Category + lazy instantiation patterns on Objective C

Question

I'm trying to use a lazy instantiation on a category but I'm stuck on how to do that without enter in a obvious infinity loop. Here some code for ilustrate:

@implementation User (Extras)

- (CacheControl *)cache
{
    CacheControl *_cache = (CacheControl *)[self valueForKey:@"cache"];
    if(!_cache){
        [self setCache:(CacheControl *)[NSEntityDescription insertNewObjectForEntityForName:@"CacheControl" inManagedObjectContext:self.managedObjectContext]];
    }
    return _cache;
}
@end

Any Ideas how to address this situation or should I just don't do that at all?

I think we must assume so but the question doesn't make this clear. — fatuhoku, Mar 03 '15 at 12:17
Yes it does, User has a cache property, the accepted answer from Martin R solved the problem. — Adriano Spadoni, Mar 03 '15 at 12:24

Martin R · Accepted Answer · 2014-04-24T11:57:59.793

To avoid infinite recursion in the getter method, you have to use the "primitive accessor" Core Data accessor methods:

- (CacheControl *) cache {
    [self willAccessValueForKey:@"cache"];
    CacheControl * cache = [self primitiveValueForKey:@"cache"];
    [self didAccessValueForKey:@"cache"];

    if (cache == nil) {
        cache = [NSEntityDescription insertNewObjectForEntityForName:@"CacheControl" inManagedObjectContext:self.managedObjectContext];
        [self setPrimitiveValue:cache forKey:@"cache"];
    }
    return cache;
}

Similar examples can be found in the "Core Data Programming Guide" and in the sectionIdentifier method of the "Custom Section Titles with NSFetchedResultsController" sample project.

Category + lazy instantiation patterns on Objective C

1 Answers1