Question

Here is what I’m calling in cellForRowAtIndexPath:

   NSManagedObjectContext *context = [[AppDelegate sharedAppDelegate] managedObjectContext];
    NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
    NSEntityDescription *entity = [NSEntityDescription
                                   entityForName:@"Favorites" inManagedObjectContext:context];
    [fetchRequest setEntity:entity];

    NSPredicate *requestPredicate = [NSPredicate predicateWithFormat:[NSString stringWithFormat:@"(link like '%@')",articleLink]];
    [fetchRequest setPredicate:requestPredicate];

    [fetchRequest setFetchBatchSize:1];
    NSError *error = nil;
    NSUInteger count = [context countForFetchRequest:fetchRequest error:&error];
    NSLog(@"Count - %d", count);
    if (count == 0)
        return NO;
    else
        return YES;

Basically it checks if a item exists in my model, or not. Pretty useful, but I thought is there any problems making so much fetch requests in such a tiny amount of time?

Any problems with battery management, is it reading from disk every time this is called?

Anything better?

Thanks!

Answer 1

There are several problems with what you’re doing. First of all, fetches are very slow since it will indeed read from your persistent store, which is on disk. Your fetch will only become slower the larger your database is and the more complicated your query (your NSPredicate) becomes.

Second, you have no control on how many times cellForRowAtIndexPath gets called, you can only assume it gets called once for each cell as it becomes visible, but it may potentially get called in other situations as well. You also have no control over how the user will use your ScrollView, he may try scrolling like mad. Everything inside that method has to be as performant as can be and can’t slow down your main thread or your ScrollView will freeze constantly which will look very bad to the user.

Third, you’re constantly allocating and initializing new objects, in this case NSFetchRequests, which is costly. Caching will drastically improve your performance, you should always try to avoid allocating new objects if possible. Even though this already a bad example, the fetchRequest could easily be stored as a global variable and have its predicate changed before each fetch.

From what I can tell from the code, it seems like you have an Entity called Favorite which holds a link, a TableView showing links and you want to change something about your cell based on whether or not that link is a favorite.

What you should be doing is fetch ALL Favorites when your ViewController is first loaded, store all their links in an array and in cellForRowAtIndexPath you check if articleLink exists in that array. That way you only read from memory when the user scrolls through your ScrollView and your performance will be optimal.