base-line metric exposure for <Text>

Summary:
Now <Text> exposes base-line metric to Yoga.

Before:
https://cl.ly/1F0B0D430U3e

After:
https://cl.ly/0G1Q29450O0y

Reviewed By: yungsters

Differential Revision: D6957055

fbshipit-source-id: 04c1df693915e294b54e3c33e0aea21611dcc232

Libraries/Text/Text/RCTTextShadowView.m

@@ -31,6 +31,7 @@ - (instancetype)initWithBridge:(RCTBridge *)bridge
     _cachedTextStorages = [NSMapTable strongToStrongObjectsMapTable];
     _needsUpdateView = YES;
     YGNodeSetMeasureFunc(self.yogaNode, RCTTextShadowViewMeasure);
+    YGNodeSetBaselineFunc(self.yogaNode, RCTTextShadowViewBaseline);
   }
 
   return self;
@@ -307,6 +308,27 @@ - (void)layoutSubviewsWithContext:(RCTLayoutContext)layoutContext
   ];
 }
 
+- (CGFloat)lastBaselineForSize:(CGSize)size
+{
+  NSAttributedString *attributedText =
+    [self textStorageAndLayoutManagerThatFitsSize:size exclusiveOwnership:NO];
+
+  __block CGFloat maximumDescender = 0.0;
+
+  [attributedText enumerateAttribute:NSFontAttributeName
+                             inRange:NSMakeRange(0, attributedText.length)
+                             options:NSAttributedStringEnumerationLongestEffectiveRangeNotRequired
+                          usingBlock:
+    ^(UIFont *font, NSRange range, __unused BOOL *stop) {
+      if (maximumDescender > font.descender) {
+        maximumDescender = font.descender;
+      }
+    }
+  ];
+
+  return size.height + maximumDescender;
+}
+
 static YGSize RCTTextShadowViewMeasure(YGNodeRef node, float width, YGMeasureMode widthMode, float height, YGMeasureMode heightMode)
 {
   CGSize maximumSize = (CGSize){
@@ -341,4 +363,18 @@ static YGSize RCTTextShadowViewMeasure(YGNodeRef node, float width, YGMeasureMod
   };
 }
 
+static float RCTTextShadowViewBaseline(YGNodeRef node, const float width, const float height)
+{
+  RCTTextShadowView *shadowTextView = (__bridge RCTTextShadowView *)YGNodeGetContext(node);
+
+  CGSize size = (CGSize){
+    RCTCoreGraphicsFloatFromYogaFloat(width),
+    RCTCoreGraphicsFloatFromYogaFloat(height)
+  };
+
+  CGFloat lastBaseline = [shadowTextView lastBaselineForSize:size];
+
+  return RCTYogaFloatFromCoreGraphicsFloat(lastBaseline);
+}
+
 @end