NSCalendar in Swift - init can return nil, but isn't optional

Question

NSCalendar(calendarIdentifier: calendarName) can return nil if calendarName is not valid - this is the original Objective-C behaviour, and is also true in Swift. However, it appears that the compiler believes the initializer returns an NSCalendar rather than an NSCalendar?, as follows:

let c1 = NSCalendar(calendarIdentifier: "gregorian")// _NSCopyOnWriteCalendarWrapper
let c2 = NSCalendar(calendarIdentifier: "buddhist")// _NSCopyOnWriteCalendarWrapper

//let c3:NSCalendar = NSCalendar(calendarIdentifier: "rubbish") // run-time error
let c3:NSCalendar? = NSCalendar(calendarIdentifier: "rubbish") // nil

So if the initializer can return nil, my understanding is that I should be able to do

if let c4 = NSCalendar(calendarIdentifier: "rubbish") as? NSCalendar { 
    //error: conditional downcast from 'NSCalendar' to 'NSCalendar' always succeeds
}

However, this is a compile-time error as shown.

What am I misunderstanding here, and how can I safely test that a named calendar actually exists?

Answer 1

Even though it looks like the initialiser should return a full NSCalendar object, it appears as though it behaves as an implicit optional (NSCalandar!). If you look at the object in the debugger it comes up as an NSCalendar:

(NSCalendar) $R1 = 0x0000000000000000 { ObjectiveC.NSObject = parent is NULL }

Even more strange is that the following code does not produce any runtime errors, when accessing a nil NSCalendar - at least for me:

    let x = NSCalendar(calendarIdentifier: "asdasda")
    let y = NSCalendar(calendarIdentifier: NSCalendarIdentifierGregorian)
    if (x.isDateInToday(NSDate())) {
        println("x works")
    }

    if (y.isDateInToday(NSDate())) {
        println("y works")
    }
    if (x == nil) {
        println("x not calendar")
    }

    if (y == nil) {
        println("y not calendar")
    }

For me this outputs 'y works' and 'x is not a calendar'. This would in pure Swift have a runtime error if using a nil implicit optional but pqnet points out that calling a method on a nil pointer in objective c doesn't produce an error, it is just ignored - which is what seems to be doing. So strange, but it does appear in this case safe to do this:

    let x = NSCalendar(calendarIdentifier: "asdasda")
    if (x != nil) {
        //do what you need with the calendar
    }

However a more Swift way would be to use an explicit optional - i.e. do this:

    let x : NSCalendar? = NSCalendar(calendarIdentifier: "asdasda")
    if let cal = x {
        //do what you need with the calendar as cal
    }

You could also use an implicit optional let x : NSCalendar =... and check with if x != nil as above - your choice, it seems sometimes different contexts cause implicit or explicit optionals...

Edit:

From the Xcode release notes:

Swift does not support object initializers that fail by returning null. (16480364)! Workaround: If there is a factory method, use it instead. Otherwise, capture the result in an optional. For example:

let url: NSURL? = NSURL(string: "not a url")