Implementing [NSCell copyWithZone:] in Swift

TableViewTextFieldCell.swift

// Swift 3.0
class TableViewTextFieldCell: NSTextFieldCell {
    
    private var previousTextColor: NSColor?
    
    override func copy(with zone: NSZone? = nil) -> Any {
        let result: TableViewTextFieldCell = super .copy(with: zone) as! TableViewTextFieldCell
        if let previousTextColor = result.previousTextColor {
            // Add the needed retain now
            let _ = Unmanaged<NSColor>.passRetained(previousTextColor)
        }
        return result
    }
}