[swift-users] make a static/class method return type be the subclass it is called with

[Pierre Monod-Broca]

Hi again,

You might want to look at Self requirement in protocols for exemple:
protocol P {
  func items(/*...*/) -> [Self]
}
class C: P {
  func items(/*...*/) -> [C] {
    ...
  }
}

However it might not always work as you expect.

I can't say which is the better. Using associated type might be more flexible. 

Pierre

> Le 5 janv. 2017 à 21:10, davelist at mac.com a écrit :
> 
> Yes, it does seem to work. I just wondered if there was a better way to do it. Perhaps by writing the method in a base class (but I couldn't get that to work) instead of using an extension. Then I could just inherit from that base class rather than needing to write:
> 
> extension Event: DDRCoreData {
>  typealias Entity = Event
> }
> 
> in addition to class Event: NSManagedObject . . .
> 
> Yes, I know in Xcode 8, that last part can now be written for you.
> 
> Again, if this is the "right way to do it", I'm ok with that, I just wondered if there is a better way to do it.
> 
> Thanks,
> Dave Reed
> 
> 
>> On Jan 5, 2017, at 2:35 PM, Pierre Monod-Broca <pierremonodbroca at gmail.com> wrote:
>> 
>> Hello,
>> 
>> It looks that you have what you wanted because Event.Entity is an alias of Event. 
>> 
>> Pierre
>> 
>>> Le 5 janv. 2017 à 16:47, Dave Reed via swift-users <swift-users at swift.org> a écrit :
>>> 
>>> Is there a way to make a static or class method specify the return type be the actual class it is called with?
>>> 
>>> The example code below using protocols/extensions mostly works (the type is [Event.Entity] not [Event] but it seems to work.
>>> 
>>> If I try to make DDRCoreData a base class and Event subclass it, I'm only able to get the return type to be [DDRCoreData], not [Event] when I call it with Event.items(in: moc)
>>> 
>>> Here is the code that mostly works using protocols. Is there a better way to do this?
>>> 
>>> protocol DDRCoreData {
>>>  associatedtype Entity: NSManagedObject
>>> 
>>>  static func items(in context: NSManagedObjectContext, matching predicate: NSPredicate?, sortedBy sorters: [NSSortDescriptor]?) -> [Entity]
>>> }
>>> 
>>> extension DDRCoreData {
>>>  static func items(in context: NSManagedObjectContext, matching predicate: NSPredicate? = nil, sortedBy sorters: [NSSortDescriptor]? = nil) -> [Entity] {
>>>      var items: [Entity] = []
>>>      context.performAndWait {
>>>          let fetchRequest: NSFetchRequest<Entity> = Entity.fetchRequest() as! NSFetchRequest<Entity>
>>>          fetchRequest.predicate = predicate
>>>          fetchRequest.sortDescriptors = sorters
>>>          do {
>>>              items = try fetchRequest.execute() as [Entity]
>>>          } catch {
>>> 
>>>          }
>>>      }
>>>      return items
>>>  }
>>> }
>>> 
>>> @objc(Event)
>>> public class Event: NSManagedObject {
>>> 
>>> }
>>> 
>>> extension Event: DDRCoreData {
>>>  typealias Entity = Event
>>> }
>>> 
>>> // compiler says items is of type [Event.Entity]
>>> let items = Event.items(in: controller.managedObjectContext!)
>>> 
>>> Thanks,
>>> Dave Reed
>>> 
>>> 
>>> _______________________________________________
>>> swift-users mailing list
>>> swift-users at swift.org
>>> https://lists.swift.org/mailman/listinfo/swift-users
>