摘自Flutter中文网：官方使用的是用dart io中的HttpClient发起的请求，但HttpClient本身功能较弱，很多常用功能都不支持。我们建议您使用dio 来发起网络请求，它是一个强大易用的dart http请求库，支持Restful API、FormData、拦截器、请求取消、Cookie管理、文件上传/下载……详情请查看github dio.

这边会简单使用一下官方网络请求， 然后重点说明dio网络的封装及使用

1.Flutter官方HttpClient网络请求

//flutter原生网络请求，简单处理get请求
  _flutterReuest() async {
    var httpClient = HttpClient();

    try {
      var uri = Uri.http('m.devapi.haoshiqi.net', '/common/index_v2');
      var request = await httpClient.getUrl(uri);
      var response = await request.close();
      if (response.statusCode == HttpStatus.ok) {
        var json = await response.transform(utf8.decoder).join();
        var data = jsonDecode(json);
        print(data);
      }
    } catch (e) {
      print(e);
    }
  }

2.Dio网络请求的封装及使用

• 添加引用：

dependencies:
  dio: ^2.1.0

• 一个简单的示例

import 'package:dio/dio.dart';
void getHttp() async {
  try {
    Response response = await Dio().get("http://www.google.com");
    print(response);
  } catch (e) {
    print(e);
  }
}

• 代码封装：这里处理方式是GET、POST分两个方法处理的（还有一个想法是封装一个方法，通过'POST /path'，这样的方式来处理）

import 'dart:convert';
import 'dart:io';
import 'package:dio/dio.dart';

class DioHttpRequest {
  //get请求
  static Future<ResponseData> getReqeust(String path, [dynamic params]) async {
    return _request('GET', path, params);
  }

  //post请求
  static Future<ResponseData> postRequest(String path,
      [dynamic params]) async {
    return _request('POST', path, params);
  }

  static Future<ResponseData> _request(String method, String path,
      [dynamic params]) async {

    BaseOptions options = BaseOptions(
      baseUrl: 'https://m.devapi.haoshiqi.net',
    );
    Dio dio = Dio(options);

    try {
      Response response;
      if (method == 'POST') {
        response = await dio.post(path, queryParameters: params);
      }else {
        response = await dio.get(path, queryParameters: params);
      }

      if (response.statusCode == 200) {
        print(response.data);
        return ResponseData.parse(response.data);
      } else {
        ResponseData responseData = ResponseData();
        responseData.errno = response.statusCode;
        responseData.errmsg = '服务器异常';
        return responseData;
      }
    } on DioError catch (e) {
      ResponseData responseData = ResponseData();
      responseData.errmsg = _errmsg(e.type);
      //类型判断 is
      if (e.error is SocketException) {
        SocketException error = e.error;
        responseData.errno = error.osError.errorCode;
      } else {
        responseData.errno = -1;
      }

      return responseData;
    }
  }

  //错误码处理
  static String _errmsg(DioErrorType type) {
    switch (type) {
      case DioErrorType.CONNECT_TIMEOUT:
        return '连接超时';
        break;
      case DioErrorType.SEND_TIMEOUT:
        return '请求超时';
        break;
      case DioErrorType.RECEIVE_TIMEOUT:
        return '响应超时';
        break;
      case DioErrorType.RESPONSE:
        return '出现异常';
        break;
      case DioErrorType.CANCEL:
        return '请求取消';
        break;
      default:
        return '无网络';
        break;
    }
  }
}

• 返回数据处理

//返回数据处理
class ResponseData {
  String errmsg;
  int errno;
  int timestamp;
  Map data;
  static ResponseData parse(dynamic data) {
    Map map;
    if (data is Map) {
      map = data;
    }else {
      map = jsonDecode(data);
    }
    ResponseData responseData = ResponseData();
    responseData.errmsg = map['errmsg'];
    responseData.errno = map['errno'];
    responseData.timestamp = map['timestamp'];
    responseData.data = map['data'];
    return responseData;
  }
}

• 使用方式

//dio网络请求（推荐）
  _httpReqeust() {
    var request = DioHttpRequest.getReqeust('/common/index_v2');
    request.then((response) {
      if (response.errno == 0) {
        //成功
        print('成功');
      } else {
        //失败
        print('失败');
      }
      setState(() {
        errmsg = 'errmsg: ${response.errmsg}';
      });
    });
  }

Demo地址：SwiftFlutterDemo

PS:我也是初学者，如有问题，请请教！