需要导入的库
dio: ^1.0.13
json_serializable: ^2.0.0 # 解析
build_runner: ^1.1.2 # 解析

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

class HttpGo {
  static final String GET = "get";
  static final String POST = "post";
  static final String DATA = "data";
  static final String CODE = "errorCode";

  Dio dio;
  static HttpGo _instance;

  static HttpGo getInstance() {
    if (_instance == null) {
      _instance = HttpGo();
    }
    return _instance;
  }

  HttpGo() {
    dio = Dio(Options(
      /*baseUrl: "http://192.168.5.6:8085",*/
      headers: {'platform': 'android', 'version': 11.0},
      connectTimeout: 5000,
      receiveTimeout: 100000,
    ));
  }

  //get请求
  get(String url, Function successCallBack,
      {params, Function errorCallBack}) async {
    _requstHttp(url, successCallBack, GET, params, errorCallBack);
  }

  //post请求
  post(String url, Function successCallBack,
      {params, Function errorCallBack}) async {
    _requstHttp(url, successCallBack, POST, params, errorCallBack);
  }

  _requstHttp(String url, Function successCallBack,
      [String method, FormData params, Function errorCallBack]) async {
    String errorMsg = '';
    int code;

    try {
      Response response;
      _addStartHttpInterceptor(dio); //添加请求之前的拦截器
      if (method == GET) {
        if (params != null && params.isNotEmpty) {
          response = await dio.get(url, data: params);
        } else {
          response = await dio.get(url);
        }
      } else if (method == POST) {
        if (params != null && params.isNotEmpty) {
          response = await dio.post(url, data: params);
        } else {
          response = await dio.post(url);
        }
      }

      code = response.statusCode;
      if (code != 200) {
        errorMsg = '错误码：' + code.toString() + '，' + response.data.toString();
        _error(errorCallBack, errorMsg);
        return;
      }

      String dataStr = json.encode(response.data);
      Map<String, dynamic> dataMap = json.decode(dataStr);
      if (dataMap != null && dataMap[CODE] != 0) {
        errorMsg =
            '错误码：' + dataMap[CODE].toString() + '，' + response.data.toString();
        _error(errorCallBack, errorMsg);
        return;
      }

      if (successCallBack != null) {
        successCallBack(dataMap[DATA]);
      }
    } catch (exception) {
      _error(errorCallBack, exception.toString());
    }
  }

  _error(Function errorCallBack, String error) {
    Fluttertoast.showToast(
        msg: error.toString(),
        toastLength: Toast.LENGTH_SHORT,
        gravity: ToastGravity.CENTER);
    if (errorCallBack != null) {
      errorCallBack(error);
    }
  }

  _addStartHttpInterceptor(Dio dio) {
    dio.interceptor.request.onSend = (Options options) {
      // 在请求被发送之前做一些事情   比如加密的一些操作 或者添加token等参数 对head 或者请求参数进行加工处理
      Map<String, dynamic> headers = options.headers;
      Map<String, dynamic> body = options.data;
      /*request['token'] = '1111111111';
      headers['addParam'] = 'aaaaaaaaaaaaaaa';*/
      return options;
    };
  }
}

用法

 var getUrl = 'http://www.wanandroid.com/article/list/0/json';
  getHttp() {
    FormData formData = FormData.from({'cid': 60});
    HttpGo.getInstance().get(
      getUrl,
      (data) {
        News news = News.fromJson(data);
        setState(() {
          httpContent = data.toString();
        });
      },
      params: formData,
    );
  }

var postUrl = 'http://www.wanandroid.com/user/login';
  postHttp() {
    FormData p =
        FormData.from({'username': '15660010019', 'password': '123456'});
    HttpGo.getInstance().post(
      postUrl,
      (data) {
        UserDetail userDetail = UserDetail.fromJson(data);
        var name = userDetail.username;
        setState(() {
          httpContent = data.toString();
        });
      },
      params: p,
    );
  }

链接：https://github.com/yzxzm/flutter_ydd/blob/master/lib/utils/HttpGo.dart