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Possibly undocumented behaviour in ListView.builder #58371
Description
It seems that ListView.builder only builds items until it's itemBuilder callback returns null, ignoring everything else afterwards. In other words, the itemBuilder callback must ALWAYS return a non-null Widget or else it will not display any meaningful data that may come after any uninitialized Widget. Is this intended behaviour? I think it would be better if itemBuilder just carried on until it reached itemCount
Here is a short program to illustrate this behaviour:
import 'package:flutter/material.dart';
void main() {
runApp(MyApp());
}
class MyApp extends StatelessWidget {
// This widget is the root of your application.
@override
Widget build(BuildContext context) {
return MaterialApp(
title: 'Flutter Demo',
theme: ThemeData(
primarySwatch: Colors.blue,
visualDensity: VisualDensity.adaptivePlatformDensity,
),
home: MyHomePage(title: 'Listview bug?'),
);
}
}
class MyHomePage extends StatefulWidget {
MyHomePage({Key key, this.title}) : super(key: key);
final String title;
@override
_MyHomePageState createState() => _MyHomePageState();
}
class _MyHomePageState extends State<MyHomePage> {
List<String> items = List<String>();
void addItemToList(){
setState(() {
items.add("test");
});
}
void addIgnoredItemToList() {
setState(() {
items.add("ignored");
});
}
@override
Widget build(BuildContext context) {
return Scaffold(
appBar: AppBar(
title: Text(widget.title),
actions: <Widget>[
FlatButton(
child: Text(
"add item",
),
onPressed: addItemToList,
),
FlatButton(
child: Text(
"add ignored item",
),
onPressed: addIgnoredItemToList,
),
],
),
body: Center(
child: ListView.builder(
itemCount: items.length,
itemBuilder: (context, index) {
Widget output;
if (items[index] == "ignored") {
return output;
} else {
output = Text(items[index]);
return output;
}
},
),
),
floatingActionButton: FloatingActionButton(
child: Icon(Icons.clear),
tooltip: 'Reset',
onPressed: (){
setState(() {
items.clear();
});
},
),
);
}
}
Here is a video demonstrating this behaviour:
https://imgur.com/a/FBupWR5
Flutter --version:
Flutter 1.17.1 • channel stable • https://github.com/flutter/flutter.git
Framework • revision f7a6a7906b (3 weeks ago) • 2020-05-12 18:39:00 -0700
Engine • revision 6bc433c6b6
Tools • Dart 2.8.2
Activity
iapicca commentedon Jun 1, 2020
Hi @nojusr
the issue issue is present with the latest master
doctor
goderbauer commentedon Jun 3, 2020
This is expected and documented in the lower levels, but we should also document that on
ListView.builder.github-actions commentedon Aug 19, 2021
This thread has been automatically locked since there has not been any recent activity after it was closed. If you are still experiencing a similar issue, please open a new bug, including the output of
flutter doctor -vand a minimal reproduction of the issue.