Disable font scaling when an explicit font handler is set

Reviewed By: sahrens

Differential Revision: D7003464

fbshipit-source-id: f36ff344c50a9c63af6c852138041c1c918259c8

Author: Mehdi Mulani

Libraries/Text/RCTTextAttributes.m

@@ -190,7 +190,7 @@ - (UIFont *)effectiveFont
 
 - (CGFloat)effectiveFontSizeMultiplier
 {
-  return _allowFontScaling && !isnan(_fontSizeMultiplier) ? _fontSizeMultiplier : 1.0;
+  return !RCTHasFontHandlerSet() && _allowFontScaling && !isnan(_fontSizeMultiplier) ? _fontSizeMultiplier : 1.0;
 }
 
 - (UIColor *)effectiveForegroundColor

React/Views/RCTFont.h

@@ -20,6 +20,7 @@ typedef UIFont *(^RCTFontHandler)(CGFloat fontSize, NSString *fontWeightDescript
  * "semibold", "extrabold", "bold", "heavy", or "black".
  */
 RCT_EXTERN void RCTSetDefaultFontHandler(RCTFontHandler handler);
+RCT_EXTERN BOOL RCTHasFontHandlerSet();
 
 @interface RCTFont : NSObject
 

React/Views/RCTFont.mm

@@ -103,6 +103,10 @@ void RCTSetDefaultFontHandler(RCTFontHandler handler) {
   defaultFontHandler = handler;
 }
 
+BOOL RCTHasFontHandlerSet() {
+  return defaultFontHandler != nil;
+}
+
 // We pass a string description of the font weight to the defaultFontHandler because UIFontWeight
 // is not defined pre-iOS 8.2.
 // Furthermore, UIFontWeight's are lossy floats, so we must use an inexact compare to figure out