get amount of NSCalendarUnits in NSDateComponents

i want to write a function, which returns specific NSCalendarUnits from a String. My function therefore takes as many NSCalendarUnits as arguments as the user wants to.

public extension String{
    func calendarComponents(units: NSCalendarUnit, separateString: String) -> NSDateComponents{
    // now i want to know which units the user specified 
    // so i can scan the string for integerValues and add them to 
    // the corresponding component
    }
}

as a result i want the user to define a String like var myStringDate = "4/5" and then use the function

 myStringDate.calendarComponents(units: .CalendarUnitMonth |    
 .CalendarUnitWeekday, "/")

and then get a NSDateComponent object with .month = 4 and .weekday = 5.

i do know how to scan the string for all the needed Values, but i don't know, how to add those values to the right component attribute.