Objective c protocol generics

Question

Can Objective-C protocol be generic?

Following this tutorial, I'm basically looking for something like that:

@protocol ItemsStore<__covariant ObjectType> <NSObject>

-(NSArray <ObjectType> *)items;

@end

Which is a generic protocol for some ObjectType that "implements" ("inherits") another protocol NSObject

Answer 1

As @rmaddy suggested, and as referred to this questions, it is NOT possible. Shame, moving to Swift then...

Answer 2

Probably, this question has exactly the same source problem as mine. Well, the idea of the design is great but doesn't work with ObjC. I also was wondering about that. I thought it could work some way like that:

@protocol Prototype<__covariant OtherProtocolOrClass> <NSObject>
/// Construct an object of the desired class or protocol
@property (nonatomic, nonnull, readonly) <OtherProtocolOrClass> objectFromPrototype;
@end

(I have not tested @protocol Prototype <NSObject,__covariant OtherProtocolOrClass> yet, but think it will fail.)

Another object in my framework (it is a collection) states, it can auto-construct an NSArray<ObjectType>* of an object type, if there is a Prototype that returns instances, like this:

@interface ValueProto : NSObject <Prototype<id<Value>>>
@end

@implementation ValueProto
-(id<Value>)objectFromPrototype {
    return [Value new];
}
@end

Im my dreams, the collection was constructed like this:

MyCollection<id<Value>>* const collection = [MyCollection new];
collection.prototype = [ValuesProto new];

And if you then access the collection's property, your array of id<Value> objects is constructed on the fly:

-(NSArray<id<Value>>*)values {
    NSArray*const sourceCollection = ...
    NSMutableArray<id<Value>>* const result = [NSMutableArray arrayWithCapacity:sourceCollection.count];
    for (id o in sourceCollection) {
        id<Value> v = self.prototype.objectFromPrototype;
        v.content = o;
        [result addObject:v];
    }
    return result;
}

Instead one of my class' objects must be the prototype itself:

-(id)objectFromPrototype {
    return [self.class new];
}

That clashes with my so-called 'Injector', which constructs and returns objects by protocols instead of classes.

If any Apple engineer is reading this:

Please, provide protocol covariants for ObjC. It's not yet dead! :-)

Answer 3

Why not use generic abstract class?

@interface AbstractItemStore <__covariant ObjectType> : NSObject

- (NSArray<ObjectType> *)items;

@end

@implementation AbstractItemStore

- (NSArray<id> *)items {
    NSParameterAssert("Not implemented!");
    return nil;
}

@end

Answer 4

Maybe you could just redefine it as generic in the interface.

@protocol ItemsStore <NSObject>

- (NSArray *)items;

@end

@interface MyItemsStore<ObjectType> : NSObject <ItemsStore>

- (NSArray <ObjectType> *)items;

@end

This seems an unlikely scenario though. You might be better just defining the type of the items in each subclass. Like what Apple do with NSFetchRequest in their core data model generation.