swift - How to use generic types to get object with same type

Question

I have extension for NSManagedObject that should help me to transfer objects between contexts:

extension NSManagedObject {

    func transferTo(#context: NSManagedObjectContext) -> NSManagedObject? {

        return context.objectWithID(objectID)
    }

}

for now it return object of NSManagedObject and i should cast it to class what i want, like this:

let someEntity: MyEntity = // ...create someEntity
let entity: MyEntity = someEntity.transferTo(context: newContext) as? MyEntity

Is there a way in Swift to avoid that useless casting and if i call transferTo(context: ...) from object of class MyEntity make it return type to MyEntity?

Answer 1

I've liked Martin's solution for a long time, but I recently ran into trouble with it. If the object has been KVO observed, then this will crash. Self in that case is the KVO subclass, and the result of objectWithID is not that subclass, so you'll get a crash like "Could not cast value of type 'myapp.Thing' (0xdeadbeef) to 'myapp.Thing' (0xfdfdfdfd)." There are two classes that call themselves myapp.Thing, and as! uses the actual class object. So Swift is not fooled by the noble lies of KVO classes.

The solution is to replace Self with a static type parameter by moving this to the context:

extension NSManagedObjectContext {
    func transferredObject<T: NSManagedObject>(object: T) -> T {
        return objectWithID(object.objectID) as! T
    }
}

T is purely defined at compile-time, so this works even if object is secretly a subclass of T.