Flutter/Dart Android 应用 NoSuchMethodError

Question

我有一个关于我正在制作的颤振/飞镖应用程序抛出这个特定错误的快速问题。

这与我的 showadddialog 类有关。当我在 _showAddDialog() 中按下带有文本“保存”的平面按钮时，它可以正常工作，但是如果我在不输入任何内容的情况下轻按警报对话框窗口，或者如果我按下名为“删除”的平面按钮，并且这两个操作都给出了同样的错误。但是，当我重新启动时，我可以看到删除按钮仍然可以从共享首选项中删除事件，然后它就崩溃了。什么可能导致我的代码中出现这种情况？ Idk 它可以在 null 上调用地图...

截图参考：https://gyazo.com/f894ae742ea50cd714026b1bbe753678

════════ Exception caught by widgets library ═══════════════════════════════════
The following NoSuchMethodError was thrown building HomePage(dirty, dependencies: [_LocalizationsScope-[GlobalKey#42494], _InheritedTheme], state: _HomePageState#acde6):
The method 'map' was called on null.
Receiver: null
Tried calling: map<Widget>(Closure: (dynamic) => ListTile)

The relevant error-causing widget was
    HomePage 
package:hello_world/main.dart:16
When the exception was thrown, this was the stack
#0      Object.noSuchMethod  (dart:core-patch/object_patch.dart:53:5)
#1      _HomePageState.build 
package:hello_world/main.dart:135
#2      StatefulElement.build 
package:flutter/…/widgets/framework.dart:4334
#3      ComponentElement.performRebuild 
package:flutter/…/widgets/framework.dart:4223
#4      Element.rebuild 
package:flutter/…/widgets/framework.dart:3947
...
════════════════════════════════════════════════════════════════════════════════

代码在这里：

import 'dart:convert';
import 'package:flutter/material.dart';
import 'package:shared_preferences/shared_preferences.dart';
import 'package:table_calendar/table_calendar.dart';

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Calendar',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: HomePage(),
    );
  }
}

class HomePage extends StatefulWidget {
  @override
  _HomePageState createState() => _HomePageState();
}

class _HomePageState extends State<HomePage> {
  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 = [];
    initPrefs();
  }

  initPrefs() async {
    prefs = await SharedPreferences.getInstance();
    setState(() {
      _events = Map<DateTime, List<dynamic>>.from(
        decodeMap(json.decode(prefs.getString("events") ?? "{}"))
      );
    });
  }

  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.sunday,
              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);
                    } else {
                      _events[_controller.selectedDay] = [
                        _eventController.text
                      ];
                    }
                    prefs.setString("events", json.encode(encodeMap(_events)));
                    _eventController.clear();
                    Navigator.pop(context);
                  },
                ),
                FlatButton(
                  child: Text("Delete Events"),
                  onPressed: () {
                    setState(() {
                      _events.remove(_controller.selectedDay);
                      prefs.setString("events", json.encode(encodeMap(_events)));
                      _eventController.clear();
                      Navigator.pop(context);
                    },
                );
                  }
                )
              ],
        )
      );
    setState(() {
      _selectedEvents = _events[_controller.selectedDay];
    });
  }
}

Answer 1

我已经查看了您的代码，并按照以下方式处理了删除事件 null 异常。
使用以下更改您的最后一个 setState 代码：

setState(() {
   _selectedEvents = _events[_controller.selectedDay] ?? [];
});

结论：
_selectedEvents 空值可以由您的代码中的?? [] 处理。