Dynamic Type system fonts with custom point sizes, weight, and italics

UIFont+CustomizedDynamicType.m

static const CGFloat kFontWeightEpsilon = FLT_EPSILON;

@implementation UIFont (CustomizedDynamicType)

+ (nonnull UIFont *)preferredFontWithDefaultSize:(CGFloat)size
                                       textStyle:(nonnull UIFontTextStyle)textStyle {
  return [self preferredFontWithDefaultSize:size
                                  textStyle:textStyle
                                 fontWeight:UIFontWeightRegular
                                     italic:NO];
}

+ (nonnull UIFont *)preferredFontWithDefaultSize:(CGFloat)size
                                       textStyle:(nonnull UIFontTextStyle)textStyle
                                      fontWeight:(UIFontWeight)fontWeight
                                          italic:(BOOL)italic {
  UIFontDescriptor *fontDescriptor =
      [UIFontDescriptor preferredFontDescriptorWithTextStyle:textStyle];
  BOOL weighted = fabs(fontWeight) > kFontWeightEpsilon;
  if (@available(iOS 14, *)) {
    if (weighted) {
      fontDescriptor = [fontDescriptor fontDescriptorByAddingAttributes:@{
        UIFontDescriptorTraitsAttribute : @{UIFontWeightTrait : @(fontWeight)}
      }];
    }
    if (italic) {
      fontDescriptor = [fontDescriptor fontDescriptorByAddingAttributes:@{
        UIFontDescriptorTraitsAttribute : @{UIFontSymbolicTrait : @(UIFontDescriptorTraitItalic)}
      }];
    }
  } else {
    if (italic) {
      BOOL isBolded = fontWeight > 0;
      if (isBolded) {
        // Pre-iOS 14 does not support weighted italic fonts for any weight other than bold, and the
        // only way to configure a font in that way is to use symbolic traits, so we short-circuit
        // the case of bold-italic fonts here.
        UIFontDescriptor *boldItalicFontDescriptor =
            [fontDescriptor fontDescriptorWithSymbolicTraits:UIFontDescriptorTraitItalic |
                                                             UIFontDescriptorTraitBold];
        if (boldItalicFontDescriptor != nil) {
          fontDescriptor = boldItalicFontDescriptor;
        } else {
          // Bold italic didn't work, fall back to at least bold then.
          UIFontDescriptor *boldFontDescriptor =
              [fontDescriptor fontDescriptorWithSymbolicTraits:UIFontDescriptorTraitBold];
          if (boldFontDescriptor != nil) {
            fontDescriptor = boldFontDescriptor;
          }
        }
      } else {
        // Treat all other weights as simply italic.
        UIFontDescriptor *italicFontDescriptor =
            [fontDescriptor fontDescriptorWithSymbolicTraits:UIFontDescriptorTraitItalic];
        if (italicFontDescriptor != nil) {
          fontDescriptor = italicFontDescriptor;
        }
      }
    } else if (weighted) {
      if (@available(iOS 13, *)) {
        // iOS 13 temporarily broke support for making bold fonts using UIFontWeightTrait, so we
        // fall back to the system font API. This means we lose leading adjustments, but are able to
        // preserve the expected weighting.
        fontDescriptor = [[UIFont systemFontOfSize:size weight:fontWeight] fontDescriptor];
      } else {
        // On iOS 12 we can customize font descriptors using UIFontWeightBold, so we use this
        // approach in order to maintain UIKit's tweaked leading.
        if (fabs(fontWeight - UIFontWeightBold) < kFontWeightEpsilon) {
          fontDescriptor = [fontDescriptor fontDescriptorByAddingAttributes:@{
            UIFontDescriptorTraitsAttribute : @{UIFontWeightTrait : @(UIFontWeightBold)}
          }];
        } else {
          // But for all other weights, we need to fall back to the system font API in order to
          // receive a font of the requested weight. This means we lose the tweaked leading values,
          // but it's more important that we respect the requested weighting.
          fontDescriptor = [[UIFont systemFontOfSize:size weight:fontWeight] fontDescriptor];
        }
      }
    }
  }
  UIFont *font = [UIFont fontWithDescriptor:fontDescriptor size:size];
  UIFontMetrics *fontMetrics = [UIFontMetrics metricsForTextStyle:textStyle];
  return [fontMetrics scaledFontForFont:font];
}

@end