Fix styling of system fonts

Summary:
When you call `-[RCTConvert UIFont:withFamily:...]` with a non-nil font object, we'll try to use the existing font object for system information. On iOS9+ which uses the San Francisco font, `[UIFont fontNamesForFamilyName:]` doesn't return anything useful, so we need to make sure that we detect this as a system font and use the appropriate methods. This issues is made worse by the fact that RCTTextView and friends recreate the font property for every attribute that is set (pretty horrible perf-wise).

This fixes #2140

Reviewed By: sahrens

Differential Revision: D3662751

fbshipit-source-id: c528e8945ed361a922c03f861d3c0b584658573b

React/Base/RCTConvert.m

@@ -481,24 +481,6 @@ + (CGColorRef)CGColor:(id)json
   return [self UIColor:json].CGColor;
 }
 
-#if !defined(__IPHONE_8_2) || __IPHONE_OS_VERSION_MIN_REQUIRED < __IPHONE_8_2
-
-// These constants are defined in iPhone SDK 8.2, but the app cannot run on
-// iOS < 8.2 unless we redefine them here. If you target iOS 8.2 or above
-// as a base target, the standard constants will be used instead.
-
-#define UIFontWeightUltraLight -0.8
-#define UIFontWeightThin -0.6
-#define UIFontWeightLight -0.4
-#define UIFontWeightRegular 0
-#define UIFontWeightMedium 0.23
-#define UIFontWeightSemibold 0.3
-#define UIFontWeightBold 0.4
-#define UIFontWeightHeavy 0.56
-#define UIFontWeightBlack 0.62
-
-#endif
-
 typedef CGFloat RCTFontWeight;
 RCT_ENUM_CONVERTER(RCTFontWeight, (@{
   @"normal": @(UIFontWeightRegular),
@@ -575,49 +557,52 @@ + (UIFont *)UIFont:(id)json
                  size:json[@"fontSize"]
                weight:json[@"fontWeight"]
                 style:json[@"fontStyle"]
-          scaleMultiplier:1.0f];
+      scaleMultiplier:1];
 }
 
 + (UIFont *)UIFont:(UIFont *)font withSize:(id)json
 {
-  return [self UIFont:font withFamily:nil size:json weight:nil style:nil scaleMultiplier:1.0];
+  return [self UIFont:font withFamily:nil size:json weight:nil style:nil scaleMultiplier:1];
 }
 
 + (UIFont *)UIFont:(UIFont *)font withWeight:(id)json
 {
-  return [self UIFont:font withFamily:nil size:nil weight:json style:nil scaleMultiplier:1.0];
+  return [self UIFont:font withFamily:nil size:nil weight:json style:nil scaleMultiplier:1];
 }
 
 + (UIFont *)UIFont:(UIFont *)font withStyle:(id)json
 {
-  return [self UIFont:font withFamily:nil size:nil weight:nil style:json scaleMultiplier:1.0];
+  return [self UIFont:font withFamily:nil size:nil weight:nil style:json scaleMultiplier:1];
 }
 
 + (UIFont *)UIFont:(UIFont *)font withFamily:(id)json
 {
-  return [self UIFont:font withFamily:json size:nil weight:nil style:nil scaleMultiplier:1.0];
+  return [self UIFont:font withFamily:json size:nil weight:nil style:nil scaleMultiplier:1];
 }
 
 + (UIFont *)UIFont:(UIFont *)font withFamily:(id)family
               size:(id)size weight:(id)weight style:(id)style
    scaleMultiplier:(CGFloat)scaleMultiplier
 {
   // Defaults
-  NSString *const RCTDefaultFontFamily = @"System";
-  NSString *const RCTIOS8SystemFontFamily = @"Helvetica Neue";
-  const RCTFontWeight RCTDefaultFontWeight = UIFontWeightRegular;
-  const CGFloat RCTDefaultFontSize = 14;
+  static NSString *defaultFontFamily;
+  static dispatch_once_t onceToken;
+  dispatch_once(&onceToken, ^{
+    defaultFontFamily = [UIFont systemFontOfSize:14].familyName;
+  });
+  const RCTFontWeight defaultFontWeight = UIFontWeightRegular;
+  const CGFloat defaultFontSize = 14;
 
   // Initialize properties to defaults
-  CGFloat fontSize = RCTDefaultFontSize;
-  RCTFontWeight fontWeight = RCTDefaultFontWeight;
-  NSString *familyName = RCTDefaultFontFamily;
+  CGFloat fontSize = defaultFontSize;
+  RCTFontWeight fontWeight = defaultFontWeight;
+  NSString *familyName = defaultFontFamily;
   BOOL isItalic = NO;
   BOOL isCondensed = NO;
 
   if (font) {
-    familyName = font.familyName ?: RCTDefaultFontFamily;
-    fontSize = font.pointSize ?: RCTDefaultFontSize;
+    familyName = font.familyName ?: defaultFontFamily;
+    fontSize = font.pointSize ?: defaultFontSize;
     fontWeight = RCTWeightOfFont(font);
     isItalic = RCTFontIsItalic(font);
     isCondensed = RCTFontIsCondensed(font);
@@ -634,7 +619,7 @@ + (UIFont *)UIFont:(UIFont *)font withFamily:(id)family
 
   // Handle system font as special case. This ensures that we preserve
   // the specific metrics of the standard system font as closely as possible.
-  if ([familyName isEqual:RCTDefaultFontFamily]) {
+  if ([familyName isEqual:defaultFontFamily] || [familyName isEqualToString:@"System"]) {
     if ([UIFont respondsToSelector:@selector(systemFontOfSize:weight:)]) {
       font = [UIFont systemFontOfSize:fontSize weight:fontWeight];
       if (isItalic || isCondensed) {
@@ -650,10 +635,6 @@ + (UIFont *)UIFont:(UIFont *)font withFamily:(id)family
         font = [UIFont fontWithDescriptor:fontDescriptor size:fontSize];
       }
       return font;
-    } else {
-      // systemFontOfSize:weight: isn't available prior to iOS 8.2, so we
-      // fall back to finding the correct font manually, by linear search.
-      familyName = RCTIOS8SystemFontFamily;
     }
   }