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

[davelist at mac.com]

Thanks to both of you for the follow-up.

For my use, the class being final is probably ok.

Dave Reed

> On Jan 10, 2017, at 3:05 AM, Slava Pestov <spestov at apple.com> wrote:
> 
>> 
>> 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
>>>>> 
>>>>> 
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