managed object creation extension

NSManagedObject.swift

protocol ManagedObject {
    
    static var entityName: String { get }

}

extension ManagedObject where Self: NSManagedObject {
    
    static func createWithContext(context: NSManagedObjectContext) -> Self {
        return Self.createWithContext(context, entityName: self.entityName)
    }
}

extension NSManagedObject {
    
    private
    class func createWithContext<T: NSManagedObject>(context: NSManagedObjectContext, entityName: String) -> T {
        let obj = NSEntityDescription.insertNewObjectForEntityForName(entityName, inManagedObjectContext: context)
        return obj as! T
    }

}

The Self cast works because it's in a protocol extension and not a class extension!

A variation of your code with a sample subclass. It's not completely awful, I guess:

protocol ManagedObject {
    static var entityName: String { get }
}

extension ManagedObject  {
    static func createWithContext(moc: NSManagedObjectContext) -> Self {
        return NSEntityDescription.insertNewObjectForEntityForName(entityName, inManagedObjectContext: moc) as! Self
    }
}

// Add conformance to managed object subclass.
extension Event: ManagedObject {
    static var entityName: String {
        return "Event"
    }
}

Yeah, that's better, though I plan on adding more NSManagedObject-specific methods to the extension eventually.

You can simplify the entityName param in the Event object:

extension Event: ManagedObject {
    static var entityName = "Event"
}