获取网络数据

对于大部分应用来说，获取网络数据都是必不可少的一个功能。幸运的是，Dart 和 Flutter 就为我们提供了这样的工具。

Fetching data from the internet is necessary for most apps. Luckily, Dart and Flutter provide tools, such as the http package, for this type of work.

这个教程包含以下步骤：

This recipe uses the following steps:

添加 http 包
Add the http package.
使用 http 包进行网络请求
Make a network request using the http package.
将返回的响应转换成一个自定义的 Dart 对象
Convert the response into a custom Dart object.
使用 Flutter 对数据进行获取和展示
Fetch and display the data with Flutter.

1. 添加 `http` 包

1. Add the `http` package

http 包为我们提供了获取网络数据最简单的方法。

The http package provides the simplest way to fetch data from the internet.

安装 http 包之前，你必须先把它添加到 pubspec.yaml 的依赖区域。你可以在 [pub.dev 找到 http 包的最新版本][http package]。

To install the http package, add it to the dependencies section of the pubspec.yaml file. You can find the latest version of the http package the pub.dev.

dependencies:
  http: <latest_version>

Import the http package.

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

2. 进行网络请求

2. Make a network request

在这里，你可以使用 http.get() 方法从 JSONPlaceholder 上获取到一个样本相册数据。

This recipe covers how to fetch a sample album from the JSONPlaceholder using the http.get() method.

Future<http.Response> fetchAlbum() {
  return http.get('https://jsonplaceholder.typicode.com/albums/1');
}

这个 http.get() 方法会返回一个包含 Response 的 Future。

The http.get() method returns a Future that contains a Response.

Future 是 Dart 用来处理异步操作的一个核心类，它通常代表一个可能的值或者将来或许会用到的错误。
Future is a core Dart class for working with async operations. A Future object represents a potential value or error that will be available at some time in the future.
http.Response 类包含成功的 http 请求接收到的数据。
The http.Response class contains the data received from a successful http call.

3. 将返回的响应转换成一个自定义的 Dart 对象

3. Convert the response into a custom Dart object

虽然进行网络请求很容易，但是处理 Future<http.Response> 却并不简单，为了后续处理起来更加方便，我们需要将 http.Response 转换成一个 Dart 对象。

While it’s easy to make a network request, working with a raw Future<http.Response> isn’t very convenient. To make your life easier, convert the http.Response into a Dart object.

创建一个 `Album` 类

Create an `Album` class

首先，创建一个包含网络请求返回数据的 Album 类，而且这个类还需要一个可以利用 json 创建 Album 的工厂构造器。

First, create an Album class that contains the data from the network request. It includes a factory constructor that creates an Album from JSON.

手动转换 JSON 是我们目前唯一的选项。想了解更多，请查看完整的文档 JSON 和序列化数据。

Converting JSON by hand is only one option. For more information, see the full article on JSON and serialization.

class Album {
  final int userId;
  final int id;
  final String title;

  Album({this.userId, this.id, this.title});

  factory Album.fromJson(Map<String, dynamic> json) {
    return Album(
      userId: json['userId'],
      id: json['id'],
      title: json['title'],
    );
  }
}

将 `http.Response` 转换成 `Album`

Convert the `http.Response` to an `Album`

现在，我们需要更新 fetchPost() 函数并返回 Future<Album>，为了实现这个目标，我们需要做以下几步：

Now, use the following steps to update the fetchAlbum() function to return a Future<Album>:

用 dart:convert 包将响应体转换成一个 json Map。
Convert the response body into a JSON Map with the dart:convert package.
如果服务器返回了一个状态码为 200 的 “OK” 响应，那么就使用 fromJson 工厂方法将 json Map 转换成 Album。
If the server does return an OK response with a status code of 200, then convert the JSON Map into an Album using the fromJson() factory method.
如果服务器返回的不是我们预期的响应（返回一个OK，Http Header 是 200），那么就抛出异常。服务器如若返回 404 Not Found 错误，也同样要抛出异常，而不是返回一个 null，在检查如下所示的 snapshot 值的时候，这一点相当重要。
If the server does not return an OK response with a status code of 200, then throw an exception. (Even in the case of a “404 Not Found” server response, throw an exception. Do not return null. This is important when examining the data in snapshot, as shown below.)

Future<Album> fetchAlbum() async {
  final response = await http.get('https://jsonplaceholder.typicode.com/albums/1');

  if (response.statusCode == 200) {
    // If the server did return a 200 OK response,
    // then parse the JSON.
    return Album.fromJson(json.decode(response.body));
  } else {
    // If the server did not return a 200 OK response,
    // then throw an exception.
    throw Exception('Failed to load album');
  }
}

太棒了！现在你就拥有了一个可以获取网络数据的完整函数啦。

Hooray! Now you’ve got a function that fetches an album from the internet.

4. 获取数据

4. Fetch the data

在 initState() 或 didChangeDependencies() 方法中调用获取数据的方法 fetch()。

Call the fetch() method in either the initState() or didChangeDependencies() methods.

initState() 方法仅会被调用一次。如果你想要响应 InheritedWidget 改变以重新加载 API 的话，请在 didChangeDependencies() 方法中进行调用，你可以在 State 文档里了解更多。

The initState() method is called exactly once and then never again. If you want to have the option of reloading the API in response to an InheritedWidget changing, put the call into the didChangeDependencies() method. See State for more details.

class _MyAppState extends State<MyApp> {
  Future<Album> futureAlbum;

  @override
  void initState() {
    super.initState();
    futureAlbum = fetchAlbum();
  }

我们将会在下一步中使用这个 Future。

This Future is used in the next step.

5. 显示数据

5. Display the data

为了能够获取数据并在屏幕上展示它，你可以使用 FutureBuilder widget。这个由 Flutter 提供的 FutureBuilder 组件可以让处理异步数据源变的非常简单。

To display the data on screen, use the FutureBuilder widget. The FutureBuilder widget comes with Flutter and makes it easy to work with asynchronous data sources.

此时，你必须要提供两个参数：

You must provide two parameters:

你想要处理的 Future，在这个例子中就是 fetchAlbum() 返回的 future。
The Future you want to work with. In this case, the future returned from the fetchAlbum() function.
一个告诉 Flutter 渲染哪些内容的 builder 函数，同时这也依赖于 Future 的状态：loading、success 或者是 error。
A builder function that tells Flutter what to render, depending on the state of the Future: loading, success, or error.

需要注意的是：当快照包含非空数据值，snapshot.hasData 将只返回 true，这就是为什么要在服务端返回 404 状态码的时候要让 fetchAlbum 方法抛出异常。如果 fetchAlbum 返回 null 的话，spinner 会显示不正常。

Note that snapshot.hasData only returns true when the snapshot contains a non-null data value. This is why the fetchAlbum function should throw an exception even in the case of a “404 Not Found” server response. If fetchAlbum returns null then the spinner displays indefinitely.

FutureBuilder<Album>(
  future: futureAlbum,
  builder: (context, snapshot) {
    if (snapshot.hasData) {
      return Text(snapshot.data.title);
    } else if (snapshot.hasError) {
      return Text("${snapshot.error}");
    }

    // By default, show a loading spinner.
    return CircularProgressIndicator();
  },
);

为何要在 initState() 中调用 fetchPost()？

Why is fetchAlbum() called in initState()?

虽然这样会比较方便，但是我们仍然不推荐将 API 调用置于 build() 方法内部。

Although it’s convenient, it’s not recommended to put an API call in a build() method.

每当 Flutter 需要改变视图中的一些内容时（这个发生的频率非常高），就会调用 build() 方法。因此，如果你将数据请求置于 build() 内部，就会造成大量的无效调用，同时还会拖慢应用程序的速度。

Flutter calls the build() method every time it needs to change anything in the view, and this happens surprisingly often. Leaving the fetch call in your build() method floods the API with unnecessary calls and slows down your app.

关于如何在页面初始化的时候，只调用 API，下面有一些更好的选择。

Here are some better options so it only hits the API when the page is initially loaded.

传入 `StatelessWidget`

Pass it into a `StatelessWidget`

使用这种策略的话，相当于父组件负责调用数据获取方法，存储结果并传入你的组件中。

With this strategy, the parent widget is responsible for calling the fetch method, storing its result, and then passing it to your widget.

class MyApp extends StatelessWidget {
  final Future<Post> post;

  MyApp({Key key, this.post}) : super(key: key);

你可以在下面看到一个关于这种策略的完整代码示例。

You can see a working example of this in the complete example below.

在 `StatefulWidget` 状态的生命周期中调用

Call it in the lifecycle of a `StatefulWidget`’s state

如果你的组件是有状态的，你可以在 initState() 或者 didChangeDependencies() 方法中调用 fetch 方法。

If your widget is stateful, call the fetch method in either the initState() or didChangeDependencies() methods.

initState() 只会被调用一次而且再也不会被调用。如果你需要在 InheritedWidget 改变的时候可以重新载入的话，可以把数据调用放在 didChangeDependencies() 方法中。想了解更多详细内容请查看 State 文档。

class _MyAppState extends State<MyApp> {
  Future<Post> post;

  @override
  void initState() {
    super.initState();
    post = fetchPost();
  }

测试

Testing

关于如何测试这个功能，请查看下面的说明：

For information on how to test this functionality, see the following recipes:

完整样例

Complete example

import 'dart:async';
import 'dart:convert';

import 'package:flutter/material.dart';
import 'package:http/http.dart' as http;

Future<Album> fetchAlbum() async {
  final response =
      await http.get('https://jsonplaceholder.typicode.com/albums/1');

  if (response.statusCode == 200) {
    // If the server did return a 200 OK response,
    // then parse the JSON.
    return Album.fromJson(json.decode(response.body));
  } else {
    // If the server did not return a 200 OK response,
    // then throw an exception.
    throw Exception('Failed to load album');
  }
}

class Album {
  final int userId;
  final int id;
  final String title;

  Album({this.userId, this.id, this.title});

  factory Album.fromJson(Map<String, dynamic> json) {
    return Album(
      userId: json['userId'],
      id: json['id'],
      title: json['title'],
    );
  }
}

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

class MyApp extends StatefulWidget {
  MyApp({Key key}) : super(key: key);

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

class _MyAppState extends State<MyApp> {
  Future<Album> futureAlbum;

  @override
  void initState() {
    super.initState();
    futureAlbum = fetchAlbum();
  }

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Fetch Data Example',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: Scaffold(
        appBar: AppBar(
          title: Text('Fetch Data Example'),
        ),
        body: Center(
          child: FutureBuilder<Album>(
            future: futureAlbum,
            builder: (context, snapshot) {
              if (snapshot.hasData) {
                return Text(snapshot.data.title);
              } else if (snapshot.hasError) {
                return Text("${snapshot.error}");
              }

              // By default, show a loading spinner.
              return CircularProgressIndicator();
            },
          ),
        ),
      ),
    );
  }
}

1. 添加 http 包

1. Add the http package