颤振不良状态无元素 [英] Flutter Bad State No Element

问题描述

我正在尝试从数据库中删除我的数据，如果成功则继续导航到我的主页.

I am trying to delete my data from a database, and proceed to navigate to my homepage if it succeeds.

以下是我的代码:

StatelessWidget 由 deleteFromDatabase 方法组成，该方法传递一个 Id(String)，一个 context:

StatelessWidget that consist of deleteFromDatabase method which passed an Id(String), an a context:

Consumer<SettingsProvider>(
      builder: (context, settingsProvider, child) {
        final exerciseSettings = settingsProvider.findById(id);
        if (exerciseSettings == null) {
          return Center(
            child: CircularProgressIndicator(),
          );
        }
        return PreviewExerciseItem(
          exerciseSettings: exerciseSettings,
          id: id,
          navigateToEdit: () =>
              _navigateToEditPage(exerciseSettings, context),
          deleteFromDatabase: () => _deleteFromDatabase(id, context),
          navigateToCountDown: () =>
              navigateToCountDownPage(exerciseSettings, context),
        );
      },
    ),

_deleteFromDatabase 方法从 StatelessWidget 调用并显示 AlertDialog 以确认删除:

_deleteFromDatabase method called from StatelessWidget and shows an AlertDialog to confirm deletion:

void _deleteFromDatabase(String id, context) async {
await showDialog(
  context: context,
  builder: (context) => new AlertDialog(
    title: new Text("Are you sure you want to delete?"),
    actions: <Widget>[
      new FlatButton(
        onPressed: () => Navigator.of(context).pop(false),
        child: new Text('No'),
      ),
      new FlatButton(
        onPressed: () async {
          try {
            Navigator.of(context).pop(true);
            await Provider.of<SettingsProvider>(context, listen: false)
                .deleteFromList(id);
            Navigator
                .pushNamedAndRemoveUntil(context,HomePage.routeName, (Route route) => route.isFirst);
          } catch (e) {
            print(e);
          }
        },
        child: new Text('Yes'),
      ),
    ],
  ),
 );
}

deleteFromList 方法来自 My Provider 类:

deleteFromList method From My Provider class:

Future<void> deleteFromList(String id) async{
try{
  final _itemIndex = _items.indexWhere((item) => item.id == id);
  await _databaseHelper.deleteExercises(id);
  _items.removeAt(_itemIndex);
  notifyListeners();
}catch(e){
  print(e);
 }
}

来自提供者类的 findById:

findById from Provider Class:

CustomExercise findById(String id) {
  return _items.firstWhere((prod) => prod.id == id);
}

注意:我可以成功地从我的数据库中删除我的数据，但是就在它导航到我的主页之前，一瞬间会以红屏的形式弹出错误:错误状态:无元素

Note: I am able to delete my data successfully from my database, however right before it navigates to my HomePage, an error pops out for a split second as a form of Red Screen: Bad State: No Element

以下是我的日志中的完整错误消息:

Below are the full error message from my Log:

以下 StateError 在构建 Consumer(dirty, dependencies: [_InheritedProviderScope, _InheritedTheme, _LocalizationsScope-[GlobalKey#5ce12]]) 时被抛出:错误状态:没有元素

The following StateError was thrown building Consumer(dirty, dependencies: [_InheritedProviderScope, _InheritedTheme, _LocalizationsScope-[GlobalKey#5ce12]]): Bad state: No element

相关的导致错误的小部件是:

The relevant error-causing widget was:

Consumer<SettingsProvider>

当抛出异常时，这是堆栈:

When the exception was thrown, this was the stack:

#0 ListMixin.firstWhere (dart:collection/list.dart:150:5)

#0 ListMixin.firstWhere (dart:collection/list.dart:150:5)

#1 SettingsProvider.findById (package:workoutapp/providers/settings_provider.dart:12:19)

#1 SettingsProvider.findById (package:workoutapp/providers/settings_provider.dart:12:19)

#2 PreviewExercisePage.build.<匿名闭包>(package:workoutapp/pages/preview_exercise_page.dart:68:55)

#2 PreviewExercisePage.build.<anonymous closure> (package:workoutapp/pages/preview_exercise_page.dart:68:55)

#3 Consumer.buildWithChild (package:provider/src/consumer.dart:175:19)

#3 Consumer.buildWithChild (package:provider/src/consumer.dart:175:19)

#4 SingleChildStatelessWidget.build (package:nested/nested.dart:260:41)

#4 SingleChildStatelessWidget.build (package:nested/nested.dart:260:41)

推荐答案

当列表为空或者第一个元素为空时会发生这种情况，因此您应该检查列表是否为空.

This is happens when the list is empty or maybe the first element is empty, so you should check the list is not empty.

List list = [];

print(list[0])

确定您会收到这样的消息:

is sure you'll receive like this message:

Unhandled exception:
Bad state: No element
#0      List.first (dart:core-patch/growable_array.dart:332:5)
#1      main (file:///C:/Users/onbody/AndroidStudioProjects/nmae/bin/name.dart:9:14)
#2      _delayEntrypointInvocation.<anonymous closure> (dart:isolate-patch/isolate_patch.dart:281:32)
#3      _RawReceivePortImpl._handleMessage (dart:isolate-patch/isolate_patch.dart:184:12)

解决办法是:

List list = [];

if (list.isNotEmpty) {
    print(list[0]);
  } else {
    print('the list is empty'!);
  }

我希望这对某人有帮助谢谢！

I hope this is helpful for someone Thanks!

这篇关于颤振不良状态无元素的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！