In Objective-C, I might use +[NSValue valueWithNonretainedObject:] to keep a unique ID of an object, where I don't wish to retain the object itself. Seems like that's deprecated for Swift.
How to do in Swift?
It's still there. It's just been adjusted to be one of NSValue's initializers:
let anObject = "Hello!"
let value = NSValue(nonretainedObject: anObject)