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

[Slava Pestov]

> On Jan 9, 2017, at 11:59 PM, Pierre Monod-Broca via swift-users <swift-users at swift.org> wrote:
> 
> 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] {
>    ...
>  }
> }

FWIW, this requires that ‘C’ is final. Otherwise, a subclass ‘D’ of ‘C’ won’t satisfy the requirement, because C.items() still returns a ‘C’ and not a ‘D’.

> 
> 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
>> 
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