fontChange.swift

func toggleBoldOverRange(range:NSRange) {
        
        if let string = textView?.textStorage?.attributedSubstringFromRange(range) {
            
            let attribs = string.fontAttributesInRange(NSRange(location: 0, length: string.length))
            
            if let font = attribs["NSFont"] as? NSFont {
                
                if font.isBold() {
                    if textView!.shouldChangeTextInRange(range, replacementString: string.string) {
                        textView?.textStorage?.beginEditing()
                        textView?.textStorage?.applyFontTraits(.UnboldFontMask, range: range)
                        textView?.textStorage?.endEditing()

                    }
                }else {
                    
                    if textView!.shouldChangeTextInRange(range, replacementString: string.string) {
                        textView?.textStorage?.beginEditing()
                        textView?.textStorage?.applyFontTraits(.BoldFontMask, range: range)
                        textView?.textStorage?.endEditing()

                    }
                }
            }
        }
    }

foo.swift

ffunc toggleTaskListOverRange(range:NSRange) {

        if let lines = textView?.textStorage?.attributedSubstringFromRange(range).mutableLines() where !lines.isEmpty {
            
            let replacementString = NSMutableAttributedString()
            
            lines.forEach { (attributedString) in
                
                if attributedString.hasCheckBox() {
                    
                    attributedString.deleteCharactersInRange(NSRange(location: 0, length: 2))
//                    let newLine = NSAttributedString(string: "\n")
//                    attributedString.appendAttributedString(newLine)
                    replacementString.appendAttributedString(attributedString)
                    
                }else {
                    
                    if !attributedString.string.isEmpty {
                        var mutableAttribs = attributedString.attributesAtIndex(0, effectiveRange: nil)
                        
                        if let currentFont = mutableAttribs[NSFontAttributeName] as? NSFont {
                            
                            let size = currentFont.pointSize
                            mutableAttribs[NSFontAttributeName] = NSFont(name: "Hellvetica", size: size)
                            let box = NSAttributedString.taskUncheckedStringWith(mutableAttribs)
                            attributedString.insertAttributedString(box, atIndex: 0)
//                            let newLine = NSAttributedString(string: "\n")
//                            attributedString.appendAttributedString(newLine)
                            replacementString.appendAttributedString(attributedString)
                        }

                    }
                }
            }
            
            if ((textView?.shouldChangeTextInRange(range, replacementString: replacementString.string)) != nil) {
                textView?.textStorage?.beginEditing()
                textView?.textStorage?.replaceCharactersInRange(range, withAttributedString: replacementString)
                textView?.textStorage?.endEditing()
                textView?.didChangeText()
            }
        }
    }

NSFontExtensions.swift

extension NSFont {
    
    func isBold() -> Bool {
        return NSFontManager.sharedFontManager().traitsOfFont(self).contains(.BoldFontMask)
    }
    
    func isItalic() -> Bool {
        return NSFontManager.sharedFontManager().traitsOfFont(self).contains(.ItalicFontMask)
    }
    
    func boldFont() -> NSFont {
        return NSFontManager.sharedFontManager().convertFont(self, toHaveTrait: .BoldFontMask)
    }
    
    func italicFont() -> NSFont {
        return NSFontManager.sharedFontManager().convertFont(self, toHaveTrait: .ItalicFontMask)
    }
    
    func nonBoldFont() -> NSFont {
        return NSFontManager.sharedFontManager().convertFont(self, toHaveTrait: .UnboldFontMask)
    }
    
    func nonItalicFont() -> NSFont {
        return NSFontManager.sharedFontManager().convertFont(self, toHaveTrait: .UnitalicFontMask)
    }
    
}