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

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

@@ -11,6 +11,7 @@
 
 #import <React/RCTUtils.h>
 #import <React/UIView+React.h>
+#import <react/featureflags/ReactNativeFeatureFlags.h>
 
 #import <React/RCTTextShadowView.h>
 
@@ -98,6 +99,36 @@ - (void)setTextStorage:(NSTextStorage *)textStorage
   [self setNeedsDisplay];
 }
 
+- (CGPoint)calculateDrawingPointWithTextStorage:(NSTextStorage *)textStorage
+                                   contentFrame:(CGRect)contentFrame {
+  UIFont *font = [textStorage attribute:NSFontAttributeName atIndex:0 effectiveRange:NULL];
+  if (!font) {
+      font = [UIFont systemFontOfSize:14];
+  }
+
+  NSParagraphStyle *paragraphStyle = [textStorage attribute:NSParagraphStyleAttributeName atIndex:0 effectiveRange:NULL];
+
+  CGFloat lineHeight = font.lineHeight;
+  if (paragraphStyle && paragraphStyle.minimumLineHeight > 0) {
+      lineHeight = paragraphStyle.minimumLineHeight;
+  }
+
+  CGFloat ascent = font.ascender;
+  CGFloat descent = fabs(font.descender);
+  CGFloat textHeight = ascent + descent;
+
+  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);
+  }
+
+  return CGPointMake(contentFrame.origin.x, contentFrame.origin.y + verticalOffset);
+}
+
 - (void)drawRect:(CGRect)rect
 {
   [super drawRect:rect];
@@ -118,8 +149,15 @@ - (void)drawRect:(CGRect)rect
 #endif
 
   NSRange glyphRange = [layoutManager glyphRangeForTextContainer:textContainer];
-  [layoutManager drawBackgroundForGlyphRange:glyphRange atPoint:_contentFrame.origin];
-  [layoutManager drawGlyphsForGlyphRange:glyphRange atPoint:_contentFrame.origin];
+
+  if (ReactNativeFeatureFlags::enableLineHeightCentering()) {
+    CGPoint drawingPoint = [self calculateDrawingPointWithTextStorage:_textStorage contentFrame:_contentFrame];
+    [layoutManager drawBackgroundForGlyphRange:glyphRange atPoint:drawingPoint];
+    [layoutManager drawGlyphsForGlyphRange:glyphRange atPoint:drawingPoint];
+  } else {
+    [layoutManager drawBackgroundForGlyphRange:glyphRange atPoint:_contentFrame.origin];
+    [layoutManager drawGlyphsForGlyphRange:glyphRange atPoint:_contentFrame.origin];
+  }
 
   __block UIBezierPath *highlightPath = nil;
   NSRange characterRange = [layoutManager characterRangeForGlyphRange:glyphRange actualGlyphRange:NULL];

packages/react-native/React/Fabric/Mounting/ComponentViews/Text/RCTParagraphComponentView.mm

@@ -9,6 +9,7 @@
 #import "RCTParagraphComponentAccessibilityProvider.h"
 
 #import <MobileCoreServices/UTCoreTypes.h>
+#import <react/featureflags/ReactNativeFeatureFlags.h>
 #import <react/renderer/components/text/ParagraphComponentDescriptor.h>
 #import <react/renderer/components/text/ParagraphProps.h>
 #import <react/renderer/components/text/ParagraphState.h>
@@ -326,6 +327,38 @@ @implementation RCTParagraphTextView {
   CAShapeLayer *_highlightLayer;
 }
 
+- (CGRect)calculateCenteredFrameWithAttributedText:(NSAttributedString *)attributedText
+                                             frame:(CGRect)frame {
+  UIFont *font = [attributedText attribute:NSFontAttributeName atIndex:0 effectiveRange:NULL];
+  if (!font) {
+      font = [UIFont systemFontOfSize:14];
+  }
+
+  NSParagraphStyle *paragraphStyle = [attributedText attribute:NSParagraphStyleAttributeName atIndex:0 effectiveRange:NULL];
+  CGFloat lineHeight = font.lineHeight;
+
+  if (paragraphStyle && paragraphStyle.minimumLineHeight > 0) {
+      lineHeight = paragraphStyle.minimumLineHeight;
+  }
+
+  CGFloat ascent = font.ascender;
+  CGFloat descent = fabs(font.descender);
+  CGFloat textHeight = ascent + descent;
+
+  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);
+  }
+
+  frame.origin.y += verticalOffset;
+
+  return frame;
+}
+
 - (void)drawRect:(CGRect)rect
 {
   if (!_state) {
@@ -343,6 +376,11 @@ - (void)drawRect:(CGRect)rect
 
   CGRect frame = RCTCGRectFromRect(_layoutMetrics.getContentFrame());
 
+  if (ReactNativeFeatureFlags::enableLineHeightCentering()) {
+    NSAttributedString *attributedText = RCTNSAttributedStringFromAttributedString(_state->getData().attributedString);
+    frame = [self calculateCenteredFrameWithAttributedText:attributedText frame:frame];
+  }
+
   [nativeTextLayoutManager drawAttributedString:_state->getData().attributedString
                             paragraphAttributes:_paragraphAttributes
                                           frame:frame

packages/react-native/ReactAndroid/src/main/java/com/facebook/react/internal/featureflags/ReactNativeFeatureFlags.kt

@@ -4,7 +4,7 @@
  * This source code is licensed under the MIT license found in the
  * LICENSE file in the root directory of this source tree.
  *
- * @generated SignedSource<<7d80322a6a37083c5e52e6914de49ce2>>
+ * @generated SignedSource<<ac76caa581b2c41bc95ed973bbd27b99>>
  */
 
 /**
@@ -58,12 +58,6 @@ public object ReactNativeFeatureFlags {
   @JvmStatic
   public fun enableAlignItemsBaselineOnFabricIOS(): Boolean = accessor.enableAlignItemsBaselineOnFabricIOS()
 
-  /**
-   * When enabled, custom line height calculation will be centered from top to bottom.
-   */
-  @JvmStatic
-  public fun enableAndroidLineHeightCentering(): Boolean = accessor.enableAndroidLineHeightCentering()
-
   /**
    * Feature flag to enable the new bridgeless architecture. Note: Enabling this will force enable the following flags: `useTurboModules` & `enableFabricRenderer.
    */
@@ -130,6 +124,12 @@ public object ReactNativeFeatureFlags {
   @JvmStatic
   public fun enableLayoutAnimationsOnIOS(): Boolean = accessor.enableLayoutAnimationsOnIOS()
 
+  /**
+   * When enabled, custom line height calculation will be centered from top to bottom.
+   */
+  @JvmStatic
+  public fun enableLineHeightCentering(): Boolean = accessor.enableLineHeightCentering()
+
   /**
    * Enables the reporting of long tasks through `PerformanceObserver`. Only works if the event loop is enabled.
    */

packages/react-native/ReactAndroid/src/main/java/com/facebook/react/internal/featureflags/ReactNativeFeatureFlagsCxxAccessor.kt

@@ -4,7 +4,7 @@
  * This source code is licensed under the MIT license found in the
  * LICENSE file in the root directory of this source tree.
  *
- * @generated SignedSource<<761d3e7b100a4f5ee6f8bda71f84918b>>
+ * @generated SignedSource<<ee55d752b258637159b99ba75e217395>>
  */
 
 /**
@@ -25,7 +25,6 @@ public class ReactNativeFeatureFlagsCxxAccessor : ReactNativeFeatureFlagsAccesso
   private var batchRenderingUpdatesInEventLoopCache: Boolean? = null
   private var completeReactInstanceCreationOnBgThreadOnAndroidCache: Boolean? = null
   private var enableAlignItemsBaselineOnFabricIOSCache: Boolean? = null
-  private var enableAndroidLineHeightCenteringCache: Boolean? = null
   private var enableBridgelessArchitectureCache: Boolean? = null
   private var enableCleanTextInputYogaNodeCache: Boolean? = null
   private var enableDeletionOfUnmountedViewsCache: Boolean? = null
@@ -37,6 +36,7 @@ public class ReactNativeFeatureFlagsCxxAccessor : ReactNativeFeatureFlagsAccesso
   private var enableGranularShadowTreeStateReconciliationCache: Boolean? = null
   private var enableIOSViewClipToPaddingBoxCache: Boolean? = null
   private var enableLayoutAnimationsOnIOSCache: Boolean? = null
+  private var enableLineHeightCenteringCache: Boolean? = null
   private var enableLongTaskAPICache: Boolean? = null
   private var enableMicrotasksCache: Boolean? = null
   private var enablePreciseSchedulingForPremountItemsOnAndroidCache: Boolean? = null
@@ -114,15 +114,6 @@ public class ReactNativeFeatureFlagsCxxAccessor : ReactNativeFeatureFlagsAccesso
     return cached
   }
 
-  override fun enableAndroidLineHeightCentering(): Boolean {
-    var cached = enableAndroidLineHeightCenteringCache
-    if (cached == null) {
-      cached = ReactNativeFeatureFlagsCxxInterop.enableAndroidLineHeightCentering()
-      enableAndroidLineHeightCenteringCache = cached
-    }
-    return cached
-  }
-
   override fun enableBridgelessArchitecture(): Boolean {
     var cached = enableBridgelessArchitectureCache
     if (cached == null) {
@@ -222,6 +213,15 @@ public class ReactNativeFeatureFlagsCxxAccessor : ReactNativeFeatureFlagsAccesso
     return cached
   }
 
+  override fun enableLineHeightCentering(): Boolean {
+    var cached = enableLineHeightCenteringCache
+    if (cached == null) {
+      cached = ReactNativeFeatureFlagsCxxInterop.enableLineHeightCentering()
+      enableLineHeightCenteringCache = cached
+    }
+    return cached
+  }
+
   override fun enableLongTaskAPI(): Boolean {
     var cached = enableLongTaskAPICache
     if (cached == null) {

packages/react-native/ReactAndroid/src/main/java/com/facebook/react/internal/featureflags/ReactNativeFeatureFlagsCxxInterop.kt

@@ -4,7 +4,7 @@
  * This source code is licensed under the MIT license found in the
  * LICENSE file in the root directory of this source tree.
  *
- * @generated SignedSource<<1ed46e0bf712406c1bf6159e1bca3c15>>
+ * @generated SignedSource<<eb10b9718add54a31ee41200c8b48076>>
  */
 
 /**
@@ -38,8 +38,6 @@ public object ReactNativeFeatureFlagsCxxInterop {
 
   @DoNotStrip @JvmStatic public external fun enableAlignItemsBaselineOnFabricIOS(): Boolean
 
-  @DoNotStrip @JvmStatic public external fun enableAndroidLineHeightCentering(): Boolean
-
   @DoNotStrip @JvmStatic public external fun enableBridgelessArchitecture(): Boolean
 
   @DoNotStrip @JvmStatic public external fun enableCleanTextInputYogaNode(): Boolean
@@ -62,6 +60,8 @@ public object ReactNativeFeatureFlagsCxxInterop {
 
   @DoNotStrip @JvmStatic public external fun enableLayoutAnimationsOnIOS(): Boolean
 
+  @DoNotStrip @JvmStatic public external fun enableLineHeightCentering(): Boolean
+
   @DoNotStrip @JvmStatic public external fun enableLongTaskAPI(): Boolean
 
   @DoNotStrip @JvmStatic public external fun enableMicrotasks(): Boolean

packages/react-native/ReactAndroid/src/main/java/com/facebook/react/internal/featureflags/ReactNativeFeatureFlagsDefaults.kt

@@ -4,7 +4,7 @@
  * This source code is licensed under the MIT license found in the
  * LICENSE file in the root directory of this source tree.
  *
- * @generated SignedSource<<19cf402242ebd8b3a08dfb7c755b801b>>
+ * @generated SignedSource<<3c8abaac37fc7644f7ace96919390074>>
  */
 
 /**
@@ -33,8 +33,6 @@ public open class ReactNativeFeatureFlagsDefaults : ReactNativeFeatureFlagsProvi
 
   override fun enableAlignItemsBaselineOnFabricIOS(): Boolean = true
 
-  override fun enableAndroidLineHeightCentering(): Boolean = false
-
   override fun enableBridgelessArchitecture(): Boolean = false
 
   override fun enableCleanTextInputYogaNode(): Boolean = false
@@ -57,6 +55,8 @@ public open class ReactNativeFeatureFlagsDefaults : ReactNativeFeatureFlagsProvi
 
   override fun enableLayoutAnimationsOnIOS(): Boolean = true
 
+  override fun enableLineHeightCentering(): Boolean = false
+
   override fun enableLongTaskAPI(): Boolean = false
 
   override fun enableMicrotasks(): Boolean = false

packages/react-native/ReactAndroid/src/main/java/com/facebook/react/internal/featureflags/ReactNativeFeatureFlagsLocalAccessor.kt

@@ -4,7 +4,7 @@
  * This source code is licensed under the MIT license found in the
  * LICENSE file in the root directory of this source tree.
  *
- * @generated SignedSource<<a307a1f9b5064547ce32f9519bbf27c4>>
+ * @generated SignedSource<<f726bd566cebccbd10dac4ec6d2b974d>>
  */
 
 /**
@@ -29,7 +29,6 @@ public class ReactNativeFeatureFlagsLocalAccessor : ReactNativeFeatureFlagsAcces
   private var batchRenderingUpdatesInEventLoopCache: Boolean? = null
   private var completeReactInstanceCreationOnBgThreadOnAndroidCache: Boolean? = null
   private var enableAlignItemsBaselineOnFabricIOSCache: Boolean? = null
-  private var enableAndroidLineHeightCenteringCache: Boolean? = null
   private var enableBridgelessArchitectureCache: Boolean? = null
   private var enableCleanTextInputYogaNodeCache: Boolean? = null
   private var enableDeletionOfUnmountedViewsCache: Boolean? = null
@@ -41,6 +40,7 @@ public class ReactNativeFeatureFlagsLocalAccessor : ReactNativeFeatureFlagsAcces
   private var enableGranularShadowTreeStateReconciliationCache: Boolean? = null
   private var enableIOSViewClipToPaddingBoxCache: Boolean? = null
   private var enableLayoutAnimationsOnIOSCache: Boolean? = null
+  private var enableLineHeightCenteringCache: Boolean? = null
   private var enableLongTaskAPICache: Boolean? = null
   private var enableMicrotasksCache: Boolean? = null
   private var enablePreciseSchedulingForPremountItemsOnAndroidCache: Boolean? = null
@@ -123,16 +123,6 @@ public class ReactNativeFeatureFlagsLocalAccessor : ReactNativeFeatureFlagsAcces
     return cached
   }
 
-  override fun enableAndroidLineHeightCentering(): Boolean {
-    var cached = enableAndroidLineHeightCenteringCache
-    if (cached == null) {
-      cached = currentProvider.enableAndroidLineHeightCentering()
-      accessedFeatureFlags.add("enableAndroidLineHeightCentering")
-      enableAndroidLineHeightCenteringCache = cached
-    }
-    return cached
-  }
-
   override fun enableBridgelessArchitecture(): Boolean {
     var cached = enableBridgelessArchitectureCache
     if (cached == null) {
@@ -243,6 +233,16 @@ public class ReactNativeFeatureFlagsLocalAccessor : ReactNativeFeatureFlagsAcces
     return cached
   }
 
+  override fun enableLineHeightCentering(): Boolean {
+    var cached = enableLineHeightCenteringCache
+    if (cached == null) {
+      cached = currentProvider.enableLineHeightCentering()
+      accessedFeatureFlags.add("enableLineHeightCentering")
+      enableLineHeightCenteringCache = cached
+    }
+    return cached
+  }
+
   override fun enableLongTaskAPI(): Boolean {
     var cached = enableLongTaskAPICache
     if (cached == null) {

packages/react-native/ReactAndroid/src/main/java/com/facebook/react/internal/featureflags/ReactNativeFeatureFlagsProvider.kt

@@ -4,7 +4,7 @@
  * This source code is licensed under the MIT license found in the
  * LICENSE file in the root directory of this source tree.
  *
- * @generated SignedSource<<bda9e94a5fe61a16e7d001ea3acfed0c>>
+ * @generated SignedSource<<73688196634e93c5b6ea63181e13f738>>
  */
 
 /**
@@ -33,8 +33,6 @@ public interface ReactNativeFeatureFlagsProvider {
 
   @DoNotStrip public fun enableAlignItemsBaselineOnFabricIOS(): Boolean
 
-  @DoNotStrip public fun enableAndroidLineHeightCentering(): Boolean
-
   @DoNotStrip public fun enableBridgelessArchitecture(): Boolean
 
   @DoNotStrip public fun enableCleanTextInputYogaNode(): Boolean
@@ -57,6 +55,8 @@ public interface ReactNativeFeatureFlagsProvider {
 
   @DoNotStrip public fun enableLayoutAnimationsOnIOS(): Boolean
 
+  @DoNotStrip public fun enableLineHeightCentering(): Boolean
+
   @DoNotStrip public fun enableLongTaskAPI(): Boolean
 
   @DoNotStrip public fun enableMicrotasks(): Boolean

packages/react-native/ReactAndroid/src/main/java/com/facebook/react/views/text/internal/span/CustomLineHeightSpan.kt

@@ -107,7 +107,7 @@ public class CustomLineHeightSpan(height: Float) : LineHeightSpan, ReactSpan {
       v: Int,
       fm: FontMetricsInt,
   ) {
-    if (ReactNativeFeatureFlags.enableAndroidLineHeightCentering()) chooseCenteredHeight(fm)
+    if (ReactNativeFeatureFlags.enableLineHeightCentering()) chooseCenteredHeight(fm)
     else chooseOriginalHeight(fm)
   }
 }