Merging main and private contexts with Core Data - ios

Question

I'm creating temportary contexts in a private queue to asynchronously update the data I persist with Core Data:
NSManagedObjectContext *privateContext = [[NSManagedObjectContext alloc] initWithConcurrencyType:NSPrivateQueueConcurrencyType];
AppDelegate *appDelegate = [[UIApplication sharedApplication] delegate];
privateContext.persistentStoreCoordinator = appDelegate.persistentStoreCoordinator;
[privateContext performBlock: ^{
// Parse files and/or call services and parse
// their responses
dispatch_async(dispatch_get_main_queue(), ^{
// Notify update to user
});
}];
Then, once I've got the new data, I need to merge the changes with my main context. I know this is a common scenario, but I'm not sure how should I proceed... This Apple's Core Data documentation section talks about setting a merge policy and I don't fully understand the way to handle that. On the other hand, I found this link, where my scenario is described in its "Stack #2" section and what it says looks simpler, and it doesn't talk about merge policies at all...
Which the correct or most appropriate way should be? I'd really appreciate an explanation and/or simple example of how to manage this scenario.
Thanks in advance.

Answer 1

What you have there looks pretty good.
You are using a private queue to do your work, and it's being saved to the persistent store.
If you only have a small number of changes, then you will be fine. In that case, you want to handle the NSManagedObjectContextDidSaveNotification for your context, and merge the changes into your other context with
[context mergeChangesFromContextDidSaveNotification:notification];
However, if you are really doing a lot of changes, you probably want a separate persistent store coordinator (attached to the same store). By doing this, you can write to the store, while MOCs on the other PSC are reading. If you share the PSC with other readers, only one will get access at a time and you could cause the readers to block until your write has finished.
Also, if you are doing lots of changes, make sure you do them in small batches, inside an autoreleasepool, saving after each batch. Take a look at this similar question: Where should NSManagedObjectContext be created?
Finally, if you are doing lots of changes, it may be more efficient to just refetch your data than it will be to process all the merges. In that case, you don't need to observe the notification, and you don't need to do the merge. It's pretty easy. Note, that if you do this, you really should have a separate PSC... especially if you want to save and notify in small-ish batches.
[privateContext performBlock: ^{
// Parse files and/or call services and parse
// their responses
dispatch_async(dispatch_get_main_queue(), ^{
// Refetch the data you want... if on iOS, this is likely as simple
// as telling the fetched results controller to refetch, and
// reloading your table view (or whatever else is using the data).
});
}];