换方式进行切换。它需要一个 WidgetBuilder 参数。

You can also use the push method of Navigator which adds the given route to the history of the navigator that most tightly encloses the given context, and transitions to it. In the following example, the MaterialPageRoute widget is a modal route that replaces the entire screen with a platform-adaptive transition. It takes a WidgetBuilder as a required parameter.

Navigator.push(context, MaterialPageRoute(builder: (BuildContext context)
 => UsualNavscreen()));

如何使用 tab 导航和 drawer 导航？

How do I use tab navigation and drawer navigation?

在 Material Design 应用程序中，Flutter 的导航形式主要有两种：tab 和 drawer。如果没有足够的 widget 可以容纳 tab，drawer 就是个不错的选择。

In Material Design apps, there are two primary options for Flutter navigation: tabs and drawers. When there is insufficient space to support tabs, drawers provide a good alternative.

Tab navigation

Tab 导航

在 React Native 中，createBottomTabNavigator 和 TabNavigation 用来显示 tab 和 tab 导航。

In React Native, createBottomTabNavigator and TabNavigation are used to show tabs and for tab navigation.

// React Native
import { createBottomTabNavigator } from 'react-navigation';

const MyApp = TabNavigator(
  { Home: { screen: HomeScreen }, Notifications: { screen: tabNavScreen } },
  { tabBarOptions: { activeTintColor: '#e91e63' } }
);

Flutter 针对 drawer 和 tab 导航提供几种专用的 widget：

Flutter provides several specialized widgets for drawer and tab navigation:

• TabController—将 tab 与 TabBar 和 TabBarView 结合起来使用。

  TabController—Coordinates the tab selection between a TabBar and a TabBarView.

• TabBar—水平显示一行 tab。

  TabBar—Displays a horizontal row of tabs.

• Tab—创建一个 material design 风格的 TabBar 中的 tab。

  Tab—Creates a material design TabBar tab.

• TabBarView—显示目前所选 tab 所对应的 widget。

  TabBarView—Displays the widget that corresponds to the currently selected tab.

// Flutter
TabController controller=TabController(length: 2, vsync: this);

TabBar(
  tabs: <Tab>[
    Tab(icon: Icon(Icons.person),),
    Tab(icon: Icon(Icons.email),),
  ],
  controller: controller,
),

要将 tab 选项与 TabBar 和 TabBarView 结合起来使用就需要 TabController。 TabController 的构造函数中的 length 参数定义了 tab 的总数。当状态变化时，需要使用 TickerProvider 来触发通知。TickerProvider 是 vsync。当你需要创建新的 TabController 时，将 vsync: this 作为构造函数的参数即可。

A TabController is required to coordinate the tab selection between a TabBar and a TabBarView. The TabController constructor length argument is the total number of tabs. A TickerProvider is required to trigger the notification whenever a frame triggers a state change. The TickerProvider is vsync. Pass the vsync: this argument to the TabController constructor whenever you create a new TabController.

TickerProvider 接口可以用于生成 Ticker 对象。当有对象被触发通知后会用到 Tickers，不过它通常都是被 AnimationController 间接调用。 AnimationControllers 需要 TickerProvider 来获得对应的 Ticker。如果你通过 State 创建了一个 AnimationController，那么你就可以使用 TickerProviderStateMixin 或者 SingleTickerProviderStateMixin 来获得对应的 TickerProvider。

The TickerProvider is an interface implemented by classes that can vend Ticker objects. Tickers can be used by any object that must be notified whenever a frame triggers, but they’re most commonly used indirectly via an AnimationController. AnimationControllers need a TickerProvider to obtain their Ticker. If you are creating an AnimationController from a State, then you can use the TickerProviderStateMixin or SingleTickerProviderStateMixin classes to obtain a suitable TickerProvider.

Scaffold 封装了一个新的 TabBar widget，其中包含两个 tab。TabBarView 作为 body 参数传递到 Scaffold 中。所有和 TabBar 中的 tab 相关的页面均是 TabBarView 的子 widget，并且都对应同一个 TabController。

The Scaffold widget wraps a new TabBar widget and creates two tabs. The TabBarView widget is passed as the body parameter of the Scaffold widget. All screens corresponding to the TabBar widget’s tabs are children to the TabBarView widget along with the same TabController.

// Flutter

class _NavigationHomePageState extends State<NavigationHomePage> with SingleTickerProviderStateMixin {
  TabController controller=TabController(length: 2, vsync: this);
  @override
 widget build(BuildContext context) {
    return Scaffold(
      bottomNavigationBar: Material (
        child: TabBar(
          tabs: <Tab> [
            Tab(icon: Icon(Icons.person),)
            Tab(icon: Icon(Icons.email),),
          ],
          controller: controller,
        ),
        color: Colors.blue,
      ),
      body: TabBarView(
        children: <Widget> [
          home.homeScreen(),
          tabScreen.tabScreen()
        ],
        controller: controller,
      )
    );
  }
}

Drawer 导航

Drawer navigation

在 React Native 中，导入所需的 react-navigation 包，然后使用 createDrawerNavigator 和 DrawerNavigation 实现。

In React Native, import the needed react-navigation packages and then use createDrawerNavigator and DrawerNavigation.

// React Native
export default (MyApp1 = DrawerNavigator({
  Home: {
    screen: SimpleApp
  },
  Screen2: {
    screen: drawerScreen
  }
}));

在 Flutter 中，我们可以结合 Drawer 和 Scaffold 一起使用来实现 Material Design 风格的 drawer 布局。如果要在应用程序中添加 Drawer，可以将它封装在 Scaffold widget 中。Scaffold widget 提供了一种一致的界面风格，它遵循 Material Design 的设计原则。同时它还支持一些特殊的 Material Design 组件，比如 Drawers，AppBars，和 SnackBars。

In Flutter, we can use the Drawer widget in combination with a Scaffold to create a layout with a Material Design drawer. To add a Drawer to an app, wrap it in a Scaffold widget. The Scaffold widget provides a consistent visual structure to apps that follow the Material Design guidelines. It also supports special Material Design components, such as Drawers, AppBars, and SnackBars.

Drawer 就是一个 Material Design 窗格，它可以从 Scaffold 边缘水平滑动显示应用程序的导航选项。你可以在里面添加 Button， Text。或者添加一个列表的元素作为 Drawer 的子 widget。在下面的例子中，ListTile 提供了点击导航。###

The Drawer widget is a Material Design panel that slides in horizontally from the edge of a Scaffold to show navigation links in an application. You can provide a Button, a Text widget, or a list of items to display as the child to the Drawer widget. In the following example, the ListTile widget provides the navigation on tap.

// Flutter
Drawer(
  child:ListTile(
    leading: Icon(Icons.change_history),
    title: Text('Screen2'),
    onTap: () {
      Navigator.of(context).pushNamed('/b');
    },
  ),
  elevation: 20.0,
),

Scaffold 还包含一个 AppBar。它会自动显示一个图标按钮来表明 Scaffold 中有一个Drawer。Scaffold 会自动处理边缘的滑动手势来显示 Drawer。

The Scaffold widget also includes an AppBar widget that automatically displays an appropriate IconButton to show the Drawer when a Drawer is available in the Scaffold. The Scaffold automatically handles the edge-swipe gesture to show the Drawer.

// Flutter
@override
Widget build(BuildContext context) {
  return Scaffold(
    drawer: Drawer(
      child: ListTile(
        leading: Icon(Icons.change_history),
        title: Text('Screen2'),
        onTap: () {
          Navigator.of(context).pushNamed('/b');
        },
      ),
      elevation: 20.0,
    ),
    appBar: AppBar(
      title: Text('Home'),
    ),
    body: Container(),
  );
}

手势检测和触摸事件处理

Gesture detection and touch event handling

Flutter 支持点击、拖拽和缩放手势来监听和相应手势操作。Flutter 中的手势处理有两个独立的层。第一层是指针事件，指针事件定义了指针在屏幕上的位置和动作，比如触摸、鼠标和触摸笔。第二层指手势，主要是语义层面的动作，里面包含一种或者多种指针动作。

To listen for and respond to gestures, Flutter supports taps, drags, and scaling. The gesture system in Flutter has two separate layers. The first layer includes raw pointer events, which describe the location and movement of pointers, (such as touches, mice, and styli movements), across the screen. The second layer includes gestures, which describe semantic actions that consist of one or more pointer movements.

如何为 widget 添加点击或者按压的监听器？

How do I add a click or press listeners to a widget?

在 React Native 中，使用 PanResponder 或者 Touchable 组件来添加监听器。

In React Native, listeners are added to components using PanResponder or the Touchable components.

// React Native
<TouchableOpacity
  onPress={() => {
    console.log('Press');
  }}
  onLongPress={() => {
    console.log('Long Press');
  }}
>
  <Text>Tap or Long Press</Text>
</TouchableOpacity>

对于更加复杂手势以及将多个触摸添加到单独的一个手势中，可以使用 PanResponder。

For more complex gestures and combining several touches into a single gesture, PanResponder is used.

// React Native
class App extends Component {

  componentWillMount() {
    this._panResponder = PanResponder.create({
      onMoveShouldSetPanResponder: (event, gestureState) =>
        !!getDirection(gestureState),
      onPanResponderMove: (event, gestureState) => true,
      onPanResponderRelease: (event, gestureState) => {
        const drag = getDirection(gestureState);
      },
      onPanResponderTerminationRequest: (event, gestureState) => true
    });
  }

  render() {
    return (
      <View style={styles.container} {...this._panResponder.panHandlers}>
        <View style={styles.center}>
          <Text>Swipe Horizontally or Vertically</Text>
        </View>
      </View>
    );
  }
}

在 Flutter 中，要为 widget 添加点击或者按压监听器，使用带有 onPress: field 的按钮或者可触摸 widget 即可。或者，用任何 widget 封装 GestureDetector，在其中添加手势检测。

In Flutter, to add a click (or press) listener to a widget, use a button or a touchable widget that has an onPress: field. Or, add gesture detection to any widget by wrapping it in a GestureDetector.

// Flutter
GestureDetector(
  child: Scaffold(
    appBar: AppBar(
      title: Text('Gestures'),
    ),
    body: Center(
      child: Column(
        mainAxisAlignment: MainAxisAlignment.center,
        children: <Widget>[
          Text('Tap, Long Press, Swipe Horizontally or Vertically '),
        ],
      )
    ),
  ),
  onTap: () {
    print('Tapped');
  },
  onLongPress: () {
    print('Long Pressed');
  },
  onVerticalDragEnd: (DragEndDetails value) {
    print('Swiped Vertically');
  },
  onHorizontalDragEnd: (DragEndDetails value) {
    print('Swiped Horizontally');
  },
);

如果想要了解更多详细内容，包括 Flutter 的 GestureDetector 回调函数的列表，请查看页面 GestureDetector class。

For more information, including a list of Flutter GestureDetector callbacks, see the GestureDetector class.

发起 HTTP 网络请求

Making HTTP network requests

对于大多数应用程序来说都需要从互联网上获取数据。在 Flutter 中，http 包提供了从互联网获取数据的最简单的途径。

Fetching data from the internet is common for most apps. And in Flutter, the http package provides the simplest way to fetch data from the internet.

如何通过 API 调用来获得数据呢？

How do I fetch data from API calls?

React Native 提供 Fetch API 实现网络编程，你可以发起请求，然后接收响应来获得数据。

React Native provides the Fetch API for networking—you make a fetch request and then receive the response to get the data.

// React Native
_getIPAddress = () => {
  fetch('https://httpbin.org/ip')
    .then(response => response.json())
    .then(responseJson => {
      this.setState({ _ipAddress: responseJson.origin });
    })
    .catch(error => {
      console.error(error);
    });
};

Flutter 使用 http 包。如果要安装 http 包，将它添加到 pubspec.yaml 的 dependencies 部分。

Flutter uses the http package. To install the http package, add it to the dependencies section of our pubspec.yaml.

dependencies:
  flutter:
    sdk: flutter
  http: <latest_version>

Flutter 使用 dart:io 提供核心的 HTTP 客户端支持，要创建一个 HTTP 客户端，引用 dart:io。

Flutter uses the dart:io core HTTP support client. To create an HTTP Client, import dart:io.

import 'dart:io';

客户端支持如下所列的 HTTP 操作：GET, POST, PUT 和 DELETE。

The client supports the following HTTP operations: GET, POST, PUT, and DELETE.

// Flutter
final url = Uri.https('httpbin.org', 'ip');
final httpClient = HttpClient();
_getIPAddress() async {
  var request = await httpClient.getUrl(url);
  var response = await request.close();
  var responseBody = await response.transform(utf8.decoder).join();
  String ip = jsonDecode(responseBody)['origin'];
  setState(() {
    _ipAddress = ip;
  });
}

输入表单

Form input

TextField 用于在应用程序中输入文本，这样就可以实现创建表单、短消息应用、搜索框等等功能。Flutter 提供两个核心文本输入 widget ： TextField 和 TextFormField.

Text fields allow users to type text into your app so they can be used to build forms, messaging apps, search experiences, and more. Flutter provides two core text field widgets: TextField and TextFormField.

如何使用文本输入 widget ？

How do I use text field widgets?

在 React Native 里，可以使用 TextInput 组件来输入文本，它会显示一个输入框，然后通过回调函数来传递输入值。

In React Native, to enter text you use a TextInput component to show a text input box and then use the callback to store the value in a variable.

// React Native
<TextInput
  placeholder="Enter your Password"
  onChangeText={password => this.setState({ password })}
 />
<Button title="Submit" onPress={this.validate} />

在 Flutter 中，使用 TextEditingController 类来管理 TextField widget。当用户修改文本的时候，controller 会通知监听器。

In Flutter, use the TextEditingController class to manage a TextField widget. Whenever the text field is modified, the controller notifies its listeners.

监听器读取文本和选项属性来获知用户所输入的内容。你可以通过 TextField 中的 text 属性获得用户输入的文本数据。

Listeners read the text and selection properties to learn what the user typed into the field. You can access the text in TextField by the text property of the controller.

// Flutter
final TextEditingController _controller = TextEditingController();
      ...
TextField(
  controller: _controller,
  decoration: InputDecoration(
    hintText: 'Type something', labelText: 'Text Field '
  ),
),
RaisedButton(
  child: Text('Submit'),
  onPressed: () {
    showDialog(
      context: context,
        child: AlertDialog(
          title: Text('Alert'),
          content: Text('You typed ${_controller.text}'),
        ),
     );
   },
 ),
)

在这个例子中，当用户点击提交按钮的时候，会弹出窗口显示当前输入的文本内容。可以使用 alertDialog widget 显示提示信息，TextField 的文本通过 text 属性来获得，该属性属于 TextEditingController。

In this example, when a user clicks on the submit button an alert dialog displays the current text entered in the text field. This is achieved using an alertDialog widget that displays the alert message, and the text from the TextField is accessed by the text property of the TextEditingController.

如何使用 Form widget 呢？

How do I use Form widgets?

在 Flutter 中，当需要使用带有提交按钮和 TextFormField 组件的复合 widget 时，就会用到 Form。 TextFormField 内含一个 onSaved 参数，它可以设置一个回调函数，当表单存储的时候会回调该函数。 FormState 用于存储、重置或者验证 Form 内含的每个 FormField。你可以通过将当前表单的 context 属性赋值给 Form.of 来获得 FormState。或者在表单的构造函数里使用 GlobalKey，然后调用 GlobalKey.currentState 来获得 FormState。

In Flutter, use the Form widget where TextFormField widgets along with the submit button are passed as children. The TextFormField widget has a parameter called onSaved which takes a callback and executes when the form is saved. A FormState object is used to save, reset, or validate each FormField that is a descendant of this Form. To obtain the FormState, you can use Form.of with a context whose ancestor is the Form, or pass a GlobalKey to the Form constructor and call GlobalKey.currentState.

final formKey = GlobalKey<FormState>();

...

Form(
  key:formKey,
  child: Column(
    children: <Widget>[
      TextFormField(
        validator: (value) => !value.contains('@') ? 'Not a valid email.' : null,
        onSaved: (val) => _email = val,
        decoration: const InputDecoration(
          hintText: 'Enter your email',
          labelText: 'Email',
        ),
      ),
      RaisedButton(
        onPressed: _submit,
        child: Text('Login'),
      ),
    ],
  ),
)

下面的示例代码展示了 Form.save() 和 formKey（这个实际上是 GlobalKey）如何被用于表单提交的。

The following example shows how Form.save() and formKey (which is a GlobalKey) are used to save the form on submit.

void _submit() {
  final form = formKey.currentState;
  if (form.validate()) {
    form.save();
    showDialog(
      context: context,
      child: AlertDialog(
        title: Text('Alert'),
        content: Text('Email: $_email, password: $_password'),
      )
    );
  }
}

平台相关代码

Platform-specific code

当构建跨平台应用程序的时候，你会尽量多地复用代码。然而，根据不同的应用场景，代码会根据平台的不同有所变化。这就需要提前声明具体的平台来进行独立的实现。

When building a cross-platform app, you want to re-use as much code as possible across platforms. However, scenarios might arise where it makes sense for the code to be different depending on the OS. This requires a separate implementation by declaring a specific platform.

在 React Native 中，下面的实现代码会被用到：

In React Native, the following implementation would be used:

// React Native
if (Platform.OS === 'ios') {
  return 'iOS';
} else if (Platform.OS === 'android') {
  return 'android';
} else {
  return 'not recognised';
}

而在 Flutter 中，则是下面这样的实现：

In Flutter, use the following implementation:

// Flutter
if (Theme.of(context).platform == TargetPlatform.iOS) {
  return 'iOS';
} else if (Theme.of(context).platform == TargetPlatform.android) {
  return 'android';
} else if (Theme.of(context).platform == TargetPlatform.fuchsia) {
  return 'fuchsia';
} else {
  return 'not recognised ';
}

调试

Debugging

在运行应用程序之前，可以使用 flutter analyze 检验一下代码。 Flutter analyzer（它封装了 dartanalyzer 工具）可以验证你的代码并且帮助你定位潜在的问题。如果你使用的是启用了 Flutter 的 IDE 的话，这个过程是全自动的。

Before running your applications, verify your code with flutter analyze. The Flutter analyzer (which is a wrapper around the dartanalyzer tool) examines your code and helps identify possible issues. If you’re using a Flutter-enabled IDE, this occurs automatically.

应该使用什么工具调试我的 Flutter 应用？

What tools can I use to debug my app in Flutter?

请使用 开发者工具 debug 你的 Flutter 和 Dart 应用。

Use the DevTools suite for debugging Flutter or Dart apps.

开发者工具包含了 profiling 构建、检查堆栈、检视 widget 树、诊断信息记录、调试、执行代码行观察、调试内存泄漏和内存碎片等。有关更多信息，请参阅 开发者工具 文档。

DevTools includes support for profiling, examining the heap, inspecting the widget tree, logging diagnostics, debugging, observing executed lines of code, debugging memory leaks and memory fragmentation. For more information, see the DevTools documentation.

如何进行热重载？

How do I perform a hot reload?

Flutter 的热重载特性可以帮助你快速便捷地实验、构建 UI 和各种特性以及修复 bug。每次修改代码以后，你只需直接热重载你的应用程序即可，而无需重新进行编译。应用程序会根据你的修改进行相应的更新，而程序原有的状态则会被保留。

Flutter’s Stateful Hot Reload feature helps you quickly and easily experiment, build UIs, add features, and fix bugs. Instead of recompiling your app every time you make a change, you can hot reload your app instantly. The app is updated to reflect your change, and the current state of the app is preserved.

在 React Native 中，iOS 模拟器对应的快捷键是 ⌘R，对应 Android 模拟器的快捷键是点击两次 R。

In React Native, the shortcut is ⌘R for the iOS Simulator and tapping R twice on Android emulators.

在 Flutter 中，如果你使用的是 IntelliJ 或者 Android Studio，可以使用 Save All (⌘s/ctrl-s)，或者可以点击工具栏上的 Hot Reload 按钮。如果你是在命令行里使用 flutter run 命令运行的程序，在窗口里输入 r 即可。也可以输入 R 进行彻底的重启。

In Flutter, If you are using IntelliJ IDE or Android Studio, you can select Save All (⌘s/ctrl-s), or you can click the Hot Reload button on the toolbar. If you are running the app at the command line using flutter run, type r in the Terminal window. You can also perform a full restart by typing R in the Terminal window.

如何打开程序里的开发者菜单？

How do I access the in-app developer menu?

在 React Native 中，开发者菜单可以通过摇动设备打开：对于 iOS 模拟器的快捷键是 ⌘D 而 Android 模拟器的快捷键是 ⌘M。

In React Native, the developer menu can be accessed by shaking your device: ⌘D for the iOS Simulator or ⌘M for Android emulator.

在 Flutter 中，如果你使用 IDE，那么可以直接使用 IDE 工具。如果你是通过命令行运行 flutter run 来启动应用程序的，你可以在命令行窗口通过输入 h 来打开菜单，或者参考下面的快捷键说明：

In Flutter, if you are using an IDE, you can use the IDE tools. If you start your application using flutter run you can also access the menu by typing h in the terminal window, or type the following shortcuts:

动画

Animation

精美的动画效果会使得 UI 更加直观，可以提升整体视觉效果，使应用显得更加精致，从而提升用户体验。Flutter 的动画框架使得开发者能够更方便地实现简单和复杂的动画。Flutter SDK 含有很多 Material Design widget。其中已经包括了标准的动画效果，你可以很方便地自定义这些效果。

Well-designed animation makes a UI feel intuitive, contributes to the look and feel of a polished app, and improves the user experience. Flutter’s animation support makes it easy to implement simple and complex animations. The Flutter SDK includes many Material Design widgets that include standard motion effects and you can easily customize these effects to personalize your app.

在 React Native 中，动画 API 用于创建动画。

In React Native, Animated APIs are used to create animations.

在 Flutter 中，使用 Animation 类和 AnimationController 类实现动画。Animation 是抽象类，内含其当前的值和它的状态（已完成或者已取消）。AnimationController 类可以正向或者反向播放动画或者停止动画以及为动画设置特定值来自定义动画。

In Flutter, use the Animation class and the AnimationController class. Animation is an abstract class that understands its current value and its state (completed or dismissed). The AnimationController class lets you play an animation forward or in reverse, or stop animation and set the animation to a specific value to customize the motion.

如何添加一个简单的淡入动画效果？

How do I add a simple fade-in animation?

在下面的 React Native 示例中，有一个动画组件，也就是 FadeInView，它是使用 Animated API 创建的。定义了初始的不透明状态，最终状态和动画切换之间的时间间隔。在 Animated 中添加了动画组件，不透明状态 fadeAnim 映射到我们想要添加动画效果的文本组件上，然后在开始动画的时候调用 start()。

In the React Native example below, an animated component, FadeInView is created using the Animated API. The initial opacity state, final state, and the duration over which the transition occurs are defined. The animation component is added inside the Animated component, the opacity state fadeAnim is mapped to the opacity of the Text component that we want to animate, and then, start() is called to start the animation.

// React Native
class FadeInView extends React.Component {
  state = {
    fadeAnim: new Animated.Value(0) // Initial value for opacity: 0
  };
  componentDidMount() {
    Animated.timing(this.state.fadeAnim, {
      toValue: 1,
      duration: 10000
    }).start();
  }
  render() {
    return (
      <Animated.View style={{...this.props.style, opacity: this.state.fadeAnim }} >
        {this.props.children}
      </Animated.View>
    );
  }
}
    ...
<FadeInView>
  <Text> Fading in </Text>
</FadeInView>
    ...

要在 Flutter 中实现相同的动画效果，创建一个 AnimationController 对象，叫它 controller，并且指定时间间隔。在默认配置下， AnimationController 会在给定时间间隔线性的生成从 0.0 到 1.0 的数值。当你的程序可以显示新一帧画面的时候，AnimationController 会生成一个新的值。通常，这个频率在每秒 60 个值。

To create the same animation in Flutter, create an AnimationController object named controller and specify the duration. By default, an AnimationController linearly produces values that range from 0.0 to 1.0, during a given duration. The animation controller generates a new value whenever the device running your app is ready to display a new frame. Typically, this rate is around 60 values per second.

当定义 AnimationController 的时候，你必须传入一个 vsync 对象。vsync 会防止屏幕显示区域之外的动画消耗不必要的资源。你可以通过添加 TickerProviderStateMixin 到类定义中来使用有状态的对象。AnimationController 需要传入一个 TickerProvider，它是通过构造函数里的 vsync 参数进行配置的。

When defining an AnimationController, you must pass in a vsync object. The presence of vsync prevents offscreen animations from consuming unnecessary resources. You can use your stateful object as the vsync by adding TickerProviderStateMixin to the class definition. An AnimationController needs a TickerProvider, which is configured using the vsync argument on the constructor.

Tween 定义了起始和结束值之间或者输入段到输出段之间的过渡。如果要在动画中使用 Tween 对象，调用 Tween 对象的 animate 方法，然后把它赋给你要修改的 Animation 对象。

A Tween describes the interpolation between a beginning and ending value or the mapping from an input range to an output range. To use a Tween object with an animation, call the Tween object’s animate method and pass it the Animation object that you want to modify.

在这个例子中，用到了 FadeTransition widget，它的 opacity 属性映射到了 animation 对象上。

For this example, a FadeTransition widget is used and the opacity property is mapped to the animation object.

要开始动画，使用 controller.forward()。其它的操作也可以使用控制器里的方法，比如 fling() 或者 repeat()。这个例子里，FlutterLogo widget 被用于 FadeTransition widget 中。

To start the animation, use controller.forward(). Other operations can also be performed using the controller such as fling() or repeat(). For this example, the FlutterLogo widget is used inside the FadeTransition widget.

// Flutter
import 'package:flutter/material.dart';

void main() {
  runApp(Center(child: LogoFade()));
}

class LogoFade extends StatefulWidget {
  _LogoFadeState createState() => _LogoFadeState();
}

class _LogoFadeState extends State<LogoFade> with TickerProviderStateMixin {
  Animation animation;
  AnimationController controller;

  initState() {
    super.initState();
    controller = AnimationController(
        duration: const Duration(milliseconds: 3000), vsync: this);
    final CurvedAnimation curve =
    CurvedAnimation(parent: controller, curve: Curves.easeIn);
    animation = Tween(begin: 0.0, end: 1.0).animate(curve);
    controller.forward();
  }

 widget build(BuildContext context) {
    return FadeTransition(
      opacity: animation,
      child: Container(
        height: 300.0,
        width: 300.0,
        child: FlutterLogo(),
      ),
    );
  }

  dispose() {
    controller.dispose();
    super.dispose();
  }
}

如何为卡片添加滑动动画呢？

How do I add swipe animation to cards?

在 React Native 中，无论 PanResponder 或者第三方库都可被用于滑动动画。

In React Native, either the PanResponder or third-party libraries are used for swipe animation.

在 Flutter 中，要添加滑动动画，使用 Dismissible widget 封装其它子 widget 即可。

In Flutter, to add a swipe animation, use the Dismissible widget and nest the child widgets.

child: Dismissible(
  key: key,
  onDismissed: (DismissDirection dir) {
    cards.removeLast();
  },
  child: Container(
    ...
  ),
),

React Native 和 Flutter widget 对等的组件

React Native and Flutter widget equivalent components

下面的表格列举了通用的 React Native 组件与对应的 Flutter widget 和通用的 widget 属性。

The following table lists commonly-used React Native components mapped to the corresponding Flutter widget and common widget properties.