" '_children[j] is MaterialGap': is not true " when updating state of ExpansionPanelList #128646
Description
Is there an existing issue for this?
- I have searched the existing issues
- I have read the guide to filing a bug
Steps to reproduce
Even though #13780 has been thoroughly discussed, I am still experiencing a crash when adding/removing ExpansionPanels to an ExpansionPanelList.
To reproduce in the minimal working example:
- Expand Panel 1 so you see "This is item number 1"
- Click the buttons 1, 2 & 3 in order
Expected results
Button 1 removes the first 2 list entries.
Button 2 adds them back and Panel 1 retains its expanded state.
Button 3 removes the last list entry.
Actual results
Button 3 causes an assertion error '_children[j] is MaterialGap': is not true
Code sample
The error can be reproduced using both ExpansionPanelList and ExpansionPanelList.radio
Code sample with ExpansionPanelList
import 'package:flutter/material.dart';
/// Flutter code sample for [ExpansionPanelList].
void main() => runApp(const MyApp());
class MyApp extends StatelessWidget {
const MyApp({super.key});
@override
Widget build(BuildContext context) {
return MaterialApp(
home: Scaffold(
appBar: AppBar(title: const Text('ExpansionPanelList Sample')),
body: const ExpansionPanelListExample(),
),
);
}
}
// stores ExpansionPanel state information
class Item {
Item({
required this.expandedValue,
required this.headerValue,
});
String expandedValue;
String headerValue;
}
List<Item> generateAllPanels() {
return List<Item>.generate(3, (int index) {
return Item(
headerValue: 'Panel $index',
expandedValue: 'This is item number $index',
);
});
}
class ExpansionPanelListExample extends StatefulWidget {
const ExpansionPanelListExample({super.key});
@override
State<ExpansionPanelListExample> createState() =>
_ExpansionPanelListExampleState();
}
class _ExpansionPanelListExampleState extends State<ExpansionPanelListExample> {
final List<Item> _data = generateAllPanels();
List<String> expandedPanels = [];
void removePanel0_1() {
_data.removeRange(0, 2);
}
void readdPanel0_1() {
_data.insert(
0,
Item(
headerValue: 'Panel 1',
expandedValue: 'This is item number 1',
),
);
_data.insert(
0,
Item(
headerValue: 'Panel 0',
expandedValue: 'This is item number 0',
),
);
}
void removePanel2() {
_data.removeLast();
}
@override
Widget build(BuildContext context) {
return SingleChildScrollView(
child: Container(
child: Column(
children: [
ElevatedButton(
onPressed: () {
setState(() {
removePanel0_1();
});
},
child: const Text('Button 1 (removes panel 0+1)'),
),
ElevatedButton(
onPressed: () {
setState(() {
readdPanel0_1();
});
},
child: const Text('Button 2 (re-adds panel 0+1)'),
),
ElevatedButton(
onPressed: () {
setState(() {
removePanel2();
});
},
child: const Text('Button 3 (removes panel 2)'),
),
_buildPanel(),
],
),
),
);
}
Widget _buildPanel() {
return ExpansionPanelList(
expansionCallback: (int index, bool isExpanded) {
setState(() {
if (expandedPanels.contains(_data[index].headerValue)) {
expandedPanels.remove(_data[index].headerValue);
} else {
expandedPanels.add(_data[index].headerValue);
}
});
},
children: _data.map<ExpansionPanel>((Item item) {
return ExpansionPanel(
canTapOnHeader: true,
//value: item.headerValue,
isExpanded: expandedPanels.contains(item.headerValue),
headerBuilder: (BuildContext context, bool isExpanded) {
return ListTile(
title: Text(item.headerValue),
);
},
body: ListTile(
title: Text(item.expandedValue),
),
);
}).toList(),
);
}
}
Code sample with ExpansionPanelList.radio
import 'package:flutter/material.dart';
/// Flutter code sample for [ExpansionPanelList].
void main() => runApp(const MyApp());
class MyApp extends StatelessWidget {
const MyApp({super.key});
@override
Widget build(BuildContext context) {
return MaterialApp(
home: Scaffold(
appBar: AppBar(title: const Text('ExpansionPanelList Sample')),
body: const ExpansionPanelListExample(),
),
);
}
}
// stores ExpansionPanel state information
class Item {
Item({
required this.expandedValue,
required this.headerValue,
});
String expandedValue;
String headerValue;
}
List<Item> generateAllPanels() {
return List<Item>.generate(3, (int index) {
return Item(
headerValue: 'Panel $index',
expandedValue: 'This is item number $index',
);
});
}
class ExpansionPanelListExample extends StatefulWidget {
const ExpansionPanelListExample({super.key});
@override
State<ExpansionPanelListExample> createState() =>
_ExpansionPanelListExampleState();
}
class _ExpansionPanelListExampleState extends State<ExpansionPanelListExample> {
final List<Item> _data = generateAllPanels();
void removePanel0_1() {
_data.removeRange(0, 2);
}
void readdPanel0_1() {
_data.insert(
0,
Item(
headerValue: 'Panel 1',
expandedValue: 'This is item number 1',
),
);
_data.insert(
0,
Item(
headerValue: 'Panel 0',
expandedValue: 'This is item number 0',
),
);
}
void removePanel2() {
_data.removeLast();
}
@override
Widget build(BuildContext context) {
return SingleChildScrollView(
child: Container(
child: Column(
children: [
ElevatedButton(
onPressed: () {
setState(() {
removePanel0_1();
});
},
child: const Text('Button 1 (removes panel 0+1)'),
),
ElevatedButton(
onPressed: () {
setState(() {
readdPanel0_1();
});
},
child: const Text('Button 2 (re-adds panel 0+1)'),
),
ElevatedButton(
onPressed: () {
setState(() {
removePanel2();
});
},
child: const Text('Button 3 (removes panel 2)'),
),
_buildPanel(),
],
),
),
);
}
Widget _buildPanel() {
return ExpansionPanelList.radio(
children: _data.map<ExpansionPanel>((Item item) {
return ExpansionPanelRadio(
canTapOnHeader: true,
value: item.headerValue,
headerBuilder: (BuildContext context, bool isExpanded) {
return ListTile(
title: Text(item.headerValue),
);
},
body: ListTile(
title: Text(item.expandedValue),
),
);
}).toList(),
);
}
}
Screenshots or Video
Screenshots / Video demonstration
2023-06-11.03-12-09.mp4
Logs
Assertion error from this line
Logs
══╡ EXCEPTION CAUGHT BY WIDGETS LIBRARY ╞═══════════════════════════════════════════════════════════
The following assertion was thrown building ExpansionPanelList(state:
_ExpansionPanelListState#421e4):
'package:flutter/src/material/mergeable_material.dart': Failed assertion: line 462 pos 18:
'_children[j] is MaterialGap': is not true.
Either the assertion indicates an error in the framework itself, or we should provide substantially
more information in this error message to help you determine and fix the underlying cause.
In either case, please report this assertion by filing a bug on GitHub:
https://github.com/flutter/flutter/issues/new?template=2_bug.md
The relevant error-causing widget was:
ExpansionPanelList
ExpansionPanelList:file:///C:/Users/Simon/Documents/flutter_samples/bug_expansion_panel_list/lib/main.dart:116:12
When the exception was thrown, this was the stack:
#2 _MergeableMaterialState.didUpdateWidget (package:flutter/src/material/mergeable_material.dart:462:18)
#3 StatefulElement.update (package:flutter/src/widgets/framework.dart:5260:55)
#4 Element.updateChild (package:flutter/src/widgets/framework.dart:3686:15)
#5 ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:5111:16)
#6 StatefulElement.performRebuild (package:flutter/src/widgets/framework.dart:5251:11)
#7 Element.rebuild (package:flutter/src/widgets/framework.dart:4805:7)
#8 StatefulElement.update (package:flutter/src/widgets/framework.dart:5274:5)
#9 Element.updateChild (package:flutter/src/widgets/framework.dart:3686:15)
#10 RenderObjectElement.updateChildren (package:flutter/src/widgets/framework.dart:6093:32)
#11 MultiChildRenderObjectElement.update (package:flutter/src/widgets/framework.dart:6595:17)
#12 Element.updateChild (package:flutter/src/widgets/framework.dart:3686:15)
#13 ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:5111:16)
#14 Element.rebuild (package:flutter/src/widgets/framework.dart:4805:7)
#15 StatelessElement.update (package:flutter/src/widgets/framework.dart:5162:5)
#16 Element.updateChild (package:flutter/src/widgets/framework.dart:3686:15)
#17 SingleChildRenderObjectElement.update (package:flutter/src/widgets/framework.dart:6442:14)
#18 Element.updateChild (package:flutter/src/widgets/framework.dart:3686:15)
#19 SingleChildRenderObjectElement.update (package:flutter/src/widgets/framework.dart:6442:14)
#20 Element.updateChild (package:flutter/src/widgets/framework.dart:3686:15)
#21 SingleChildRenderObjectElement.update (package:flutter/src/widgets/framework.dart:6442:14)
#22 Element.updateChild (package:flutter/src/widgets/framework.dart:3686:15)
#23 SingleChildRenderObjectElement.update (package:flutter/src/widgets/framework.dart:6442:14)
#24 Element.updateChild (package:flutter/src/widgets/framework.dart:3686:15)
#25 SingleChildRenderObjectElement.update (package:flutter/src/widgets/framework.dart:6442:14)
#26 Element.updateChild (package:flutter/src/widgets/framework.dart:3686:15)
#27 ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:5111:16)
#28 StatefulElement.performRebuild (package:flutter/src/widgets/framework.dart:5251:11)
#29 Element.rebuild (package:flutter/src/widgets/framework.dart:4805:7)
#30 StatefulElement.update (package:flutter/src/widgets/framework.dart:5274:5)
#31 Element.updateChild (package:flutter/src/widgets/framework.dart:3686:15)
#32 SingleChildRenderObjectElement.update (package:flutter/src/widgets/framework.dart:6442:14)
#33 Element.updateChild (package:flutter/src/widgets/framework.dart:3686:15)
#34 ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:5111:16)
#35 Element.rebuild (package:flutter/src/widgets/framework.dart:4805:7)
#36 ProxyElement.update (package:flutter/src/widgets/framework.dart:5417:5)
#37 Element.updateChild (package:flutter/src/widgets/framework.dart:3686:15)
#38 SingleChildRenderObjectElement.update (package:flutter/src/widgets/framework.dart:6442:14)
#39 Element.updateChild (package:flutter/src/widgets/framework.dart:3686:15)
#40 ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:5111:16)
#41 Element.rebuild (package:flutter/src/widgets/framework.dart:4805:7)
#42 ProxyElement.update (package:flutter/src/widgets/framework.dart:5417:5)
#43 Element.updateChild (package:flutter/src/widgets/framework.dart:3686:15)
#44 SingleChildRenderObjectElement.update (package:flutter/src/widgets/framework.dart:6442:14)
#45 Element.updateChild (package:flutter/src/widgets/framework.dart:3686:15)
#46 SingleChildRenderObjectElement.update (package:flutter/src/widgets/framework.dart:6442:14)
#47 Element.updateChild (package:flutter/src/widgets/framework.dart:3686:15)
#48 SingleChildRenderObjectElement.update (package:flutter/src/widgets/framework.dart:6442:14)
#49 Element.updateChild (package:flutter/src/widgets/framework.dart:3686:15)
#50 SingleChildRenderObjectElement.update (package:flutter/src/widgets/framework.dart:6442:14)
#51 Element.updateChild (package:flutter/src/widgets/framework.dart:3686:15)
#52 SingleChildRenderObjectElement.update (package:flutter/src/widgets/framework.dart:6442:14)
#53 Element.updateChild (package:flutter/src/widgets/framework.dart:3686:15)
#54 ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:5111:16)
#55 StatefulElement.performRebuild (package:flutter/src/widgets/framework.dart:5251:11)
#56 Element.rebuild (package:flutter/src/widgets/framework.dart:4805:7)
#57 StatefulElement.update (package:flutter/src/widgets/framework.dart:5274:5)
#58 Element.updateChild (package:flutter/src/widgets/framework.dart:3686:15)
#59 SingleChildRenderObjectElement.update (package:flutter/src/widgets/framework.dart:6442:14)
#60 Element.updateChild (package:flutter/src/widgets/framework.dart:3686:15)
#61 SingleChildRenderObjectElement.update (package:flutter/src/widgets/framework.dart:6442:14)
#62 Element.updateChild (package:flutter/src/widgets/framework.dart:3686:15)
#63 ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:5111:16)
#64 Element.rebuild (package:flutter/src/widgets/framework.dart:4805:7)
#65 ProxyElement.update (package:flutter/src/widgets/framework.dart:5417:5)
#66 Element.updateChild (package:flutter/src/widgets/framework.dart:3686:15)
#67 ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:5111:16)
#68 Element.rebuild (package:flutter/src/widgets/framework.dart:4805:7)
#69 ProxyElement.update (package:flutter/src/widgets/framework.dart:5417:5)
#70 Element.updateChild (package:flutter/src/widgets/framework.dart:3686:15)
#71 ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:5111:16)
#72 StatefulElement.performRebuild (package:flutter/src/widgets/framework.dart:5251:11)
#73 Element.rebuild (package:flutter/src/widgets/framework.dart:4805:7)
#74 StatefulElement.update (package:flutter/src/widgets/framework.dart:5274:5)
#75 Element.updateChild (package:flutter/src/widgets/framework.dart:3686:15)
#76 ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:5111:16)
#77 Element.rebuild (package:flutter/src/widgets/framework.dart:4805:7)
#78 StatelessElement.update (package:flutter/src/widgets/framework.dart:5162:5)
#79 Element.updateChild (package:flutter/src/widgets/framework.dart:3686:15)
#80 ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:5111:16)
#81 StatefulElement.performRebuild (package:flutter/src/widgets/framework.dart:5251:11)
#82 Element.rebuild (package:flutter/src/widgets/framework.dart:4805:7)
#83 StatefulElement.update (package:flutter/src/widgets/framework.dart:5274:5)
#84 Element.updateChild (package:flutter/src/widgets/framework.dart:3686:15)
#85 ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:5111:16)
#86 Element.rebuild (package:flutter/src/widgets/framework.dart:4805:7)
#87 StatelessElement.update (package:flutter/src/widgets/framework.dart:5162:5)
#88 Element.updateChild (package:flutter/src/widgets/framework.dart:3686:15)
#89 ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:5111:16)
#90 StatefulElement.performRebuild (package:flutter/src/widgets/framework.dart:5251:11)
#91 Element.rebuild (package:flutter/src/widgets/framework.dart:4805:7)
#92 BuildOwner.buildScope (package:flutter/src/widgets/framework.dart:2780:19)
#93 WidgetsBinding.drawFrame (package:flutter/src/widgets/binding.dart:903:21)
#94 RendererBinding._handlePersistentFrameCallback (package:flutter/src/rendering/binding.dart:358:5)
#95 SchedulerBinding._invokeFrameCallback (package:flutter/src/scheduler/binding.dart:1284:15)
#96 SchedulerBinding.handleDrawFrame (package:flutter/src/scheduler/binding.dart:1214:9)
#97 SchedulerBinding._handleDrawFrame (package:flutter/src/scheduler/binding.dart:1072:5)
#98 _invoke (dart:ui/hooks.dart:142:13)
#99 PlatformDispatcher._drawFrame (dart:ui/platform_dispatcher.dart:359:5)
#100 _drawFrame (dart:ui/hooks.dart:112:31)
(elided 2 frames from class _AssertionError)
════════════════════════════════════════════════════════════════════════════════════════════════════Flutter Doctor output
Doctor output
Doctor summary (to see all details, run flutter doctor -v):
[√] Flutter (Channel stable, 3.10.2, on Microsoft Windows [Version 10.0.19045.2965], locale de-DE)
[√] Windows Version (Installed version of Windows is version 10 or higher)
[√] Android toolchain - develop for Android devices (Android SDK version 33.0.2)
[√] Chrome - develop for the web
[√] Visual Studio - develop for Windows (Visual Studio Community 2022 17.5.5)
[√] Android Studio (version 2022.2)
[√] IntelliJ IDEA Community Edition (version 2023.1)
[√] VS Code (version 1.79.0)
[√] Connected device (3 available)
[√] Network resources
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