Flutter/Dart Android应用程序NoSuchMethodError

Question

我收到了一个关于我正在制作的一个飞镖/飞镖应用程序的快速问题，该应用程序抛出了这个特定的错误。

这与我的showadddialog类有关。当我在_showAddDialog((中按下文本为"save"的平面按钮时，它工作正常，但如果我在没有输入任何内容的情况下点击警报对话框窗口，或者如果我按下名为"delete"的平面键，我的应用程序会崩溃，两个操作都会给出相同的错误。然而，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，它只是在之后崩溃了。是什么原因导致我的代码出现这种情况？Idk它可能在null上调用映射。。。

屏幕截图参考：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
...
════════════════════════════════════════════════════════════════════════════════

此处编码：

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

我已经完成了您的代码，并按照下面的步骤处理了删除事件null异常
更改您的最后一个setState代码如下：

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

结论：
_selectedEvents空值可以由代码中的?? []处理。