react / react/react-native

Nested Virtualized Lists in opposing directions firing all onViewableItemsChanged

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Component: FlatList Issue: Author Provided Repro Newer Patch Available
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Description

Description

Nested Bidirectional FlatList onViewableItemsChanged Issue

App Image Layout Image

The above images describe the layout where this problem has arisen. Essentially it's a 2D FlatList.

When the Vertical FlatList mounts and the onViewableItemsChanged prop callbacks fire for only the elements on the screen, the expected behavior is working.

However, once the nested FlatLists mount, instead of firing the onViewableItemsChanged callback for only the nested Horizontal FlatList items that are within the visible viewport, it fires for all rendered list items (including those that are out of the viewport). It seems that the correct viewability is determined on horizontal lists, however, the VirtualizedList doesn't check if those horizontal lists are vertically visible.

Below, I've illustrated the expected vs actual behavior with images/emojis

🎇 - Vertical FlatList onViewableItemsChanged callback fires
🔥 - Horizontal FlatList onViewableItemsChanged callback fires

Expected Actual
Untitled-2022-08-26-1424 Untitled-2022-08-26-1424

I provided a minimum setup repo below you can clone. Only changed the App.tsx file to recreate the above steps.

React Native Version

0.71.0

Output of npx react-native info
System:
    OS: macOS 12.6
    CPU: (10) arm64 Apple M1 Max
    Memory: 204.02 MB / 32.00 GB
    Shell: 5.8.1 - /bin/zsh
  Binaries:
    Node: 16.18.0 - ~/.nvm/versions/node/v16.18.0/bin/node
    Yarn: 1.22.19 - ~/.nvm/versions/node/v16.18.0/bin/yarn
    npm: 8.19.2 - ~/.nvm/versions/node/v16.18.0/bin/npm
    Watchman: Not Found
  Managers:
    CocoaPods: Not Found
  SDKs:
    iOS SDK:
      Platforms: DriverKit 21.4, iOS 16.0, macOS 12.3, tvOS 16.0, watchOS 9.0
    Android SDK:
      API Levels: 29, 30, 31, 32, 33
      Build Tools: 29.0.3, 30.0.2, 30.0.3, 31.0.0, 32.0.0, 33.0.0, 33.0.0
      System Images: android-29 | Intel x86 Atom_64, android-29 | Google APIs Intel x86 Atom_64, android-29 | Google Play ARM 64 v8a, android-29 | Google Play Intel x86 Atom_64, android-31 | Google Play ARM 64 v8a, android-32 | Google APIs ARM 64 v8a, android-32 | Google Play ARM 64 v8a, android-33 | Google Play ARM 64 v8a
      Android NDK: Not Found
  IDEs:
    Android Studio: 2021.3 AI-213.7172.25.2113.9014738
    Xcode: 14.0.1/14A400 - /usr/bin/xcodebuild
  Languages:
    Java: 11.0.11 - /Users/vvella/.jenv/shims/javac
  npmPackages:
    @react-native-community/cli: Not Found
    react: 18.1.0 => 18.1.0
    react-native: 0.70.4 => 0.71.0
    react-native-macos: Not Found
  npmGlobalPackages:
    *react-native*: Not Found
Steps to reproduce
  1. Create a vertical FlatList that contains a 2D data array with a length long enough to make the list run off of your screen and require scrolling
  2. Create another FlatList (horizontal) to be rendered in the renderItem prop of the parent FlatList
  3. Add an onViewableItemsChanged to the nested (horizontal) FlatList
  4. Observe that all items rendered (regardless of vertical viewability) fire their onViewableItemsChanged callback
Snack, code example, screenshot, or link to a repository

Taken from the tester app
See that InnerViewable shows "56,57,58"
All of which are horizontally viewable but not vertically shown in the viewport

Snack: https://snack.expo.dev/@vincentvella/great-almond
Repo: https://github.com/vincentvella/bidirectional-virtualized-list-issue

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

Start with App.tsx in the linked reproduction and run the Snack or clone the repository to reproduce the nested vertical and horizontal FlatLists. Then trace the React Native FlatList/VirtualizedList viewability behavior involved in nested lists. Done means the inner callback reports only items visible in both the horizontal list and the parent’s vertical viewport.

Written by the indexing model from the issue text.

Assessment

Tech stack
react-native
Domain
mobile-dev
Issue type
Bug
Difficulty
4/5
Estimated time
3-5 days
Activity status
Stale
Clarity
Mostly clear
Newbie friendliness
45/100

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