Flutter HTTP POST請求提交表單資料

https://twitter.com/search?q=COVID-19&src=typed_query

import 'dart:io';
var httpClient = new HttpClient();

post(dynamic url, { Map<String, String> headers, dynamic body, Encoding encoding }) → Future<Response>

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

await http.post('https://www.coa.gov.tw/theme_list.php?theme=news&sub_theme=agri',
           headers: headersMap,body: formdata,encoding: Utf8Codec())
    .then((http.Response response) {
      	var responseBody = response.body;
       // 處理響應數據
    
    }).catchError((error) {
      print('$error');
    });

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

String selectDivision = '*';
var response;

Future<http.Response> requestData(String division) async{
	selectDivision = division;
    var url_post = 'https://www.coa.gov.tw/theme_list.php?theme=news&sub_theme=agri';
    Map<String, String> headersMap = {
      "content-type":"application/x-www-form-urlencoded",
      "User-Agent":"Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/90.0.4430.212 Safari/537.36",
    };

    Map<String, String> formdata = {
      "division_lv1":selectDivision,
      "year":"110",
      "month":"1",
      "end_year":"110",
      "end_month":"5",
      "search_Submit":"查詢",
      "is_search":"y",
    };

    return response = await http.post(
      url_post,
      headers: headersMap,
      body: formdata,
      encoding: Utf8Codec() //注：Utf8Codec()需要 import 'dart:convert';
    );

  }

var title = new List();
var href = new List();
var date = new List();
var division = new List();
var date_reg = new RegExp(r'..-..-..'); //日期正則
var division_reg = new RegExp(r'(<td align="center">)(.+)(</td>)'); //發布機關正則

void getNews() async {
    var data = await requestData(selectDivision);

    setState(() {
      response = data;
    });
    
    if (response.statusCode == 200) {
      title.clear();
      href.clear();
      date.clear();
      division.clear();

      var document = parse(response.body);
      
      //標題和連結
      var titleElement = document.querySelectorAll('.main-c9-index');
      for (final word in titleElement) {
        title.add(word.attributes['title']);
        href.add('https://www.coa.gov.tw/' + word.attributes['href']);
      }
      
      //日期和發布機關
      var tableElement = document.querySelectorAll('.table > tbody > tr');

      for (final word in tableElement) {
        Iterable date_allMatches = date_reg.allMatches(word.innerHtml); //抓取符合日期正則的資料
        Iterable division_allMatches = division_reg.allMatches(word.innerHtml); //抓取符合發布機關正則的資料
        
        //日期
        date_allMatches.forEach((match) {
          date.add(word.innerHtml.substring(match.start, match.end));
        });
        
        //發布機關
        division_allMatches.forEach((match) {
          division.add(match.group(2));
        });
        
      }
      setState(() {
          title = title;
          href = href;
          date = date;
          division = division;
        });
    }
  }

@override
  void initState() {
    getNews();
    super.initState();
  }

Widget newsList(){
    return ListView.separated(
      itemCount: title.length,
      itemBuilder: (context, index) {
        return new ListTile(
          leading: new Icon(Icons.new_releases),
          title: new Text("${title[index]}"),
          subtitle: new Text("${device[index]} ${day[index]}"),
          contentPadding: EdgeInsets.symmetric(horizontal: 20.0),
          enabled: true,
          onTap: () => {
            //頁面傳值
            Navigator.of(context) 
                .push(new MaterialPageRoute(builder: (_) {
                  return new newsContent(
                    title: "${title[index]}",
                    url: "${href[index]}",
                    day: "${date[index]}",
                    division: "${division[index]}",
                  );
            })),
          },
        );
      }, separatorBuilder: (BuildContext context, int index) {
          return Divider();
        },
    );
  }

class ListItem {
  String id;
  String name;
  ListItem(this.id, this.name);
}

List<ListItem> _dropdownItems = [
    ListItem('*', "所有機關"),
    ListItem('coa', "農委會"),
    ListItem('afa', "農糧署"),
    ListItem('boaf', "農業金融局"),
    ListItem('forest', "林務局"),
    ListItem('swcb', "水土保持局"),
    ListItem('irrigation2', "農田水利署"),
    ListItem('tari', "農業試驗所"),
    ListItem('tfri', "林業試驗所"),
    ListItem('tactri', "農業藥物毒物試驗所"),
    ListItem('tydares', "桃園區農業改良場"),
    ListItem('mdares', "苗栗區農業改良場"),
    ListItem('tdares', "臺中區農業改良場"),
    ListItem('tndais', "臺南區農業改良場"),
    ListItem('kdais', "高雄區農業改良場"),
    ListItem('hdais', "花蓮區農業改良場"),
    ListItem('ttdares', "臺東區農業改良場"),
    ListItem('teais', "茶葉改良場"),
    ListItem('tss', "種苗改良繁殖場"),
    ListItem('pabp', "屏東農業生物技術園區"),
  ];

 List<DropdownMenuItem<ListItem>> _dropdownMenuItems;
 ListItem _selectedItem;

 List<DropdownMenuItem<ListItem>> buildDropDownMenuItems(List listItems) {
   List<DropdownMenuItem<ListItem>> items = List();
   
    for (ListItem listItem in listItems) {
      items.add(
        DropdownMenuItem(
          child: SizedBox(
              width: 200,
              child: Text(
                listItem.name,
                textAlign: TextAlign.center,
              ),
            ),
          value: listItem,
          
        ),
      );
    }
   
    return items;
 }

@override
  void initState() {
    getNews();
    _dropdownMenuItems = buildDropDownMenuItems(_dropdownItems);
    _selectedItem = _dropdownMenuItems[0].value;
    super.initState();
  }

 DropdownButton<ListItem>(
    style: TextStyle(fontSize: 17),
    value: _selectedItem,
    items: _dropdownMenuItems,
    onChanged: (value) {
        setState((){
            _selectedItem = value;
            selectDivision = value.id;
          	getNews();
        });
    }
 ),