Swift 2.0: Protocol Extension Class Method returning Self

Question

In order to extend some functionalities of my NSManagedObject subclasses, I have defined a series of protocols:

protocol ManagedObjectFindable {
    static func find(format:String, arguments: [AnyObject]?, inContext context:NSManagedObjectContext, entityName:String?) -> Self?
}

protocol UniquelyIdentifiable: ManagedObjectFindable {

    var identifier: NSNumber? { get }
    static func findWithIdentifier(identifier: Int, inContext context:NSManagedObjectContext) -> Self?
}

So then every NSManagedObject that has identifier in its data model entity can conform to UniquelyIdentifiable.

For that purpose I am utilising Swift 2.0 Protocol Extensions, where:

extension UniquelyIdentifiable {

    static func findWithIdentifier(identifier: Int, inContext context:NSManagedObjectContext) -> Self? {
        return self.find("identifier == %lld", arguments: [NSNumber(longLong: Int64(identifier))], inContext: context, entityName:nil)
    }
}

Where find is defined as:

extension NSManagedObject: ManagedObjectFindable {
    /** returns single entity if found, nil otherwise */
    class func find(format:String, arguments: [AnyObject]?, inContext context:NSManagedObjectContext, entityName:String? = nil) -> Self? {

        let objectEntityName:String
        if let name = entityName {
            objectEntityName = name
        } else {
            objectEntityName = String(self)
        }

        let fetchRequest = NSFetchRequest()
        fetchRequest.entity = NSEntityDescription.entityForName(objectEntityName, inManagedObjectContext: context)
        fetchRequest.fetchLimit = 1
        fetchRequest.predicate = NSPredicate(format: format, argumentArray: arguments)

        var persistentEntityº:NSManagedObject?
        context.performBlockAndWait {

            do {
                let fetchResults = try context.executeFetchRequest(fetchRequest)
                if (fetchResults.count != 0){
                    persistentEntityº = fetchResults.first as? NSManagedObject
                }
            } catch {}
        }

        if let persistentEntity = persistentEntityº {
            return _safeObjectSelfCast(persistentEntity)
        } else {
            return nil
        }
    }
}
func _unsafeObjectSelfCast<T>(obj: AnyObject!) -> T { return obj as! T }
func _safeObjectSelfCast<T>(obj: AnyObject) -> T?   { return obj as? T }

Now these methods correctly return Self? and compiler is silent on the coding time, however when compiling it gives me that error Method 'findWithIdentifier(_:inContext:)' in non-final class must return 'Self' to conform to protocol 'UniquelyIdentifiable'

Now the thing is that if instead of implementing that method in a protocol extension I would just extend my NSManagedObject subclass, it will go fine, but that kills the purpose of protocol extensions, when you are completely duplicating the same code across dozens of your NSManagedObject subclasses.

Any workaround, or I am really missing something?

@MartinR: try adding to NSMOSubclass: extension SomeManagedObjectSubclass: UniquelyIdentifiable {} In that case it won't... — ambientlight, Nov 19 '15 at 12:25

Said Sikira · Answer 1 · 2015-11-19T12:41:40.000

2

Short answer: Change Self? in extension to the NSManagedObject?.

Long answer: Self in protocol requirement acts as a placeholder for the class that will implement that protocol. So if you have

protocol SomeProtocol {
   func returnSomething() -> Self
}

That means that if you implement it on Int, function returnSomething() should return Int, and if you implement it on Double it should return Double.

Since you are implementing UniquelyIdentifiable on NSManagedObject and your protocol has Self? requirement, you should return NSManagedObject?.

edited Nov 19 '15 at 12:41

answered Nov 19 '15 at 12:35

Said Sikira

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In my case it should return the instance of Optional(NSManagedObject Subclass), not Optional(NSManagedObject), yes it will work but you want to return Optional(NSManagedObjectSubclass) the reason to return Self? instead of NSManagedObject? is because you can have many different NSManagedObject subclasses, and if the subclass conforms to this protocol, it will already implement findWithIdentifier, which is in protocol extension default implementation. – ambientlight Nov 19 '15 at 12:44

Swift 2.0: Protocol Extension Class Method returning Self

1 Answers1