[Text] Add a virtual fontName called "System".

Add a virtual fontName called "System" that defaults to whatever the
current system font is. (@nicklockwood #1611)

It may break UI of existing apps.

Author: dalinaum

Examples/UIExplorer/UIExplorerIntegrationTests/ReferenceImages/Examples-UIExplorer-UIExplorerApp.ios/[email protected]

@@ -41,7 +41,8 @@ - (void)DISABLED_testWeight // task #7118691
 - (void)testSize
 {
   {
-    UIFont *expected = [UIFont fontWithName:@"HelveticaNeue" size:18.5];
+    UIFont *expected =
+      [UIFont fontWithName:[UIFont systemFontOfSize:18.5].fontName size:18.5];
     UIFont *result = [RCTConvert UIFont:@{@"fontSize": @18.5}];
     RCTAssertEqualFonts(expected, result);
   }
@@ -68,13 +69,33 @@ - (void)testFamily
 
 - (void)testStyle
 {
+  NSString *systemFontName = [UIFont systemFontOfSize:14].fontName; 
   {
-    UIFont *expected = [UIFont fontWithName:@"HelveticaNeue-Italic" size:14];
+    UIFont *bestMatch = [UIFont fontWithName:systemFontName size: 14];
+    CGFloat closestWeight = INFINITY;
+
+    for (NSString *name in [UIFont fontNamesForFamilyName:systemFontName]) {
+      UIFont *match = [UIFont fontWithName:name size:14];
+      NSDictionary *traits = [match.fontDescriptor objectForKey:UIFontDescriptorTraitsAttribute];
+      UIFontDescriptorSymbolicTraits symbolicTraits =
+        [traits[UIFontSymbolicTrait] unsignedIntValue];
+      BOOL isItalic = (symbolicTraits & UIFontDescriptorTraitItalic) != 0;
+      BOOL isCondensed = (symbolicTraits & UIFontDescriptorTraitCondensed) != 0;
+      CGFloat weightOfFont = [traits[UIFontWeightTrait] doubleValue];
+
+      if (isItalic && !isCondensed) {
+        if (ABS(weightOfFont) < ABS(closestWeight)) {
+          bestMatch = match;
+          closestWeight = weightOfFont;
+        }
+      }
+    }
+    UIFont *expected = [UIFont fontWithName:bestMatch.fontName size:14];
     UIFont *result = [RCTConvert UIFont:@{@"fontStyle": @"italic"}];
     RCTAssertEqualFonts(expected, result);
   }
   {
-    UIFont *expected = [UIFont fontWithName:@"HelveticaNeue" size:14];
+    UIFont *expected = [UIFont fontWithName:systemFontName size:14];
     UIFont *result = [RCTConvert UIFont:@{@"fontStyle": @"normal"}];
     RCTAssertEqualFonts(expected, result);
   }

Examples/UIExplorer/UIExplorerIntegrationTests/ReferenceImages/Examples-UIExplorer-UIExplorerApp.ios/[email protected]

@@ -746,7 +746,7 @@ + (UIFont *)UIFont:(UIFont *)font withFamily:(id)family
               size:(id)size weight:(id)weight style:(id)style
 {
   // Defaults
-  NSString *const RCTDefaultFontFamily = @"Helvetica Neue";
+  NSString *const RCTDefaultFontFamily = @"System";
   const RCTFontWeight RCTDefaultFontWeight = UIFontWeightRegular;
   const CGFloat RCTDefaultFontSize = 14;
 
@@ -771,6 +771,10 @@ + (UIFont *)UIFont:(UIFont *)font withFamily:(id)family
   // Get font family
   familyName = [self NSString:family] ?: familyName;
 
+  if ([familyName isEqualToString: RCTDefaultFontFamily]) {
+    familyName = [UIFont systemFontOfSize:fontSize].familyName;
+  }
+
   // Gracefully handle being given a font name rather than font family, for
   // example: "Helvetica Light Oblique" rather than just "Helvetica".
   if ([UIFont fontNamesForFamilyName:familyName].count == 0) {