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Improve heuristic that triggers onStartReached #44287

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@janicduplessis janicduplessis commented Apr 26, 2024

Summary:

onStartReached is called more than it should be since our heuristic to not re-trigger it is based on the list content size. This has worked fine for onEndReached, but for onStartReached it causes some issues.

On initial mount we receive different content size updates. I assume this is normal because of virtualization and the need to measure content. However for onStartReached since we usually start at scroll position 0 we are already in the threshold so any change to content size causes us to trigger a onStartReached event.

For example using this code we get 2 onStartReached calls on mount. On top of this every time we add items at the end of the list onStartReached is called another time.

To improve this I suggest using firstVisibleItemKey that we use for maintainVisibleContentPosition to track if we should re-trigger onStartReached. This means that it will be only re-triggered if new items are added at the start of the list or if we leave the threshold.

Changelog:

[GENERAL] [FIXED] - Improve heuristic that triggers onStartReached

Test Plan:

Tested using this code. See that onStartReached is only called once on mount and then again only if leaving the threshold or adding new items at the start of the list. Adding items at the end of the list should also not trigger onStartReached.

@facebook-github-bot facebook-github-bot added CLA Signed This label is managed by the Facebook bot. Authors need to sign the CLA before a PR can be reviewed. Contributor A React Native contributor. labels Apr 26, 2024
@janicduplessis janicduplessis marked this pull request as ready for review April 27, 2024 17:29
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cc @NickGerleman

@facebook-github-bot facebook-github-bot added the Shared with Meta Applied via automation to indicate that an Issue or Pull Request has been shared with the team. label Apr 27, 2024
@@ -1552,16 +1552,16 @@ class VirtualizedList extends StateSafePureComponent<Props, State> {
onStartReached != null &&
this.state.cellsAroundViewport.first === 0 &&
isWithinStartThreshold &&
this._listMetrics.getContentLength() !== this._sentStartForContentLength
this.state.firstVisibleItemKey !== this._sentStartForFirstVisibleItemKey
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Comment above is now out of date it looks like

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Fixed, and only if onEndReached is not being executed that part was also out of date from a previous change.

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3 participants