iOS实现粗体斜体字体boldItalic

原来自己是用NSAttributedString,通过倾斜角度，哎，差劲！！！ stackoverflow.com/questions/4… oc版：

UILabel * label = [[UILabel alloc] init]; // use your label object instead of this
UIFontDescriptor * fontD = [label.font.fontDescriptor
            fontDescriptorWithSymbolicTraits:UIFontDescriptorTraitBold
                            | UIFontDescriptorTraitItalic];
label.font = [UIFont fontWithDescriptor:fontD size:0];

swift版：

extension UIFont {

    func withTraits(traits:UIFontDescriptorSymbolicTraits...) -> UIFont {
        let descriptor = self.fontDescriptor()
            .fontDescriptorWithSymbolicTraits(UIFontDescriptorSymbolicTraits(traits))
        return UIFont(descriptor: descriptor, size: 0)
    }

    func boldItalic() -> UIFont {
        return withTraits(.TraitBold, .TraitItalic)
    }

}