运行 时抛出 NoSuchMethodError。在 null 上调用了方法“[]”。接收者：无。尝试调用：[](0)。如何解决这个错误？
NoSuchMethodError was thrown while running. The method '[]' was called on null. Receiver: null. Tried calling: [](0). How to solve this error?
我正在使用 Flutter-SQLite-Dart 开发移动应用程序。这个应用程序基本上是一个测验应用程序。该应用程序运行良好，但最初 运行 弹出特定错误，然后按编码恢复。
从模型中调用问题索引时出现以下错误，然后应用程序正常运行。
只有第一次运行出现这个错误。据我所知，发生此错误是因为在初始化之前调用了此状态，但我已在 initState() 函数中对其进行了正确初始化。
======== Exception caught by widgets library =======================================================
The following NoSuchMethodError was thrown building:
The method '[]' was called on null.
Receiver: null
Tried calling: [](0)
When the exception was thrown, this was the stack:
#0 Object.noSuchMethod (dart:core-patch/object_patch.dart:68:5)
#1 _QuizPageState.buildNotes.<anonymous closure> (package:loginform_validation_demo/QuizApp/screens/quiz/quiz_screen_stateful.dart:97:28)
#2 SliverChildBuilderDelegate.build (package:flutter/src/widgets/sliver.dart:456:22)
#3 SliverMultiBoxAdaptorElement._build (package:flutter/src/widgets/sliver.dart:1214:28)
#4 SliverMultiBoxAdaptorElement.createChild.<anonymous closure> (package:flutter/src/widgets/sliver.dart:1227:55)
#5 BuildOwner.buildScope (package:flutter/src/widgets/framework.dart:2531:19)
#6 SliverMultiBoxAdaptorElement.createChild (package:flutter/src/widgets/sliver.dart:1220:12)
#7 RenderSliverMultiBoxAdaptor._createOrObtainChild.<anonymous closure> (package:flutter/src/rendering/sliver_multi_box_adaptor.dart:349:23)
#8 RenderObject.invokeLayoutCallback.<anonymous closure> (package:flutter/src/rendering/object.dart:1968:59)
#9 PipelineOwner._enableMutationsToDirtySubtrees (package:flutter/src/rendering/object.dart:916:15)
#10 RenderObject.invokeLayoutCallback (package:flutter/src/rendering/object.dart:1968:14)
#11 RenderSliverMultiBoxAdaptor._createOrObtainChild (package:flutter/src/rendering/sliver_multi_box_adaptor.dart:338:5)
#12 RenderSliverMultiBoxAdaptor.addInitialChild (package:flutter/src/rendering/sliver_multi_box_adaptor.dart:422:5)
#13 RenderSliverFixedExtentBoxAdaptor.performLayout (package:flutter/src/rendering/sliver_fixed_extent_list.dart:205:12)
#14 RenderObject.layout (package:flutter/src/rendering/object.dart:1858:7)
#15 RenderSliverEdgeInsetsPadding.performLayout (package:flutter/src/rendering/sliver_padding.dart:137:12)
#16 _RenderSliverFractionalPadding.performLayout (package:flutter/src/widgets/sliver_fill.dart:167:11)
#17 RenderObject.layout (package:flutter/src/rendering/object.dart:1858:7)
#18 RenderViewportBase.layoutChildSequence (package:flutter/src/rendering/viewport.dart:510:13)
#19 RenderViewport._attemptLayout (package:flutter/src/rendering/viewport.dart:1580:12)
#20 RenderViewport.performLayout (package:flutter/src/rendering/viewport.dart:1489:20)
#21 RenderObject.layout (package:flutter/src/rendering/object.dart:1858:7)
#22 RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:116:14)
#23 RenderObject.layout (package:flutter/src/rendering/object.dart:1858:7)
#24 RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:116:14)
#25 RenderObject.layout (package:flutter/src/rendering/object.dart:1858:7)
#26 RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:116:14)
#27 RenderObject.layout (package:flutter/src/rendering/object.dart:1858:7)
#28 RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:116:14)
#29 RenderObject.layout (package:flutter/src/rendering/object.dart:1858:7)
#30 RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:116:14)
#31 RenderObject.layout (package:flutter/src/rendering/object.dart:1858:7)
#32 RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:116:14)
#33 RenderObject.layout (package:flutter/src/rendering/object.dart:1858:7)
#34 RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:116:14)
#35 RenderObject.layout (package:flutter/src/rendering/object.dart:1858:7)
#36 RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:116:14)
#37 RenderCustomPaint.performLayout (package:flutter/src/rendering/custom_paint.dart:545:11)
#38 RenderObject.layout (package:flutter/src/rendering/object.dart:1858:7)
#39 RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:116:14)
#40 RenderObject.layout (package:flutter/src/rendering/object.dart:1858:7)
#41 RenderPositionedBox.performLayout (package:flutter/src/rendering/shifted_box.dart:437:14)
#42 RenderObject.layout (package:flutter/src/rendering/object.dart:1858:7)
#43 MultiChildLayoutDelegate.layoutChild (package:flutter/src/rendering/custom_layout.dart:171:12)
#44 _ScaffoldLayout.performLayout (package:flutter/src/material/scaffold.dart:983:7)
#45 MultiChildLayoutDelegate._callPerformLayout (package:flutter/src/rendering/custom_layout.dart:240:7)
#46 RenderCustomMultiChildLayoutBox.performLayout (package:flutter/src/rendering/custom_layout.dart:404:14)
#47 RenderObject.layout (package:flutter/src/rendering/object.dart:1858:7)
#48 RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:116:14)
#49 RenderObject.layout (package:flutter/src/rendering/object.dart:1858:7)
#50 RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:116:14)
#51 _RenderCustomClip.performLayout (package:flutter/src/rendering/proxy_box.dart:1376:11)
#52 RenderObject.layout (package:flutter/src/rendering/object.dart:1858:7)
#53 RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:116:14)
#54 RenderObject.layout (package:flutter/src/rendering/object.dart:1858:7)
#55 RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:116:14)
#56 RenderObject.layout (package:flutter/src/rendering/object.dart:1858:7)
#57 RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:116:14)
#58 RenderObject.layout (package:flutter/src/rendering/object.dart:1858:7)
#59 RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:116:14)
#60 RenderObject.layout (package:flutter/src/rendering/object.dart:1858:7)
#61 RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:116:14)
#62 RenderObject.layout (package:flutter/src/rendering/object.dart:1858:7)
#63 RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:116:14)
#64 RenderObject.layout (package:flutter/src/rendering/object.dart:1858:7)
#65 RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:116:14)
#66 RenderObject.layout (package:flutter/src/rendering/object.dart:1858:7)
#67 RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:116:14)
#68 RenderObject.layout (package:flutter/src/rendering/object.dart:1858:7)
#69 RenderOffstage.performLayout (package:flutter/src/rendering/proxy_box.dart:3408:14)
#70 RenderObject.layout (package:flutter/src/rendering/object.dart:1858:7)
#71 RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:116:14)
#72 RenderObject.layout (package:flutter/src/rendering/object.dart:1858:7)
#73 _RenderTheatre.performLayout (package:flutter/src/widgets/overlay.dart:751:15)
#74 RenderObject._layoutWithoutResize (package:flutter/src/rendering/object.dart:1713:7)
#75 PipelineOwner.flushLayout (package:flutter/src/rendering/object.dart:885:18)
#76 RendererBinding.drawFrame (package:flutter/src/rendering/binding.dart:453:19)
#77 WidgetsBinding.drawFrame (package:flutter/src/widgets/binding.dart:883:13)
#78 RendererBinding._handlePersistentFrameCallback (package:flutter/src/rendering/binding.dart:319:5)
#79 SchedulerBinding._invokeFrameCallback (package:flutter/src/scheduler/binding.dart:1145:15)
#80 SchedulerBinding.handleDrawFrame (package:flutter/src/scheduler/binding.dart:1082:9)
#81 SchedulerBinding._handleDrawFrame (package:flutter/src/scheduler/binding.dart:996:5)
#85 _invoke (dart:ui/hooks.dart:166:10)
#86 PlatformDispatcher._drawFrame (dart:ui/platform_dispatcher.dart:270:5)
#87 _drawFrame (dart:ui/hooks.dart:129:31)
(elided 3 frames from dart:async)
====================================================================================================
测验屏幕如下所示，
错误发生的地方在buildNotes() widget下的itemBuilder中，
itemBuilder：（上下文，索引）{
//Error occurring place
note = question[index];
.
.
}
仅在第一次编译时打印此错误。
import 'package:flutter/material.dart';
import 'package:get/get.dart';
import 'package:loginform_validation_demo/QuizApp/controllers/question_controller_copy.dart';
import 'package:loginform_validation_demo/QuizApp/database/quiz_database.dart';
import 'package:loginform_validation_demo/QuizApp/models/Album.dart';
class QuizPage extends StatefulWidget {
final BlazorQuiz quiz;
const QuizPage({Key key, this.quiz}) : super(key: key);
@override
_QuizPageState createState() => _QuizPageState();
}
class _QuizPageState extends State<QuizPage> {
List<BlazorQuiz> question;
List<BlazorQuiz> answer;
bool isLoading = false;
String title;
BlazorQuiz note;
@override
void initState() {
super.initState();
note = question as BlazorQuiz;
this.title = widget.quiz?.Question ?? 'Question';
refreshNotesBlazor();
}
@override
void dispose() {
debugPrint(" QuizCheckModel dispose called ");
QuizDatabase.instance.close();
super.dispose();
}
Future refreshNotesBlazor() async {
setState(() => isLoading = true);
this.question = (await QuizDatabase.instance.readAllNotesBlazor());
this.answer = (await QuizDatabase.instance.readAllNotesBlazorAnswer());
setState(() => isLoading = false);
debugPrint(
" Blazor QuizCheckModel refreshNotes Question : ${this.question}");
debugPrint(" Blazor QuizCheckModel refreshNotes Answer :
${this.answer}");
}
@override
Widget build(BuildContext context) => Scaffold(
appBar: AppBar(
title: Text(
'Quiz App',
style: TextStyle(fontSize: 24),
),
automaticallyImplyLeading: false,
actions: [Icon(Icons.search), SizedBox(width: 12)],
),
body: Center(child: buildNotes()),
);
QuestionControllerCopy _questionController =
Get.put(QuestionControllerCopy());
Widget buildNotes() => PageView.builder(
physics: NeverScrollableScrollPhysics(),
controller: _questionController.pageController,
itemCount: 18,
itemBuilder: (context, index) {
//Error occurring place
note = question[index];
debugPrint('QuizScreenStateful : $question');
debugPrint('QuizScreenStateful index : $index');
debugPrint('QuizScreenStateful question-index : ${question[index]}');
return SafeArea(
child: Column(
children: [
if (question[index].QnType == 'Statement') ...[
Expanded(
child: SingleChildScrollView(
scrollDirection: Axis.vertical,
child: QuestionCardWidgetStart(
note: note,
index: index,
questionLength: question.length),
),
),
] else if (question[index].QnType == 'Radio') ...[
Expanded(
child: SingleChildScrollView(
scrollDirection: Axis.vertical,
child: QuestionCardWidgetRadio(
note: note,
index: index,
questionLength: question.length),
),
),
] else if (question[index].QnType == 'Check') ...[
Expanded(
child: SingleChildScrollView(
scrollDirection: Axis.vertical,
child: QuestionCardWidgetCheck(
note: note,
index: index,
questionLength: question.length),
),
),
] else if (question[index].QnType == 'Drop') ...[
Expanded(
child: SingleChildScrollView(
scrollDirection: Axis.vertical,
child: QuestionCardWidgetDropdown(
note: note,
index: index,
questionLength: question.length),
),
),
]
else if (question[index].QnType == 'EditText') ...[
Expanded(
child: SingleChildScrollView(
scrollDirection: Axis.vertical,
child: QuestionCardWidgetEditText(
note: note,
index: index,
questionLength: question.length),
),
),
]
else if (question[index].QnType == 'End') ...[
Expanded(
child: SingleChildScrollView(
scrollDirection: Axis.vertical,
child: QuestionCardWidgetEnd(
note: note,
index: index,
questionLength: question.length),
),
),
] else ...[
Expanded(
child: SingleChildScrollView(
scrollDirection: Axis.vertical,
child: QuestionCardWidget(
note: note,
index: index,
questionLength: question.length),
),
),
]
],
),
);
},
);
}
note = question[index];
您的 question 变量是 null。为什么？好吧，我看不到一行代码将其设置为其他内容，所以它仍然是 null.
所以有两条评论来解决这个问题：
使用当前版本的 Flutter。您使用的版本已过时。当前版本不会在不警告您错误的情况下允许这样做。
确保您的 question 变量填充了不为空的内容。
只需像这样更改您的 Future 函数：
Future refreshNotesBlazor() async {
setState(() => isLoading = true);
final question = (await QuizDatabase.instance.readAllNotesBlazor());
setState(() => isLoading = false);
debugPrint(
" Blazor QuizCheckModel refreshNotes Question : ${this.question}");
return question;
}
Future refreshNotesBlazor2() async {
setState(() => isLoading = true);
final answer = (await QuizDatabase.instance.readAllNotesBlazor());
setState(() => isLoading = false);
debugPrint(" Blazor QuizCheckModel refreshNotes Answer :
${this.answer}");
return answer;
}
并将其用于您的屏幕：
Widget buildNotes() => FutureBuilder(
future: refreshNotesBlazor(),
builder: (context, snapshot) => snapshot.hasData ? PageView.builder(
physics: NeverScrollableScrollPhysics(),
controller: _questionController.pageController,
itemCount: 18,
itemBuilder: (context, index) {
//Error occurring place
note = question[index];
debugPrint('QuizScreenStateful : $question');
debugPrint('QuizScreenStateful index : $index');
debugPrint('QuizScreenStateful question-index : ${question[index]}');
return SafeArea(
child: Column(
children: [
if (question[index].QnType == 'Statement') ...[
Expanded(
child: SingleChildScrollView(
scrollDirection: Axis.vertical,
child: QuestionCardWidgetStart(
note: note,
index: index,
questionLength: question.length),
),
),
] else if (question[index].QnType == 'Radio') ...[
Expanded(
child: SingleChildScrollView(
scrollDirection: Axis.vertical,
child: QuestionCardWidgetRadio(
note: note,
index: index,
questionLength: question.length),
),
),
] else if (question[index].QnType == 'Check') ...[
Expanded(
child: SingleChildScrollView(
scrollDirection: Axis.vertical,
child: QuestionCardWidgetCheck(
note: note,
index: index,
questionLength: question.length),
),
),
] else if (question[index].QnType == 'Drop') ...[
Expanded(
child: SingleChildScrollView(
scrollDirection: Axis.vertical,
child: QuestionCardWidgetDropdown(
note: note,
index: index,
questionLength: question.length),
),
),
]
else if (question[index].QnType == 'EditText') ...[
Expanded(
child: SingleChildScrollView(
scrollDirection: Axis.vertical,
child: QuestionCardWidgetEditText(
note: note,
index: index,
questionLength: question.length),
),
),
]
else if (question[index].QnType == 'End') ...[
Expanded(
child: SingleChildScrollView(
scrollDirection: Axis.vertical,
child: QuestionCardWidgetEnd(
note: note,
index: index,
questionLength: question.length),
),
),
] else ...[
Expanded(
child: SingleChildScrollView(
scrollDirection: Axis.vertical,
child: QuestionCardWidget(
note: note,
index: index,
questionLength: question.length),
),
),
]
],
),
);
},
) : Center(child: CircularProgressIndicator(),));
我正在使用 Flutter-SQLite-Dart 开发移动应用程序。这个应用程序基本上是一个测验应用程序。该应用程序运行良好，但最初 运行 弹出特定错误，然后按编码恢复。
从模型中调用问题索引时出现以下错误，然后应用程序正常运行。
只有第一次运行出现这个错误。据我所知，发生此错误是因为在初始化之前调用了此状态，但我已在 initState() 函数中对其进行了正确初始化。
======== Exception caught by widgets library =======================================================
The following NoSuchMethodError was thrown building:
The method '[]' was called on null.
Receiver: null
Tried calling: [](0)
When the exception was thrown, this was the stack:
#0 Object.noSuchMethod (dart:core-patch/object_patch.dart:68:5)
#1 _QuizPageState.buildNotes.<anonymous closure> (package:loginform_validation_demo/QuizApp/screens/quiz/quiz_screen_stateful.dart:97:28)
#2 SliverChildBuilderDelegate.build (package:flutter/src/widgets/sliver.dart:456:22)
#3 SliverMultiBoxAdaptorElement._build (package:flutter/src/widgets/sliver.dart:1214:28)
#4 SliverMultiBoxAdaptorElement.createChild.<anonymous closure> (package:flutter/src/widgets/sliver.dart:1227:55)
#5 BuildOwner.buildScope (package:flutter/src/widgets/framework.dart:2531:19)
#6 SliverMultiBoxAdaptorElement.createChild (package:flutter/src/widgets/sliver.dart:1220:12)
#7 RenderSliverMultiBoxAdaptor._createOrObtainChild.<anonymous closure> (package:flutter/src/rendering/sliver_multi_box_adaptor.dart:349:23)
#8 RenderObject.invokeLayoutCallback.<anonymous closure> (package:flutter/src/rendering/object.dart:1968:59)
#9 PipelineOwner._enableMutationsToDirtySubtrees (package:flutter/src/rendering/object.dart:916:15)
#10 RenderObject.invokeLayoutCallback (package:flutter/src/rendering/object.dart:1968:14)
#11 RenderSliverMultiBoxAdaptor._createOrObtainChild (package:flutter/src/rendering/sliver_multi_box_adaptor.dart:338:5)
#12 RenderSliverMultiBoxAdaptor.addInitialChild (package:flutter/src/rendering/sliver_multi_box_adaptor.dart:422:5)
#13 RenderSliverFixedExtentBoxAdaptor.performLayout (package:flutter/src/rendering/sliver_fixed_extent_list.dart:205:12)
#14 RenderObject.layout (package:flutter/src/rendering/object.dart:1858:7)
#15 RenderSliverEdgeInsetsPadding.performLayout (package:flutter/src/rendering/sliver_padding.dart:137:12)
#16 _RenderSliverFractionalPadding.performLayout (package:flutter/src/widgets/sliver_fill.dart:167:11)
#17 RenderObject.layout (package:flutter/src/rendering/object.dart:1858:7)
#18 RenderViewportBase.layoutChildSequence (package:flutter/src/rendering/viewport.dart:510:13)
#19 RenderViewport._attemptLayout (package:flutter/src/rendering/viewport.dart:1580:12)
#20 RenderViewport.performLayout (package:flutter/src/rendering/viewport.dart:1489:20)
#21 RenderObject.layout (package:flutter/src/rendering/object.dart:1858:7)
#22 RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:116:14)
#23 RenderObject.layout (package:flutter/src/rendering/object.dart:1858:7)
#24 RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:116:14)
#25 RenderObject.layout (package:flutter/src/rendering/object.dart:1858:7)
#26 RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:116:14)
#27 RenderObject.layout (package:flutter/src/rendering/object.dart:1858:7)
#28 RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:116:14)
#29 RenderObject.layout (package:flutter/src/rendering/object.dart:1858:7)
#30 RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:116:14)
#31 RenderObject.layout (package:flutter/src/rendering/object.dart:1858:7)
#32 RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:116:14)
#33 RenderObject.layout (package:flutter/src/rendering/object.dart:1858:7)
#34 RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:116:14)
#35 RenderObject.layout (package:flutter/src/rendering/object.dart:1858:7)
#36 RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:116:14)
#37 RenderCustomPaint.performLayout (package:flutter/src/rendering/custom_paint.dart:545:11)
#38 RenderObject.layout (package:flutter/src/rendering/object.dart:1858:7)
#39 RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:116:14)
#40 RenderObject.layout (package:flutter/src/rendering/object.dart:1858:7)
#41 RenderPositionedBox.performLayout (package:flutter/src/rendering/shifted_box.dart:437:14)
#42 RenderObject.layout (package:flutter/src/rendering/object.dart:1858:7)
#43 MultiChildLayoutDelegate.layoutChild (package:flutter/src/rendering/custom_layout.dart:171:12)
#44 _ScaffoldLayout.performLayout (package:flutter/src/material/scaffold.dart:983:7)
#45 MultiChildLayoutDelegate._callPerformLayout (package:flutter/src/rendering/custom_layout.dart:240:7)
#46 RenderCustomMultiChildLayoutBox.performLayout (package:flutter/src/rendering/custom_layout.dart:404:14)
#47 RenderObject.layout (package:flutter/src/rendering/object.dart:1858:7)
#48 RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:116:14)
#49 RenderObject.layout (package:flutter/src/rendering/object.dart:1858:7)
#50 RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:116:14)
#51 _RenderCustomClip.performLayout (package:flutter/src/rendering/proxy_box.dart:1376:11)
#52 RenderObject.layout (package:flutter/src/rendering/object.dart:1858:7)
#53 RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:116:14)
#54 RenderObject.layout (package:flutter/src/rendering/object.dart:1858:7)
#55 RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:116:14)
#56 RenderObject.layout (package:flutter/src/rendering/object.dart:1858:7)
#57 RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:116:14)
#58 RenderObject.layout (package:flutter/src/rendering/object.dart:1858:7)
#59 RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:116:14)
#60 RenderObject.layout (package:flutter/src/rendering/object.dart:1858:7)
#61 RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:116:14)
#62 RenderObject.layout (package:flutter/src/rendering/object.dart:1858:7)
#63 RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:116:14)
#64 RenderObject.layout (package:flutter/src/rendering/object.dart:1858:7)
#65 RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:116:14)
#66 RenderObject.layout (package:flutter/src/rendering/object.dart:1858:7)
#67 RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:116:14)
#68 RenderObject.layout (package:flutter/src/rendering/object.dart:1858:7)
#69 RenderOffstage.performLayout (package:flutter/src/rendering/proxy_box.dart:3408:14)
#70 RenderObject.layout (package:flutter/src/rendering/object.dart:1858:7)
#71 RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:116:14)
#72 RenderObject.layout (package:flutter/src/rendering/object.dart:1858:7)
#73 _RenderTheatre.performLayout (package:flutter/src/widgets/overlay.dart:751:15)
#74 RenderObject._layoutWithoutResize (package:flutter/src/rendering/object.dart:1713:7)
#75 PipelineOwner.flushLayout (package:flutter/src/rendering/object.dart:885:18)
#76 RendererBinding.drawFrame (package:flutter/src/rendering/binding.dart:453:19)
#77 WidgetsBinding.drawFrame (package:flutter/src/widgets/binding.dart:883:13)
#78 RendererBinding._handlePersistentFrameCallback (package:flutter/src/rendering/binding.dart:319:5)
#79 SchedulerBinding._invokeFrameCallback (package:flutter/src/scheduler/binding.dart:1145:15)
#80 SchedulerBinding.handleDrawFrame (package:flutter/src/scheduler/binding.dart:1082:9)
#81 SchedulerBinding._handleDrawFrame (package:flutter/src/scheduler/binding.dart:996:5)
#85 _invoke (dart:ui/hooks.dart:166:10)
#86 PlatformDispatcher._drawFrame (dart:ui/platform_dispatcher.dart:270:5)
#87 _drawFrame (dart:ui/hooks.dart:129:31)
(elided 3 frames from dart:async)
====================================================================================================
测验屏幕如下所示， 错误发生的地方在buildNotes() widget下的itemBuilder中，
itemBuilder：（上下文，索引）{
//Error occurring place
note = question[index];
.
.
}
仅在第一次编译时打印此错误。
import 'package:flutter/material.dart';
import 'package:get/get.dart';
import 'package:loginform_validation_demo/QuizApp/controllers/question_controller_copy.dart';
import 'package:loginform_validation_demo/QuizApp/database/quiz_database.dart';
import 'package:loginform_validation_demo/QuizApp/models/Album.dart';
class QuizPage extends StatefulWidget {
final BlazorQuiz quiz;
const QuizPage({Key key, this.quiz}) : super(key: key);
@override
_QuizPageState createState() => _QuizPageState();
}
class _QuizPageState extends State<QuizPage> {
List<BlazorQuiz> question;
List<BlazorQuiz> answer;
bool isLoading = false;
String title;
BlazorQuiz note;
@override
void initState() {
super.initState();
note = question as BlazorQuiz;
this.title = widget.quiz?.Question ?? 'Question';
refreshNotesBlazor();
}
@override
void dispose() {
debugPrint(" QuizCheckModel dispose called ");
QuizDatabase.instance.close();
super.dispose();
}
Future refreshNotesBlazor() async {
setState(() => isLoading = true);
this.question = (await QuizDatabase.instance.readAllNotesBlazor());
this.answer = (await QuizDatabase.instance.readAllNotesBlazorAnswer());
setState(() => isLoading = false);
debugPrint(
" Blazor QuizCheckModel refreshNotes Question : ${this.question}");
debugPrint(" Blazor QuizCheckModel refreshNotes Answer :
${this.answer}");
}
@override
Widget build(BuildContext context) => Scaffold(
appBar: AppBar(
title: Text(
'Quiz App',
style: TextStyle(fontSize: 24),
),
automaticallyImplyLeading: false,
actions: [Icon(Icons.search), SizedBox(width: 12)],
),
body: Center(child: buildNotes()),
);
QuestionControllerCopy _questionController =
Get.put(QuestionControllerCopy());
Widget buildNotes() => PageView.builder(
physics: NeverScrollableScrollPhysics(),
controller: _questionController.pageController,
itemCount: 18,
itemBuilder: (context, index) {
//Error occurring place
note = question[index];
debugPrint('QuizScreenStateful : $question');
debugPrint('QuizScreenStateful index : $index');
debugPrint('QuizScreenStateful question-index : ${question[index]}');
return SafeArea(
child: Column(
children: [
if (question[index].QnType == 'Statement') ...[
Expanded(
child: SingleChildScrollView(
scrollDirection: Axis.vertical,
child: QuestionCardWidgetStart(
note: note,
index: index,
questionLength: question.length),
),
),
] else if (question[index].QnType == 'Radio') ...[
Expanded(
child: SingleChildScrollView(
scrollDirection: Axis.vertical,
child: QuestionCardWidgetRadio(
note: note,
index: index,
questionLength: question.length),
),
),
] else if (question[index].QnType == 'Check') ...[
Expanded(
child: SingleChildScrollView(
scrollDirection: Axis.vertical,
child: QuestionCardWidgetCheck(
note: note,
index: index,
questionLength: question.length),
),
),
] else if (question[index].QnType == 'Drop') ...[
Expanded(
child: SingleChildScrollView(
scrollDirection: Axis.vertical,
child: QuestionCardWidgetDropdown(
note: note,
index: index,
questionLength: question.length),
),
),
]
else if (question[index].QnType == 'EditText') ...[
Expanded(
child: SingleChildScrollView(
scrollDirection: Axis.vertical,
child: QuestionCardWidgetEditText(
note: note,
index: index,
questionLength: question.length),
),
),
]
else if (question[index].QnType == 'End') ...[
Expanded(
child: SingleChildScrollView(
scrollDirection: Axis.vertical,
child: QuestionCardWidgetEnd(
note: note,
index: index,
questionLength: question.length),
),
),
] else ...[
Expanded(
child: SingleChildScrollView(
scrollDirection: Axis.vertical,
child: QuestionCardWidget(
note: note,
index: index,
questionLength: question.length),
),
),
]
],
),
);
},
);
}
note = question[index];
您的 question 变量是 null。为什么？好吧，我看不到一行代码将其设置为其他内容，所以它仍然是 null.
所以有两条评论来解决这个问题：
使用当前版本的 Flutter。您使用的版本已过时。当前版本不会在不警告您错误的情况下允许这样做。
确保您的
question变量填充了不为空的内容。
只需像这样更改您的 Future 函数：
Future refreshNotesBlazor() async {
setState(() => isLoading = true);
final question = (await QuizDatabase.instance.readAllNotesBlazor());
setState(() => isLoading = false);
debugPrint(
" Blazor QuizCheckModel refreshNotes Question : ${this.question}");
return question;
}
Future refreshNotesBlazor2() async {
setState(() => isLoading = true);
final answer = (await QuizDatabase.instance.readAllNotesBlazor());
setState(() => isLoading = false);
debugPrint(" Blazor QuizCheckModel refreshNotes Answer :
${this.answer}");
return answer;
}
并将其用于您的屏幕：
Widget buildNotes() => FutureBuilder(
future: refreshNotesBlazor(),
builder: (context, snapshot) => snapshot.hasData ? PageView.builder(
physics: NeverScrollableScrollPhysics(),
controller: _questionController.pageController,
itemCount: 18,
itemBuilder: (context, index) {
//Error occurring place
note = question[index];
debugPrint('QuizScreenStateful : $question');
debugPrint('QuizScreenStateful index : $index');
debugPrint('QuizScreenStateful question-index : ${question[index]}');
return SafeArea(
child: Column(
children: [
if (question[index].QnType == 'Statement') ...[
Expanded(
child: SingleChildScrollView(
scrollDirection: Axis.vertical,
child: QuestionCardWidgetStart(
note: note,
index: index,
questionLength: question.length),
),
),
] else if (question[index].QnType == 'Radio') ...[
Expanded(
child: SingleChildScrollView(
scrollDirection: Axis.vertical,
child: QuestionCardWidgetRadio(
note: note,
index: index,
questionLength: question.length),
),
),
] else if (question[index].QnType == 'Check') ...[
Expanded(
child: SingleChildScrollView(
scrollDirection: Axis.vertical,
child: QuestionCardWidgetCheck(
note: note,
index: index,
questionLength: question.length),
),
),
] else if (question[index].QnType == 'Drop') ...[
Expanded(
child: SingleChildScrollView(
scrollDirection: Axis.vertical,
child: QuestionCardWidgetDropdown(
note: note,
index: index,
questionLength: question.length),
),
),
]
else if (question[index].QnType == 'EditText') ...[
Expanded(
child: SingleChildScrollView(
scrollDirection: Axis.vertical,
child: QuestionCardWidgetEditText(
note: note,
index: index,
questionLength: question.length),
),
),
]
else if (question[index].QnType == 'End') ...[
Expanded(
child: SingleChildScrollView(
scrollDirection: Axis.vertical,
child: QuestionCardWidgetEnd(
note: note,
index: index,
questionLength: question.length),
),
),
] else ...[
Expanded(
child: SingleChildScrollView(
scrollDirection: Axis.vertical,
child: QuestionCardWidget(
note: note,
index: index,
questionLength: question.length),
),
),
]
],
),
);
},
) : Center(child: CircularProgressIndicator(),));