Custom Font Sizing in Xcode 6 Size Classes not working properly with Custom Fonts

Question

Xcode 6 has a new feature where fonts and font sizes in UILabel, UITextField, and UIButton can be s

Answer 1

A combination of some of the later answers above were helpful. Here's how I solved the IB bug via a Swift UILabel extension:

import UIKit

// This extension is only required as a work-around to an interface builder bug in XCode 7.3.1
// When custom fonts are set per size class, they are reset to a small system font
// In order for this extension to work, you must set the fonts in IB to System
// We are switching any instances of ".SFUIDisplay-Bold" to "MuseoSans-700" and ".SFUIDisplay-Regular" to "MuseoSans-300" and keeping the same point size as specified in IB

extension UILabel {
    override public func traitCollectionDidChange(previousTraitCollection: UITraitCollection?) {
        super.traitCollectionDidChange(previousTraitCollection)

        if ((traitCollection.verticalSizeClass != previousTraitCollection?.verticalSizeClass) || traitCollection.horizontalSizeClass != previousTraitCollection?.horizontalSizeClass) {
            //let oldFontName = "\(font.fontName)-\(font.pointSize)"

            if (font.fontName == systemFontRegular) {
                font = UIFont(name: customFontRegular, size: (font?.pointSize)!)
                //xlog.debug("Old font: \(oldFontName) -> new Font: \(font.fontName) - \(font.pointSize)")
            }
            else if (font.fontName == systemFontBold) {
                font = UIFont(name: customFontBold, size: (font?.pointSize)!)
                //xlog.debug("Old font: \(oldFontName) -> new Font: \(font.fontName) - \(font.pointSize)")
            }
        }
    }
}