NSAttributedString.swift

extension NSMutableAttributedString {

    func add(attribute: Attribute, range: NSRange) throws {
        guard (range.location + range.length) <= length else {
            throw AttributedStringError.InvalidRange(range: range)
        }
        addAttribute(attribute.name, value: attribute.value, range: range)
    }
}

enum AttributedStringError: ErrorType {
    case InvalidRange(range: NSRange)
}

enum Attribute {
    case Font(UIFont)
    case ForegroundColor(UIColor)
    case BackgroundColor(UIColor)
    case ParagraphStyle(NSParagraphStyle)
    case Ligatures(Bool)
    case Kerning(Float)
    case StrikethroughStyle(Int)
    case StrikethroughColor(UIColor)
    case UnderlineStyle(Int)
    case UnderlineColor(UIColor)
    case StrokeWidth(Float)
    case StrokeColor(UIColor)
    case Shadow(NSShadow)
    case TextEffect(String)
    case Attachment(NSTextAttachment)
    case Link(NSURL)
    case BaselineOffset(Float)
    case Obliqueness(Float)
    case Expansion(Float)

    var name: String {
        switch self {
        case .Font:                 return NSFontAttributeName
        case .ForegroundColor:      return NSForegroundColorAttributeName
        case .BackgroundColor:      return NSBackgroundColorAttributeName
        case .ParagraphStyle:       return NSParagraphStyleAttributeName
        case .Ligatures:            return NSLigatureAttributeName
        case .Kerning:              return NSKernAttributeName
        case .StrikethroughStyle:   return NSStrikethroughStyleAttributeName
        case .UnderlineStyle:       return NSUnderlineStyleAttributeName
        case .UnderlineColor:       return NSUnderlineColorAttributeName
        case .StrokeWidth:          return NSStrokeWidthAttributeName
        case .StrokeColor:          return NSStrokeColorAttributeName
        case .Shadow:               return NSShadowAttributeName
        case .TextEffect:           return NSTextEffectAttributeName
        case .Attachment:           return NSAttachmentAttributeName
        case .Link:                 return NSLinkAttributeName
        case .BaselineOffset:       return NSBaselineOffsetAttributeName
        case .StrikethroughColor:   return NSStrikethroughColorAttributeName
        case .Obliqueness:          return NSObliquenessAttributeName
        case .Expansion:            return NSExpansionAttributeName
        }
    }

    var value: AnyObject {
        switch self {
        case .Font(let font):                   return font
        case .ForegroundColor(let color):       return color
        case .BackgroundColor(let color):       return color
        case .ParagraphStyle(let style):        return style
        case .Ligatures(let val):               return NSNumber(bool: val)
        case .Kerning(let val):                 return NSNumber(float: val)
        case .StrikethroughStyle(let val):      return NSNumber(integer: val)
        case .StrikethroughColor(let color):    return color
        case .UnderlineStyle(let val):          return NSNumber(integer: val)
        case .UnderlineColor(let color):        return color
        case .StrokeWidth(let val):             return NSNumber(float: val)
        case .StrokeColor(let color):           return color
        case .Shadow(let shadow):               return shadow
        case .TextEffect(let effect):           return effect
        case .Attachment(let attachment):       return attachment
        case .Link(let url):                    return url
        case .BaselineOffset(let val):          return NSNumber(float: val)
        case .Obliqueness(let val):             return NSNumber(float: val)
        case .Expansion(let val):               return NSNumber(float: val)
        }
    }
}