网络和Http

发起HTTP请求

http支持位于dart:io，所以要创建一个HTTP client， 我们需要添加一个导入：

import 'dart:io';

var httpClient = new HttpClient();

该 client 支持常用的HTTP操作, such as GET, POST, PUT, DELETE.

处理异步

注意，HTTP API 在返回值中使用了Dart Futures。 我们建议使用async/await语法来调用API。

网络调用通常遵循如下步骤：

其中的几个步骤使用基于Future的API。上面步骤的示例：

get() async {
  var httpClient = new HttpClient();
  var uri = new Uri.http(
      'example.com', '/path1/path2', {'param1': '42', 'param2': 'foo'});
  var request = await httpClient.getUrl(uri);
  var response = await request.close();
  var responseBody = await response.transform(UTF8.decoder).join();
}

解码和编码JSON

使用dart:convert库可以简单解码和编码JSON。 有关其他的JSON文档，请参阅JSON和序列化。

解码简单的JSON字符串并将响应解析为Map：

Map data = JSON.decode(responseBody);
// Assume the response body is something like: ['foo', { 'bar': 499 }]
int barValue = data[1]['bar']; // barValue is set to 499

要对简单的JSON进行编码，请将简单值（字符串，布尔值或数字字面量）或包含简单值的Map，list等传给encode方法：

String encodedString = JSON.encode([1, 2, { 'a': null }]);

示例: 解码 HTTPS GET请求的JSON

以下示例显示了如何在Flutter应用中对HTTPS GET请求返回的JSON数据进行解码

它调用httpbin.comWeb service测试API，请注意，使用安全网络请求（HTTPS）

运行flutter create，创建一个新的Flutter应用.

将lib/main.dart替换为一下内容:

import 'dart:convert';
import 'dart:io';

import 'package:flutter/material.dart';

void main() {
  runApp(new MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return new MaterialApp(
      home: new MyHomePage(),
    );
  }
}

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

  @override
  _MyHomePageState createState() => new _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  var _ipAddress = 'Unknown';

  _getIPAddress() async {
    var url = 'https://httpbin.org/ip';
    var httpClient = new HttpClient();

    String result;
    try {
      var request = await httpClient.getUrl(Uri.parse(url));
      var response = await request.close();
      if (response.statusCode == HttpStatus.OK) {
        var json = await response.transform(utf8.decoder).join();
        var data = jsonDecode(json);
        result = data['origin'];
      } else {
        result =
            'Error getting IP address:\nHttp status ${response.statusCode}';
      }
    } catch (exception) {
      result = 'Failed getting IP address';
    }

    // If the widget was removed from the tree while the message was in flight,
    // we want to discard the reply rather than calling setState to update our
    // non-existent appearance.
    if (!mounted) return;

    setState(() {
      _ipAddress = result;
    });
  }

  @override
  Widget build(BuildContext context) {
    var spacer = new SizedBox(height: 32.0);

    return new Scaffold(
      body: new Center(
        child: new Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            new Text('Your current IP address is:'),
            new Text('$_ipAddress.'),
            spacer,
            new RaisedButton(
              onPressed: _getIPAddress,
              child: new Text('Get IP address'),
            ),
          ],
        ),
      ),
    );
  }
}