Search code examples
swiftnscalendar

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

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?

Solution

  • Note: The following only applies to Swift 1.0. In Swift 1.1, there are failable initializers.

    This works:

    if let c4 = NSCalendar(calendarIdentifier: "rubbish") as NSCalendar? { 

}

    In Swift 1.0, there is a known issue in the Xcode release notes about Swift not supporting Objective-C initializers that return nil. Basically, what is happening is that according to the Swift language, the expression NSCalendar(...) has type NSCalendar, which is a non-optional type (cannot be nil). However, this is really an imported initializer from Objective-C, where it can return nil.

    So what currently happens is that when you call this and it returns nil, you have a value that at runtime is nil, but Swift thinks is a non-optional NSCalendar (which cannot be nil). This is a really bad situation in which you have a value that is not possible for the type. The Xcode release notes mention a "workaround" where you convert the result of the initializer to an optional type before using it. The reason that this works is that at runtime, both optional and non-optional object pointers are represented as simple pointers, where nil object pointer is the null pointer, and non-optional ones are assumed to not be null pointers. The operation of converting from a non-optional to optional object pointer is a simple assignment (in both cases it's a non-null pointer). But in the case where the value is nil, a simple assignment turns it into a (valid) nil value of optional type. So everything is happy.

    But if you don't convert it to optional first, all hell breaks loose, as you have a value that is not supposed to be possible for that type.

    Your attempt to downcast using as? from NSCalendar to NSCalendar isn't allowed, because a cast from NSCalendar to NSCalendar cannot fail (theoretically).