我有一个关于Flutter / Dart应用的快速问题，我正在抛出此特定错误。
它与我的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，但是如果我从警报对话框窗口中轻按而未输入任何内容，或者按下名为“删除”的平面按钮，并且我的应用程序崩溃，则这两个操作都会同样的错误。但是，当我重新启动时，可以看到删除按钮仍然可以从共享首选项中删除事件，此后它只是崩溃了。是什么在我的代码中引起这种情况？ Idk可能在null上调用 map 的地方...
屏幕截图引用: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];
});
}
}
最佳答案
我遍历了您的代码，并按照以下方式处理了delete event null异常。
使用以下命令更改您的最后一个setState代码:
setState(() {
_selectedEvents = _events[_controller.selectedDay] ?? [];
});
结论:
_selectedEvents空值可以由代码中的
?? []处理。关于android - Flutter/Dart Android应用程式NoSuchMethodError，我们在Stack Overflow上找到一个类似的问题：https://stackoverflow.com/questions/61565942/