@IBOutlet weak var txtScrollView: NSScrollView!

self.txtScrollView.documentView?.setString("写入第一句。")

self.txtScrollView.documentView?.textStorage??.mutableString.appendString("追加第二句。")

if let height=self.txtScrollView.documentView?.bounds.size.height{
   var y=height-self.txtScrollView.documentVisibleRect.height
   if y<0{
      y=0
   }
   self.txtScrollView.contentView.scrollPoint(NSMakePoint(0, y))

}