flutter - NoSuchMethodError:方法 'add'在null上被调用。接收者:null

Question

嗨，我正在一个项目中，我使用table_calender创建事件并将其保存在数据库中，我已经成功添加了数据库，然后在数据库中，当我必须检索它们时，当数据库中只有一个事件时，就会出现问题。根据日历显示制造商，但是当有多个重复的日期时，它应该在该日期再次添加另一个标记，但是它给出了异常(exception)，就是说当我添加另一个时， map 中的键为null在同一日期的标记。
异常(exception)在
_events [mee.dateTime] .add(meetingInfo [i] ['m_agenda']);;

for(int i=0; i<meetingInfo.length;i++)
{
  if (_events[mee.dateTime] != null) 
  {
    dateWithT=meetingInfo[i]['m_date'].replaceAll(new RegExp(r'[^\w\s]+'),'');
    dateWithT=dateWithT.replaceAll(' ', '');
    dateWithT=dateWithT+"000000";
    dateWithT =  dateWithT.substring(0, 8) + 'T' + dateWithT.substring(8);
    mee.dateTime = DateTime.parse(dateWithT);
    _events[mee.dateTime].add(meetingInfo[i]['m_agenda']);
  } 
  else 
  {
    dateWithT=meetingInfo[i]['m_date'].replaceAll(new RegExp(r'[^\w\s]+'),'');
    dateWithT=dateWithT.replaceAll(' ', '');
    dateWithT=dateWithT+"000000";
    dateWithT =  dateWithT.substring(0, 8) + 'T' + dateWithT.substring(8);
    mee.dateTime = DateTime.parse(dateWithT);
    _events[mee.dateTime] = [meetingInfo[i]['m_agenda']];
  }
}

session 箭

import 'package:flutter/material.dart';
import 'package:hexcolor/hexcolor.dart';
import 'package:intl/intl.dart';
import 'notifcation_dialog.dart';
import 'package:shared_preferences/shared_preferences.dart';
import 'package:table_calendar/table_calendar.dart';
import 'dart:convert';
import 'package:http/http.dart' as http;
import 'variables.dart';
meeting mee = new meeting();
class AddMeeting extends StatefulWidget {
  @override
  _AddMeetingeState createState() => _AddMeetingeState();
}

class _AddMeetingeState extends State<AddMeeting> {
  List meetDate;
  CalendarController _controller;
  Map<DateTime, List<dynamic>> _events;
  List<dynamic> _selectedEvents;
  TextEditingController _eventController;
  // SharedPreferences prefs;

  @override
  void initState() {
    super.initState();
    _controller = CalendarController();
    _eventController = TextEditingController();
    _events = {};
    _selectedEvents = [];
    getIdAndGetMeetings();
  }
String dateWithT; 

  String emailFromSharedPref, meetingAdded;
  Map smData;
  List meetingInfo, customerInfo;
  Future getIdAndGetMeetings() async {
    // getting email from SharedPreferences
    SharedPreferences prefs = await SharedPreferences.getInstance();
    emailFromSharedPref = prefs.getString('email');
    // getting saleManager info using email
    var url = "http://..../getSaleMangerDetal.php";
    var response = await http.post(url, body: {
      "m_email": emailFromSharedPref,
    });
    smData = jsonDecode(response.body);
    // getting customer info using saleManagerID
    var customerInfoUrl = "http://..../calender/getCustomerInfo.php";
    var customerInfoResponse = await http.post(customerInfoUrl, body: {
      "sm_id": smData['manager_id'],
    });
    // saving customer info in the list, because there are many dict in the list
    customerInfo = jsonDecode(customerInfoResponse.body);

    // get meetings details from server using saleManager Id
    var getmeetingUrl = "http://..../getMeetings.php";
    var getMeetiongResponse = await http.post(getmeetingUrl, body: {
      "manager_id": smData['manager_id'],
    });
    meetingInfo = jsonDecode(getMeetiongResponse.body);
    print(meetingInfo);




    for(int i=0; i<meetingInfo.length;i++)
    {
      if (_events[mee.dateTime] != null) 
      {
        dateWithT=meetingInfo[i]['m_date'].replaceAll(new RegExp(r'[^\w\s]+'),'');
        dateWithT=dateWithT.replaceAll(' ', '');
        dateWithT=dateWithT+"000000";
        dateWithT =  dateWithT.substring(0, 8) + 'T' + dateWithT.substring(8);
        mee.dateTime = DateTime.parse(dateWithT);
        _events[mee.dateTime].add(meetingInfo[i]['m_agenda']);
      } 
      else 
      {
        dateWithT=meetingInfo[i]['m_date'].replaceAll(new RegExp(r'[^\w\s]+'),'');
        dateWithT=dateWithT.replaceAll(' ', '');
        dateWithT=dateWithT+"000000";
        dateWithT =  dateWithT.substring(0, 8) + 'T' + dateWithT.substring(8);
        mee.dateTime = DateTime.parse(dateWithT);
        _events[mee.dateTime] = [meetingInfo[i]['m_agenda']];
      }
    }


  }

  Map<String, dynamic> encodeMap(Map<DateTime, dynamic> map) {
    Map<String, dynamic> newMap = {};
    map.forEach((key, value) {
      newMap[key.toString()] = map[key];
    });
    return newMap;
  }

  Map<DateTime, dynamic> decodeMap(Map<String, dynamic> map) {
    Map<DateTime, dynamic> newMap = {};
    map.forEach((key, value) {
      newMap[DateTime.parse(key)] = map[key];
    });
    return newMap;
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Flutter Calendar'),
      ),
      body: SingleChildScrollView(
        child: Column(
          crossAxisAlignment: CrossAxisAlignment.start,
          children: <Widget>[
            TableCalendar(
              events: _events,
              // initialCalendarFormat: CalendarFormat.week,
              calendarStyle: CalendarStyle(
                  canEventMarkersOverflow: true,
                  todayColor: Colors.orange,
                  selectedColor: Theme.of(context).primaryColor,
                  todayStyle: TextStyle(
                      fontWeight: FontWeight.bold,
                      fontSize: 18.0,
                      color: Colors.white)),
              headerStyle: HeaderStyle(
                centerHeaderTitle: true,
                formatButtonDecoration: BoxDecoration(
                  color: Colors.orange,
                  borderRadius: BorderRadius.circular(20.0),
                ),
                formatButtonTextStyle: TextStyle(color: Colors.white),
                formatButtonShowsNext: false,
              ),
              startingDayOfWeek: StartingDayOfWeek.monday,
              onDaySelected: (date, events) {
                setState(() {
                  _selectedEvents = events;
                });
              },
              builders: CalendarBuilders(
                selectedDayBuilder: (context, date, events) => Container(
                    margin: const EdgeInsets.all(4.0),
                    alignment: Alignment.center,
                    decoration: BoxDecoration(
                        color: Theme.of(context).primaryColor,
                        borderRadius: BorderRadius.circular(10.0)),
                    child: Text(
                      date.day.toString(),
                      style: TextStyle(color: Colors.white),
                    )),
                todayDayBuilder: (context, date, events) => Container(
                    margin: const EdgeInsets.all(4.0),
                    alignment: Alignment.center,
                    decoration: BoxDecoration(
                        color: Colors.orange,
                        borderRadius: BorderRadius.circular(10.0)),
                    child: Text(
                      date.day.toString(),
                      style: TextStyle(color: Colors.white),
                    )),
              ),
              calendarController: _controller,
            ),
            ..._selectedEvents.map((event) => ListTile(
                  title: Text(event),
                )),
          ],
        ),
      ),
      floatingActionButton: FloatingActionButton(
        child: Icon(Icons.add),
        onPressed: _showAddDialog,
      ),
    );
  }

  _showAddDialog() async {
    await showDialog(
        context: context,
        builder: (context) => AlertDialog(
              content: TextField(
                controller: _eventController,
              ),
              actions: <Widget>[
                FlatButton(
                  child: Text("Save"),
                  onPressed: () 
                  {
                    // if (_eventController.text.isEmpty) return;
                    // if (_events[_controller.selectedDay] != null) {
                    //   _events[_controller.selectedDay]
                    //       .add(_eventController.text);
                      String asd = _controller.selectedDay.toString();
                      meetDate = asd.split(' ');
                      addMeetingToDatabase(meetDate);
                    // } 
                    // else 
                    // {
                    //   // _events[_controller.selectedDay] = [
                    //   //   _eventController.text
                    //   // ];
                    //   String asd = _controller.selectedDay.toString();
                    //   meetDate = asd.split(' ');
                    //   addMeetingToDatabase(meetDate);
                    //   // print(_controller.selectedDay);
                    // }
                    // prefs.setString("events", json.encode(encodeMap(_events)));

                    _eventController.clear();
                    Navigator.pop(context);
                  },
                )
              ],
            ));
    setState(() {
      _selectedEvents = _events[mee.dateTime];
    });
  }

  addMeetingToDatabase(List meetDate2) async {
    // print(meetDate2[0]);
    var getmeetingUrls = "http://..../addMeeting.php";
    var getMeetiongResponses = await http.post(getmeetingUrls, body: {
      "sm_id": smData['manager_id'],
      "c_id": '2',
      "agenda": _eventController.text,
      "date": meetDate2[0],
      // "time":smData['manager_id'],
    });
    meetingAdded = jsonDecode(getMeetiongResponses.body);
    print(meetingAdded);
  }
}

Answer 1

似乎您正在尝试在Map中不存在的对象上调用方法。

 mee.dateTime = DateTime.parse(dateWithT);
 _events[mee.dateTime].add(meetingInfo[i]['m_agenda']);

在上面的代码中，如果 map 中不存在 mee.dateTime，则返回 null，这当前导致您的异常。

现在，您有一张支票

if (_events[mee.dateTime] != null) {
...
   mee.dateTime = DateTime.parse(dateWithT);
   _events[mee.dateTime].add(meetingInfo[i]['m_agenda']);
}

因此，基本上，进入此部分之前在 map 中检查过的 dateTime现在会在块内更新。现在，您不检查 map 中是否存在新的dateTime。

您的代码中存在逻辑问题。