UIFontWeightTrait and UIFontDescriptorFamilyAttribute Ignored when creating UIFont from UIFontDescriptor

I ran into the same issue, and the documentation was not much help. Eventually I figured out that using the family attribute combined with the face attribute worked:

 UIFontDescriptor* desc = [UIFontDescriptor fontDescriptorWithFontAttributes:
        @{
            UIFontDescriptorFamilyAttribute: @"Helvetica Neue",
            UIFontDescriptorFaceAttribute: @"Light"
        }
    ];

From the docs:

Font Traits Dictionary Keys

The following constants can be used as keys to retrieve information about a font descriptor from its trait dictionary.

NSString *const UIFontSymbolicTrait;
NSString *const UIFontWeightTrait;
NSString *const UIFontWidthTrait;
NSString *const UIFontSlantTrait;

This reads to me like these keys are designed only for getting information from a font descriptor—not for setting it. For example you could do something like this:

UIFont *font = [UIFont preferredFontForTextStyle:UIFontTextStyleHeadline];
NSDictionary *traits = [font.fontDescriptor objectForKey:UIFontDescriptorTraitsAttribute];
CGFloat weight = [traits[UIFontWeightTrait] floatValue];

Which would show you that the font you got back is a little bit heavier than the normal weight. This doesn't seem nearly as useful as being able to ask for a lighter weight of a font without having to give an exact name—but it seems to be the intended usage.

When looking for a bold or italic font, symbolic traits at least do appear to be respected. So this works nicely:

+ (UIFont*)fontWithFamily:(NSString*)family bold:(BOOL)bold italic:(BOOL)italic size:(CGFloat)pointSize {
  UIFontDescriptorSymbolicTraits traits = 0;
  if (bold)   traits |= UIFontDescriptorTraitBold;
  if (italic) traits |= UIFontDescriptorTraitItalic;
  UIFontDescriptor* fd = [UIFontDescriptor
                          fontDescriptorWithFontAttributes:@{UIFontDescriptorFamilyAttribute: family,
                                                             UIFontDescriptorTraitsAttribute: @{UIFontSymbolicTrait:
                                                                                                  [NSNumber numberWithInteger:traits]}}];
  NSArray* matches = [fd matchingFontDescriptorsWithMandatoryKeys:
                      [NSSet setWithObjects:UIFontDescriptorFamilyAttribute, UIFontDescriptorTraitsAttribute, nil]];
  if (matches.count == 0) return nil;
  return [UIFont fontWithDescriptor:matches[0] size:pointSize];
}

e.g. [MyClass fontWithFamily:@"Avenir Next Condensed" bold:YES italic:NO size:12.0f];

I don't think this helps the OP, who was looking specifically for a light font with UIFontWeightTrait, but it may help others with similar problems.