RenderSliverMainAxisGroup causes lazy children to build eagerly #142183
Description
Steps to reproduce
Add a CustomScrollView with SliverMainAxisGroups with SliverLists or SliverGrids inside. Add a simple print in the lists' item builders to print the list and item indices. Rather only the visible and first item, all or most children are instantiated during the first layout pass, even for groups far below the visible extent.
For the minimal example below, the consequences are less severe, with only 10 or so built per group, but with several groups each with a sticky SliverPersistentHeader and a SliverGrid of images to be fetched for each child, there is a quite significant performance hit (and no way to keep all those images in memory at the same time).
My investigation led me to a missing cacheExtent update in the RenderSliverMainAxisGroup's performLayout, causing it to consume no cache extent, even though it's children would. As the layout progresses, each RenderSliverMainAxisGroup has the same cacheExtent set, as it is never decreased in between.
Adding the following at the end of performLayout, in sliver_group.dart line 287, seems to fix it for me:
final double paintExtent = calculatePaintOffset(
constraints,
from: math.min(constraints.scrollOffset, 0),
to: totalScrollExtent,
);
geometry = SliverGeometry(
scrollExtent: totalScrollExtent,
paintExtent: paintExtent,
cacheExtent: paintExtent,
maxPaintExtent: maxPaintExtent,
hasVisualOverflow: totalScrollExtent > constraints.remainingPaintExtent ||
constraints.scrollOffset > 0.0,
);
}
Expected results
The children of any lists in view, and the first child for any subsequent lists should be built and laid out (for calculating max extent etc).
Actual results
All (or several) children for each list gets built immediately, regardless of whether the children or even lists are visible.
Code sample
Code sample
import 'package:flutter/material.dart';
void main() => runApp(MyApp());
class MyApp extends StatelessWidget {
@override
Widget build(BuildContext context) {
return MaterialApp(
debugShowCheckedModeBanner: false,
home: const NonLazySliverGrids(),
);
}
}
class NonLazySliverGrids extends StatelessWidget {
const NonLazySliverGrids({super.key});
@override
Widget build(BuildContext context) => CustomScrollView(
slivers: [
for (var i = 0; i < 100; i++)
SliverMainAxisGroup(
slivers: [
SliverList.builder(
itemCount: 100,
itemBuilder: (context, index) {
print('list $i, item $index');
return Container(
height: 24,
color: Colors.primaries[index % Colors.primaries.length],
);
},
),
],
),
],
);
}Screenshots or Video
No response
Logs
No response
Flutter Doctor output
Doctor output
[√] Flutter (Channel stable, 3.16.8, on Microsoft Windows [Version 10.0.22631.3007], locale en-US)
• Flutter version 3.16.8 on channel stable at C:\src\flutter
• Upstream repository https://github.com/flutter/flutter.git
• Framework revision 67457e669f (8 days ago), 2024-01-16 16:22:29 -0800
• Engine revision 6e2ea58a5c
• Dart version 3.2.5
• DevTools version 2.28.5
[√] Windows Version (Installed version of Windows is version 10 or higher)
[√] Android toolchain - develop for Android devices (Android SDK version 34.0.0)
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• Platform android-34, build-tools 34.0.0
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• Java version OpenJDK Runtime Environment (build 17.0.7+0-b2043.56-10550314)
• All Android licenses accepted.
[√] Chrome - develop for the web
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[√] Visual Studio - develop Windows apps (Visual Studio Build Tools 2022 17.7.6)
• Visual Studio at C:\Program Files (x86)\Microsoft Visual Studio\2022\BuildTools
• Visual Studio Build Tools 2022 version 17.7.34221.43
• Windows 10 SDK version 10.0.22621.0
[√] Android Studio (version 2023.1)
• Android Studio at C:\Program Files\Android\Android Studio
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• Java version OpenJDK Runtime Environment (build 17.0.7+0-b2043.56-10550314)
[√] VS Code (version 1.85.2)
• VS Code at C:\Users\~\AppData\Local\Programs\Microsoft VS Code
• Flutter extension version 3.80.0
[√] Connected device (3 available)
• Windows (desktop) • windows • windows-x64 • Microsoft Windows [Version 10.0.22631.3007]
• Chrome (web) • chrome • web-javascript • Google Chrome 120.0.6099.225
• Edge (web) • edge • web-javascript • Microsoft Edge 120.0.2210.144
[√] Network resources
• All expected network resources are available.
• No issues found!