Sum all values of a textfield of a NSTableView column

I work with swift 3 and I have a NSTableView (3 columns). I fill the data like below:

 func tableView(_ tableView: NSTableView, viewFor tableColumn: NSTableColumn?, row: Int) -> NSView? {

        var cellIdentifier = String()
        var cellText = String()

            switch tableColumn {
                case tablewView.tableColumns[0]?:
                    cellText = "100"
                    cellIdentifier = "Cell1"
                break

                case tablewView.tableColumns[1]?:
                    cellText = "100"
                    cellIdentifier = "Cell2"
                break

                case tablewView.tableColumns[2]?:
                    cellText = "100"
                    cellIdentifier = "Cell3"
                break

                default: break
            }

            if let view = tableView.make(withIdentifier: cellIdentifier, owner: nil) as? NSTableCellView {
                view.textField?.stringValue = cellText
                return view
            }

        return nil
    }

Now I would like to sum all values of column 1, every time when the selection will change. how can I realize this?