My Flutter application is throwing an exception (ScrollController attached to multiple scroll views) when navigating away from a page that has a ScrollController to control a NestedScrollView and I am not sure what I am doing wrong.
I have recreated the exception with a simple example below. I can navigate from FirstPage to SecondPage (and optionally back) just fine, but when I navigate from SecondPage to ThirdPage the exception is thrown. The SecondPage is the page that contains my custom CollapsingAppBarPage widget which I assume has the problem. CollapsingAppBarPage is simplified in this example, but in my real app it changes colors/sizes of components based on the scroll position. In this example, the exception happens when _scrollController.offset is called as the navigation begins to the ThirdPage. Also, I know FirstPage and ThirdPage could be Stateless instead of StatefulWidgets but I wanted to make it as much like my app as possible. Here is my fully working example.
import 'package:flutter/material.dart';
void main() => runApp(MyApp());
class MyApp extends StatelessWidget {
@override
Widget build(BuildContext context) {
return MaterialApp(
title: 'Flutter Demo',
home: FirstPage(),
);
}
}
class FirstPage extends StatefulWidget {
@override
_FirstPageState createState() => _FirstPageState();
}
class _FirstPageState extends State<FirstPage> {
@override
Widget build(BuildContext context) {
return Scaffold(
appBar: AppBar(
title: Text("First Page"),
),
body: Center(
child: RaisedButton(
child: Text("Navigate Next"),
onPressed: () async {
await Navigator.push(context,
MaterialPageRoute(builder: (context) => SecondPage())
);
},
),
),
);
}
}
class SecondPage extends StatefulWidget {
@override
_SecondPageState createState() => _SecondPageState();
}
class _SecondPageState extends State<SecondPage> {
@override
Widget build(BuildContext context) {
return Material(
child: CollapsingAppBarPage(
titleText: "Second Page",
bodyCreator: (context) {
return ListView(
children: <Widget>[
Center(
child: RaisedButton(
child: Text("Navigate Next"),
onPressed: () async {
await Navigator.push(context,
MaterialPageRoute(builder: (context) => ThirdPage())
);
},
),
)
],
);
},
),
);
}
}
class ThirdPage extends StatefulWidget {
@override
_ThirdPageState createState() => _ThirdPageState();
}
class _ThirdPageState extends State<ThirdPage> {
@override
Widget build(BuildContext context) {
return Scaffold(
appBar: AppBar(
title: Text("Third Page")
),
body: Container()
);
}
}
typedef CollapsingAppBarBodyCreator = Widget Function(BuildContext context);
class CollapsingAppBarPage extends StatefulWidget {
final String titleText;
final CollapsingAppBarBodyCreator bodyCreator;
CollapsingAppBarPage({
Key key,
this.titleText,
@required this.bodyCreator,
}) : super(key: key);
@override
_CollapsingAppBarPageState createState() => _CollapsingAppBarPageState();
}
class _CollapsingAppBarPageState extends State<CollapsingAppBarPage> {
static const _kExpandedHeight = 200.0;
ScrollController _scrollController;
@override
void initState() {
super.initState();
_scrollController = ScrollController()
..addListener(() {
setState(() {
// force a refresh so the app bar can be updated
});
});
}
@override
void dispose() {
_scrollController.dispose();
super.dispose();
}
@override
Widget build(BuildContext context) {
return NestedScrollView(
controller: _scrollController,
headerSliverBuilder: _createSliverAppBar,
body: widget.bodyCreator(context),
);
}
List<Widget> _createSliverAppBar(BuildContext context, bool innerBoxIsScrolled) {
// change the icon color as the page scrolls
var collapsePercent = _getAppBarCollapsePercent();
int rgb = ((1.0 - collapsePercent) * 255).round();
var color = Color.fromARGB(255, rgb, rgb, rgb);
return <Widget>[
SliverAppBar(
expandedHeight: _kExpandedHeight,
pinned: true,
iconTheme: IconThemeData(color: color),
title: Text(widget.titleText),
)
];
}
double _getAppBarCollapsePercent() {
if (!_scrollController.hasClients)
return 0.0;
return (_scrollController.offset / (_kExpandedHeight - kToolbarHeight)).clamp(0.0, 1.0);
}
}
Here is a truncated trace of the exception:
flutter: ══╡ EXCEPTION CAUGHT BY WIDGETS LIBRARY ╞═══════════════════════════════════════════════════════════
flutter: The following assertion was thrown building Builder(dirty):
flutter: ScrollController attached to multiple scroll views.
flutter: 'package:flutter/src/widgets/scroll_controller.dart': Failed assertion: line 111 pos 12:
flutter: '_positions.length == 1'
flutter:
flutter: Either the assertion indicates an error in the framework itself, or we should provide substantially
flutter: more information in this error message to help you determine and fix the underlying cause.
flutter: In either case, please report this assertion by filing a bug on GitHub:
flutter: https://github.com/flutter/flutter/issues/new?template=BUG.md
flutter:
flutter: When the exception was thrown, this was the stack:
flutter: #2 ScrollController.position (package:flutter/src/widgets/scroll_controller.dart:111:12)
flutter: #3 ScrollController.offset (package:flutter/src/widgets/scroll_controller.dart:118:24)
flutter: #4 _CollapsingAppBarPageState._getAppBarCollapsePercent (package:flutter_scroll_test/main.dart:160:31)
flutter: #5 _CollapsingAppBarPageState._createSliverAppBar (package:flutter_scroll_test/main.dart:142:27)
flutter: #6 NestedScrollView._buildSlivers (package:flutter/src/widgets/nested_scroll_view.dart:271:20)
flutter: #7 _NestedScrollViewState.build.<anonymous closure> (package:flutter/src/widgets/nested_scroll_view.dart:347:29)
flutter: #8 Builder.build (package:flutter/src/widgets/basic.dart:5736:41)
flutter: #9 StatelessElement.build (package:flutter/src/widgets/framework.dart:3774:28)
flutter: #10 ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:3721:15)
Don't use _scrollController.offset directly as pushing routes apparently attach a scroll position to be retrieved when you approach scrolling when you pop again
so we need to transfer it into a piece of state
this is the complete revised code.
import 'package:flutter/material.dart';
void main() => runApp(MyApp());
class MyApp extends StatelessWidget {
@override
Widget build(BuildContext context) {
return MaterialApp(
title: 'Flutter Demo',
home: FirstPage(),
);
}
}
class FirstPage extends StatefulWidget {
@override
_FirstPageState createState() => _FirstPageState();
}
class _FirstPageState extends State<FirstPage> {
@override
Widget build(BuildContext context) {
return Scaffold(
appBar: AppBar(
title: Text("First Page"),
),
body: Center(
child: RaisedButton(
child: Text("Navigate Next"),
onPressed: () async {
await Navigator.push(context,
MaterialPageRoute(builder: (context) => SecondPage())
);
},
),
),
);
}
}
class SecondPage extends StatefulWidget {
@override
_SecondPageState createState() => _SecondPageState();
}
class _SecondPageState extends State<SecondPage> {
@override
Widget build(BuildContext context) {
return Material(
child: CollapsingAppBarPage(
titleText: "Second Page",
bodyCreator: (context) {
return ListView(
children: <Widget>[
Center(
child: RaisedButton(
child: Text("Navigate Next"),
onPressed: () async {
await Navigator.push(context,
MaterialPageRoute(builder: (context) => ThirdPage())
);
},
),
)
],
);
},
),
);
}
}
class ThirdPage extends StatefulWidget {
@override
_ThirdPageState createState() => _ThirdPageState();
}
class _ThirdPageState extends State<ThirdPage> {
@override
Widget build(BuildContext context) {
return Scaffold(
appBar: AppBar(
title: Text("Third Page")
),
body: Container()
);
}
}
typedef CollapsingAppBarBodyCreator = Widget Function(BuildContext context);
class CollapsingAppBarPage extends StatefulWidget {
final String titleText;
final CollapsingAppBarBodyCreator bodyCreator;
CollapsingAppBarPage({
Key key,
this.titleText,
@required this.bodyCreator,
}) : super(key: key);
@override
_CollapsingAppBarPageState createState() => _CollapsingAppBarPageState();
}
class _CollapsingAppBarPageState extends State<CollapsingAppBarPage> {
static const _kExpandedHeight = 200.0;
ScrollController _scrollController;
//Offset state <-------------------------------------
double offset = 0.0 ;
@override
void initState() {
super.initState();
//print("init state is called");
_scrollController = ScrollController() //keepScrollOffset: false removed
..addListener(() {
setState(() {
//<-----------------------------
offset = _scrollController.offset;
// force a refresh so the app bar can be updated
});
});
}
@override
void dispose() {
_scrollController.dispose();
super.dispose();
}
@override
Widget build(BuildContext context) {
return NestedScrollView(
controller: _scrollController,
headerSliverBuilder: _createSliverAppBar,
body: widget.bodyCreator(context),
);
}
List<Widget> _createSliverAppBar(BuildContext context, bool innerBoxIsScrolled) {
// change the icon color as the page scrolls
print("_createSliverAppBar is called");
var collapsePercent = _getAppBarCollapsePercent();
int rgb = ((1.0 - collapsePercent) * 255).round();
var color = Color.fromARGB(255, rgb, rgb, rgb);
return <Widget>[
SliverAppBar(
expandedHeight: _kExpandedHeight,
pinned: true,
iconTheme: IconThemeData(color: color),
title: Text(widget.titleText),
)
];
}
double _getAppBarCollapsePercent() {
if (!_scrollController.hasClients ){
print("positions is ${_scrollController.positions.length}");
return 0.0;
}
//print("offset is${_scrollController.offset} and positions is ${_scrollController.positions.length}");
return (offset / (_kExpandedHeight - kToolbarHeight)).clamp(0.0, 1.0);
}
}