Scrolling a 'moving' FlatList causes it to stutter or block scrolling completely on Android
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Description
Description
When you create a FlatList that moves as it's being scrolled (change in offset for the FlatList is a change in top property of FlatList's style) it stutters heavily. This behavior is much less disruptive when movement is scaled down compared to scrolling and becomes app-breaking when upscaled. See provided videos:
- Multiplier: 1.0
mult1.webm - Mutliplier: 2.0
mult2.webm - Multiplier: 0.5
mult0.5.webm
It was originally an issue in react-native-reanimated but after digging into it I narrowed it down to react-native and rewrote the code to not use react-native-reanimated. It seems to had been in the code for a long time, since this posts that dates 3 years back is about the same thing.
I checked it on versions 0.72, 0.71 and 0.70. On both Fabric and Paper and the result is always the same (although it feels a bit like Fabric behaviour is worse).
iOS works properly on each version too.
It seems like there is a problem when calculating layout and touch events - that's what I conducted due to multiplier making it better or worse. After moving a FlatList twice the distance it scrolled and having the component moved React Native seems to 'think' that the finger is now further than it should be and lowers the offset? That's just my guess.
React Native Version
0.72.3
Output of npx react-native info
System:
OS: macOS 13.4.1
CPU: (10) arm64 Apple M2 Pro
Memory: 49.31 MB / 16.00 GB
Shell:
version: "5.9"
path: /bin/zsh
Binaries:
Node:
version: 18.16.0
path: /var/folders/jg/m839qn593nn7w_h3n0r9k25c0000gn/T/yarn--1689593442476-0.9098900178237601/node
Yarn:
version: 1.22.19
path: /var/folders/jg/m839qn593nn7w_h3n0r9k25c0000gn/T/yarn--1689593442476-0.9098900178237601/yarn
npm:
version: 9.5.1
path: ~/.nvm/versions/node/v18.16.0/bin/npm
Watchman:
version: 2023.06.12.00
path: /opt/homebrew/bin/watchman
Managers:
CocoaPods:
version: 1.12.1
path: /Users/user/.rbenv/shims/pod
SDKs:
iOS SDK:
Platforms:
- DriverKit 22.4
- iOS 16.4
- macOS 13.3
- tvOS 16.4
- watchOS 9.4
Android SDK: Not Found
IDEs:
Android Studio: 2022.1 AI-221.6008.13.2211.9619390
Xcode:
version: 14.3.1/14E300c
path: /usr/bin/xcodebuild
Languages:
Java:
version: 11.0.19
path: /usr/bin/javac
Ruby:
version: 3.2.2
path: /Users/user/.rbenv/shims/ruby
npmPackages:
"@react-native-community/cli": Not Found
react:
installed: 18.2.0
wanted: 18.2.0
react-native:
installed: 0.72.3
wanted: 0.72.3
react-native-macos: Not Found
npmGlobalPackages:
"react-native": Not Found
Android:
hermesEnabled: true
newArchEnabled: true
iOS:
hermesEnabled: true
newArchEnabled: false
Steps to reproduce
Just need to run the provided code snippet.
Snack, code example, screenshot, or link to a repository
https://github.com/tjzel/ReactNativeMovingFlatList
import React from 'react';
import {Dimensions, FlatList, StyleSheet, Text, View} from 'react-native';
const {height: SCREEN_HEIGHT} = Dimensions.get('screen');
const data = Array.from({length: 30}, (_, index) => ({
id: `${index + 1}`,
text: `Item ${index + 1}`,
}));
const multiplier = 1.0;
export default function App() {
const [animatedVerticalScroll, setAnimatedVerticalScroll] = React.useState(0);
// used to detect stutters
const previousValue = React.useRef(0);
const listAnimatedStyle = {
top:
animatedVerticalScroll * multiplier > SCREEN_HEIGHT * 0.7
? 0
: SCREEN_HEIGHT * 0.7 - animatedVerticalScroll * multiplier,
};
const renderItem = ({item}) => (
<View style={styles.item}>
<Text>{item.text}</Text>
</View>
);
const scrollHandler = event => {
// logger to detect if there was a stutter
if (previousValue.current > event.nativeEvent.contentOffset.y) {
console.log(
`\nScrolling stuttered!\nPrevious offset was ${previousValue.current}\nCurrent offset is ${event.nativeEvent.contentOffset.y}\n`,
);
} else {
console.log(event.nativeEvent.contentOffset.y);
}
setAnimatedVerticalScroll(event.nativeEvent.contentOffset.y);
previousValue.current = event.nativeEvent.contentOffset.y;
};
return (
<View style={styles.container}>
<View style={styles.containerBehind}>
<Text>Multiplier: {multiplier}</Text>
</View>
<FlatList
data={data}
renderItem={renderItem}
style={[styles.listContainer, listAnimatedStyle]}
onScroll={scrollHandler}
/>
</View>
);
}
const styles = StyleSheet.create({
container: {
flex: 1,
},
containerBehind: {
width: '100%',
height: '100%',
flex: 1,
backgroundColor: 'red',
alignItems: 'center',
justifyContent: 'center',
},
listContainer: {
top: 0,
left: 0,
position: 'absolute',
zIndex: 10,
width: '100%',
height: '100%',
backgroundColor: 'green',
},
item: {
height: 50,
},
});
Contributor guide
First steps
- Read the whole issue, then the project's contributing guide.
- Comment on the issue to say you are picking it up — it saves two people doing the same work.
- Fork the repository and make your change on a branch.
- Open a pull request that references the issue number.
Research direction
Start by running the provided ReactNativeMovingFlatList reproduction and compare Android scrolling with the moving FlatList across the listed multipliers. The issue names no React Native source file or test; done means scrolling remains smooth and is not blocked when the FlatList's top style changes, including under Fabric and Paper.
Written by the indexing model from the issue text.
Assessment
- Tech stack
- android, react-native, typescript
- Domain
- mobile, performance
- Issue type
- Bug
- Difficulty
- 4/5
- Estimated time
- 3-5 days
- Activity status
- Stale
- Clarity
- Needs clarification
- Newbie friendliness
- 35/100