react / react/react-native

React Native onEndReached FlatList trigger error

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Component: FlatList Needs: Triage :mag: Newer Patch Available
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Description

Description

I'm developing a react native app using the version 0.71.3 and I'm having some problems with the extremely discussed onEndReached callback.

React Native Version

0.71.3

Output of npx react-native info

System:
OS: macOS 12.6.3
CPU: (4) x64 Intel(R) Core(TM) i5-6267U CPU @ 2.90GHz
Memory: 228.86 MB / 8.00 GB
Shell: 5.8.1 - /bin/zsh
Binaries:
Node: 18.14.0 - /usr/local/bin/node
Yarn: 1.22.19 - /usr/local/bin/yarn
npm: 9.5.0 - /usr/local/bin/npm
Watchman: Not Found
Managers:
CocoaPods: 1.12.0 - /usr/local/bin/pod
SDKs:
iOS SDK:
Platforms: DriverKit 22.2, iOS 16.2, macOS 13.1, tvOS 16.1, watchOS 9.1
Android SDK: Not Found
IDEs:
Android Studio: 2021.3 AI-213.7172.25.2113.9123335
Xcode: 14.2/14C18 - /usr/bin/xcodebuild
Languages:
Java: 17.0.2 - /usr/bin/javac
npmPackages:
@react-native-community/cli: Not Found
react: 18.2.0 => 18.2.0
react-native: 0.71.3 => 0.71.3
react-native-macos: Not Found
npmGlobalPackages:
react-native: Not Found

Steps to reproduce

Here's my onEndReached re-implementation to prevent the callback to be triggered even if the list is empty and to trigger it only once:

if (!endReached.current && dataSize > 0) {
  endReached.current = true;
  onEndReached?.();
}

And every time the data size changes, the endReached is set to false again:

useEffect(() => {
  endReached.current = false;
}, [dataSize]);

And then the list:

<FlatList onEndReachedThreshold={0.2} />

Now the problem arise when loading the list for the first time, the onEndReached callback is triggered even if the list is empty and once the list is populated, downloading items asynchronously, and reach the end the callback it's not triggered.

But if the list data area cached, the second time I load the list the onEndReached is not triggered at the beginning but on the right way.

It seems that the error, or I think the bug, arise when the list is empty because data are loading, indeed if I set a condition to render the list only after the data are fully loaded, the onEndReached is trigged correctly.

Contributor guide

Open the contributing guide

First steps

  1. Read the whole issue, then the project's contributing guide.
  2. Comment on the issue to say you are picking it up — it saves two people doing the same work.
  3. Fork the repository and make your change on a branch.
  4. Open a pull request that references the issue number.

Research direction

Reproduce the first-load sequence using FlatList, onEndReachedThreshold={0.2}, and asynchronously populated data from the issue. Compare it with the cached-data case and the conditional-rendering workaround, then trace the onEndReached entry point. Done means an initially empty list does not consume the callback before data arrives, while reaching the end afterward still triggers it.

Written by the indexing model from the issue text.

Assessment

Tech stack
javascript, react-native
Domain
mobile
Issue type
Bug
Difficulty
3/5
Estimated time
1-2 days
Activity status
Active
Clarity
Mostly clear
Newbie friendliness
55/100

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