Ordering Matters in UIFontDescriptor, a bug

FontDescriptorBug.swift

import UIKit

/*:
 # Ordering Matters in UIFontDescriptor
 Oct 21, 2017, iOS 11

 When creating a derived font descriptor with a font family, existing traits
 such as italic or bold are overwritten or ignored.
*/

/*:
 Here I attempt to build a font with an italic trait from the Avenir family.
 */

var wrongFontDescriptor = UIFontDescriptor()
wrongFontDescriptor = wrongFontDescriptor.withSymbolicTraits([.traitItalic])!
wrongFontDescriptor = wrongFontDescriptor.withFamily("Avenir")

/*:
 The resulting font is "Avenir-Book", which is not what I would expect.
 */

var wrongFont = UIFont(descriptor: wrongFontDescriptor, size: 12)
wrongFont.fontName // "Avenir-Book"

/*:
 If symbolic traits are added after the font family, the resulting font is as
 expected.
 */

var rightFontDescriptor = UIFontDescriptor()
rightFontDescriptor = rightFontDescriptor.withFamily("Avenir")
rightFontDescriptor = rightFontDescriptor.withSymbolicTraits([.traitItalic])!
var rightFont = UIFont(descriptor: rightFontDescriptor, size: 12)
rightFont.fontName // "Avenir-BookOblique"

/*:
 I would expect that setting the font family would not override traits
 unecessarily. This would allow the construction of fonts from their
 characteristics in any order.
 
 Alternatively, I would expect the documentation for `UIFontDescriptor` to be
 clear about the behavior of `-fontDescriptorWithFamily:`.
 
 The documentation states only:
 
 > Returns a new font descriptor whose attributes are the same as the receiver
 > but from the specified family.
 */

/*:
 ## Additional Examples
 
 The same can be shown for the Bold trait and combined traits.
 
 Traits before family results in a wrong font:
 */

wrongFontDescriptor = UIFontDescriptor()
wrongFontDescriptor = wrongFontDescriptor.withSymbolicTraits([.traitBold])!
wrongFontDescriptor = wrongFontDescriptor.withFamily("Avenir")
wrongFont = UIFont(descriptor: wrongFontDescriptor, size: 12)
wrongFont.fontName // "Avenir-Book"

wrongFontDescriptor = UIFontDescriptor()
wrongFontDescriptor = wrongFontDescriptor.withSymbolicTraits([.traitBold, .traitItalic])!
wrongFontDescriptor = wrongFontDescriptor.withFamily("Avenir")
wrongFont = UIFont(descriptor: wrongFontDescriptor, size: 12)
wrongFont.fontName // "Avenir-Book"

/*:
 Traits after family is correct
 */

rightFontDescriptor = UIFontDescriptor()
rightFontDescriptor = rightFontDescriptor.withFamily("Avenir")
rightFontDescriptor = rightFontDescriptor.withSymbolicTraits([.traitBold])!
rightFont = UIFont(descriptor: rightFontDescriptor, size: 12)
rightFont.fontName // "Avenir-Heavy"

rightFontDescriptor = UIFontDescriptor()
rightFontDescriptor = rightFontDescriptor.withFamily("Avenir")
rightFontDescriptor = rightFontDescriptor.withSymbolicTraits([.traitItalic, .traitBold])!
rightFont = UIFont(descriptor: rightFontDescriptor, size: 12)
rightFont.fontName // "Avenir-HeavyOblique"