iOS: updating a core data db require a lot of time

Question

In my project I should download a JSON file made in this way:

[{"id":"2","n":"One"},{"id":"2","n":"Two"},{"id":"2","n":"Three"},...]

my code is this:

- (void) startPopulate:(NSArray *)array{

    NSManagedObjectContext *context = [[self sharedAppDelegate] managedObjectContext];

    NSFetchRequest *fetchRequest=[NSFetchRequest fetchRequestWithEntityName:@"Myentity"];
    NSError *error = nil;

    for (id element in array){

        [fetchRequest setPredicate:[NSPredicate predicateWithFormat:@"id == %@",[element objectForKey:@"id"]]];
        Myentity *myE = [[context executeFetchRequest:fetchRequest error:&error] lastObject];

        //update
        if (myE != nil){

            myE.id_e = [element objectForKey:@"id"];
            myE.name = [element objectForKey:@"n"];
        }

        //new element
        else{

            Myentity *myE = [NSEntityDescription insertNewObjectForEntityForName:@"Myentity" inManagedObjectContext:context];
            myE.name = [element objectForKey:@"n"];
            myE.id_e = [element objectForKey:@"id"];
        }
    }

    if (![context save:&error]) {
        NSLog(@"couldn't save: %@", [error localizedDescription]);
    }
    else{

        NSLog(@"DB UPDATED");
    }
}

As you can see I pass at the method the array of dictionaries and I check if the entity exist or not. It work fine and I have not particulars problems. The 'problem', if it's the way to call it, is that I have 12000 element and this method run for about 53 seconds. It's very very slow. What type of solution can I adopt to make it more quick? Or I should just put this method inside a background process? thanks

There are some [techniques described in Apple's docs](https://developer.apple.com/library/ios/documentation/Cocoa/Conceptual/CoreData/Articles/cdImporting.html) that may help you. — Alladinian, Apr 01 '14 at 08:56
possible duplicate of [What is the most efficient way to sync Core Data with JSON API](http://stackoverflow.com/questions/14109501/what-is-the-most-efficient-way-to-sync-core-data-with-json-api) — Lorenzo B, Apr 01 '14 at 09:09

score 0 · Accepted Answer · answered Apr 01 '14 at 08:55

Should you put it in a background process?

Absolutely. Start a background thread and create a child context from your main context to do all the work in.

What else can be done

Every unsaved change you make to a managed object context is stored in memory. Making thousands of changes before saving is inadvisable as it will take a lot of memory but also the save take will take a long time.

You could batch the updates into say 100 changes at a time and save each block of 100.

Alternatives

If you are always loading in these items for the app then you could also preload them into a DB and bundle it into the app itself.

That way you don't need to save them at run time as they are already in there.

do you have any suggest to create a background thread with core data? — cyclingIsBetter, Apr 01 '14 at 09:38

score 0 · Answer 2 · answered Apr 02 '14 at 10:53

Here's how you create a background thread while using core data:

dispatch_queue_t fetchQ = dispatch_queue_create("JSON Fetcher", NULL);
    dispatch_async(fetchQ, ^{
        NSManagedObjectContext *context = [[NSManagedObjectContext alloc] initWithConcurrencyType:NSPrivateQueueConcurrencyType];
        context.parentContext = self.mainMOC;

        [context performBlock:^{
            // implement your own background code here

            NSError *error;
            if(![context save:&error]) {
                // handle error
            }
        }];
});

You also need to save the results so they get propagated to the mainMOC. Hope it helps!

iOS: updating a core data db require a lot of time

2 Answers2