[iOS] fix: incorrect ScrollView offset on update

[rnike]

Summary

This pull request is to fix #30258.

After an investigation, I found out the scroll offset of the list seems to be calculated incorrectly due to a workaround in the source code.

Instead of fixing the calculateOffsetForContentSize, I chose to remove it, the reason why I do so is because this workaround is for fixing the offset when contentSize is set manually, but according to the source code, there is no interface for a react-native user to set the contentSize of ScrollView, it is just set to GCSizeZero and will never be changed (ref).

Also I changed the function name from updateContentOffsetIfNeeded to updateContentSizeIfNeeded according what the function is actually doing.

Changelog

[iOS] [Fixed] - Incorrect ScrollView offset on update

Test Plan

1. Create a fresh new project named testapp using npx create-react-native-app
2. Apply example code to testapp from https://snack.expo.io/@lokomass/flatlist
3. Run testapp on iOS emulator and reproduce the bug
4. Make changes to files in testapp/node_modules/react-native/
5. Rebuild testapp and run on iOS emulator again, the bug is no more exist
6. Apply changes from step 4 to react-native, make a pull request.

Screenshots

Before: The scroll offset is incorrect after children of FlatList has changed

Before.mov

After: No more incorrect scroll offset if children of FlatList has changed

After.mov

[analysis-bot]

Platform	Engine	Arch	Size (bytes)	Diff
ios	-	universal	n/a	--

Base commit: 89beefa

[facebook-github-bot]

@shergin has imported this pull request. If you are a Facebook employee, you can view this diff on Phabricator.

[shergin]

Love this PR, I hope this works out!

The contentSize property is declared as assignable (@property (nonatomic, assign) CGSize contentSize;) but the custom setter is missing. We need to fix this first.

[shergin]

See my last comment.

[rnike]

@shergin I'm not sure what you mean by fixing the contentSize property, I am just removing it as well, because the contentContainerStyle prop is already there to let us manually set the content size, seems that contentSize property is no longer needed.

Please let me know if I am on the right or wrong direction of doing this.

[rnike]

Have manually checked on react-native v0.63.4, it works fine.

Wanted to try on v0.64.0-rc.2 but facing a build fail issue:

Cannot check if it works on RN64, I assume that it works as same as v0.63.4.

[facebook-github-bot]

@shergin has imported this pull request. If you are a Facebook employee, you can view this diff on Phabricator.

[facebook-github-bot]

@shergin merged this pull request in a4526bc.

[lokomass]

Amazing thanks !!

[lokomass]

Sorry for this, but I have always this issue with last RN version, is it normal ?

[FERARMAO]

Just add this prop --> automaticallyAdjustContentInsets={false}