Using Swift protocols with generics - core-data

Question

I have a simple example that seems like it should work:
import CoreData
#objc protocol CoreDataModel {
#optional class func entityName() -> String
}
class AbstractModel: NSManagedObject, CoreDataModel {
class func create<T : CoreDataModel>(context:NSManagedObjectContext) -> T {
var name = T.entityName?()
var object = NSEntityDescription.insertNewObjectForEntityForName(name, inManagedObjectContext: context) as T
return object
}
}
So we have a class called AbstractModel which conforms to the protocol CoreDataModel, and CoreDataModel defines an optional class method called entityName.
However, this line:
var name = T.entityName?()
causes the error:
Expected member name or constructor call after type name
Any idea what I'm doing wrong?
Edit
Removing the word #optional from the declaration and changing the function a bit allows the code to compile, but now I get a runtime error saying that the
'Swift dynamic cast failed'
#objc protocol CoreDataModel {
class func entityName() -> String
}
class AbstractModel: NSManagedObject, CoreDataModel {
class func entityName() -> String {
return "AbstractModel"
}
class func create<T : CoreDataModel>(aClass:T.Type, context:NSManagedObjectContext) -> T {
var name = aClass.entityName()
var object = NSEntityDescription.insertNewObjectForEntityForName(name, inManagedObjectContext: context) as T
return object
}
}

Answer 1

I cannot explain why your code causes a runtime exception. But it works if you change
the function prototype
class func create<T : CoreDataModel>(aClass:T.Type, context:NSManagedObjectContext) -> T
to
class func create<T : NSManagedObject where T: CoreDataModel>(aClass:T.Type, context:NSManagedObjectContext) -> T
Assuming that your managed object subclass conforms to the protocol, for example
extension Event : CoreDataModel {
class func entityName() -> String {
return "Event"
}
}
then this works and creates a new object:
let newManagedObject = AbstractModel.create(Event.self, context: context)
Alternatively, you could use the approach from the answer to
"Swift: return Array of type self" and
define an extension to the NSManagedObjectContext class:
extension NSManagedObjectContext {
func create<T : NSManagedObject where T : CoreDataModel >(entity: T.Type) -> T {
var classname = entity.entityName()
var object = NSEntityDescription.insertNewObjectForEntityForName(classname, inManagedObjectContext: self) as T
return object
}
}
Then a new object would be created as
let newManagedObject = context.create(Event.self)

Answer 2

From "The Swift Programming Language"
Because T is a placeholder, Swift does not look for an actual type called T.
As T is not a real type, it is maybe not useful to cast to T.