fix: Improve text line height calculation to properly align text on iOS #46884

ArekChr · 2024-10-08T10:08:50Z

Summary:

This pull request addresses an issue in React Native on iOS: Text gets truncated when the line height is too small, even if the line-height matches or exceeds the font size.

The existing logic did not correctly handle font metrics (ascent, descent, top, bottom) when the total height exceeded the line height. The implementation now ensures a more balanced vertical space distribution, reducing text clipping and misalignment.

The fix mirrors a similar solution made on Android, ensuring consistency across both platforms.

For more context, see the related issue: React Native issue #29507.

Changelog:

[IOS] [FIXED] - Fixed an issue where text clipping and misalignment occurred when the line-height was smaller than the total ascent and descent of the font.

Test Plan:

The result is an improvement over the current implementation, reducing the occurrence of text truncation. While minimal truncation still exists at single-line heights, particularly with special characters, the implementation better preserves the vertical alignment of the text compared to previous behavior. However, like the Android implementation, this does not fully resolve the issue of React Native truncating content that is out of bounds of the span.

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okwasniewski · 2024-10-08T12:47:16Z

packages/react-native/Libraries/Text/Text/RCTTextView.mm

+                                   contentFrame:(CGRect)contentFrame {
+  UIFont *font = [textStorage attribute:NSFontAttributeName atIndex:0 effectiveRange:NULL];
+  if (!font) {
+      font = [UIFont systemFontOfSize:14];


[Q] Maybe we can use systemFontSize here?

https://developer.apple.com/documentation/uikit/uifont/1623395-systemfontsize?language=objc

I couldn’t find systemFontSize used anywhere else in the repo, and the default font size of 14 has been hardcoded. Also, since systemFontSize is not available on tvOS, I’m not sure if it would be a suitable replacement. Let me know your thoughts on whether we should stick with the hardcoded value for consistency or if there’s a preferred approach.

okwasniewski · 2024-10-08T12:48:54Z

packages/react-native/Libraries/Text/Text/RCTTextView.mm

+  CGFloat verticalOffset = 0;
+  if (textHeight > lineHeight) {
+      CGFloat difference = textHeight - lineHeight;
+      verticalOffset = difference / 2.0;
+  } else if (textHeight < lineHeight) {
+      CGFloat difference = lineHeight - textHeight;
+      verticalOffset = -(difference / 2.0);
+  }


I think comment explaining the calculations might be useful

Sure, added comments

mrousavy · 2024-10-08T14:52:26Z

nice!

…ributes in calculateDrawingPoint.

cipolleschi

Changes looks good. We discussed this internally and we can proceed importing it.

To set the expectation, this might take some time to merge, given that this might affect internal tests and product code and we would need some time to fix them. But thankfully, we have a feature flag to control the rollout! 😄

facebook-github-bot · 2024-10-09T12:35:08Z

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

cipolleschi · 2024-10-09T12:49:45Z

@ArekChr CI is currently red. The error is

[React-RCTText] Compiling RCTTextView.mm
Error: use of undeclared identifier 'ReactNativeFeatureFlags'; did you mean 'facebook::react::ReactNativeFeatureFlags'?
  if (ReactNativeFeatureFlags::enableLineHeightCentering()) {

Could you please verify why this is happening and fix it?
You should be able to build it locally running these commands in the react-native root folder:

yarn install
cd packages/rn-tester
bundle install
bundle exec pod install
open RNTesterPods.xcworkspace

and then build from Xcode.

If you need to run the old architecture, you can disable it by adding:

- (BOOL) newArchEnabled
{
  return NO;
}

in the AppDelegate.mm file

ArekChr · 2024-10-09T18:52:57Z

Hi @cipolleschi, the issue has been fixed, and the build runs successfully locally. Thanks!

cipolleschi · 2024-10-10T09:38:55Z

We were discussing internally that it would be better to have two separate feature flags, one for Android and one for iOS. Could I ask you to:

revert the changes you made on the preexisting feature flag, which was targeting only Android
add the iOS dual of that feature flag

The idea is to be able to control the two platforms separatedly.

Thank you so much for your help and patience

ArekChr · 2024-10-11T12:02:20Z

@cipolleschi I have an issue where the ReactNativeFeatureFlags.h header is not found when using dynamic frameworks. I’ve tried a few things but haven’t resolved the issue. Do you have any suggestions on how to fix or work around this?

improve text line height calculation on ios

f998a9a

ArekChr mentioned this pull request Oct 8, 2024

Line height is distributed unevenly when lineHeight <= fontSize #29507

Open

ArekChr changed the title ~~fix: Improve text line height calculation to properly align text~~ fix: Improve text line height calculation to properly align text on iOS Oct 8, 2024

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. Shared with Meta Applied via automation to indicate that an Issue or Pull Request has been shared with the team. labels Oct 8, 2024

okwasniewski reviewed Oct 8, 2024

View reviewed changes

ArekChr force-pushed the improve-lineheight-calc-ios branch from 246c550 to f998a9a Compare October 9, 2024 07:48

ArekChr added 4 commits October 9, 2024 09:55

add comments

8168cf0

add safeguards to prevent out-of-bounds crash when accessing text att…

164bf0c

…ributes in calculateDrawingPoint.

Merge branch 'main' into improve-lineheight-calc-ios

f79a614

codegen feature flags

b044af9

cipolleschi approved these changes Oct 9, 2024

View reviewed changes

Fix namespace for ReactNativeFeatureFlags in RCTTextView

4844731

refactor: separate feature flat per platform

76faa56

fix: add React-featureflags pod for dynamic frameworks configuration

9e5ed70

ArekChr force-pushed the improve-lineheight-calc-ios branch from fa44b24 to 9e5ed70 Compare October 14, 2024 07:01

Merge branch 'main' into improve-lineheight-calc-ios

09118be

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fix: Improve text line height calculation to properly align text on iOS #46884

fix: Improve text line height calculation to properly align text on iOS #46884

ArekChr commented Oct 8, 2024 •

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ArekChr commented Oct 11, 2024 •

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fix: Improve text line height calculation to properly align text on iOS #46884

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fix: Improve text line height calculation to properly align text on iOS #46884

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ArekChr commented Oct 8, 2024 • edited Loading

Summary:

Changelog:

Test Plan:

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Process

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