Implementing Fast and Efficient Core Data Import on iOS 5

Question

Question: How do I get my child context to see changes persisted on the parent context so that they trigger my NSFetchedResultsController to update the UI?<

Answer 1

You should probably save the master MOC in strides as well. No sense having that MOC wait until the end to save. It has its own thread, and it will help keep memory down as well.

You wrote:

Then at the end of the import process, I save on the master/parent context which, ostensibly, pushes modifications out to the other child contexts including the main context:

In your configuration, you have two children (the main MOC and the background MOC), both parented to the "master."

When you save on a child, it pushes the changes up into the parent. Other children of that MOC will see the data the next time they perform a fetch... they are not explicitly notified.

So, when BG saves, its data is pushed to MASTER. Note, however, that none of this data is on disk until MASTER saves. Furthermore, any new items will not get permanent IDs until the MASTER saves to disk.

In your scenario, you are pulling the data into the MAIN MOC by merging from the MASTER save during the DidSave notification.

That should work, so I'm curious as to where it is "hung." I will note, that you are not running on the main MOC thread in the canonical way (at least not for iOS 5).

Also, you probably only are interested in merging changes from the master MOC (though your registration looks like it is only for that anyway). If I were to use the update-on-did-save-notification, I'd do this...

- (void)contextChanged:(NSNotification*)notification {
    // Only interested in merging from master into main.
    if ([notification object] != masterManagedObjectContext) return;

    [mainManagedObjectContext performBlock:^{
        [mainManagedObjectContext mergeChangesFromContextDidSaveNotification:notification];

        // NOTE: our MOC should not be updated, but we need to reload the data as well
    }];
}

Now, for what may be your real issue regarding the hang... you show two different calls to save on the master. the first is well protected in its own performBlock, but the second is not (though you may be calling saveMasterContext in a performBlock...

However, I'd also change this code...

- (void)saveMasterContext {
    NSNotificationCenter *notificationCenter = [NSNotificationCenter defaultCenter];    
    [notificationCenter addObserver:self selector:@selector(contextChanged:) name:NSManagedObjectContextDidSaveNotification object:masterManagedObjectContext];

    // Make sure the master runs in it's own thread...
    [masterManagedObjectContext performBlock:^{
        NSError *error = nil;
        BOOL saveSuccess = [masterManagedObjectContext save:&error];
        // Handle error...
        [notificationCenter removeObserver:self name:NSManagedObjectContextDidSaveNotification object:masterManagedObjectContext];
    }];
}

However, note that the MAIN is a child of MASTER. So, it should not have to merge the changes. Instead, just watch for the DidSave on the master, and just refetch! The data is sitting in your parent already, just waiting for you to ask for it. That's one of the benefits of having the data in the parent in the first place.

Another alternative to consider (and I'd be interested to hear about your results -- that's a lot of data)...

Instead of making the background MOC a child of the MASTER, make it a child of the MAIN.

Get this. Every time the BG saves, it automatically gets pushed into the MAIN. Now, the MAIN has to call save, and then the master has to call save, but all those are doing is moving pointers... until the master saves to disk.

The beauty of that method is that the data goes from the background MOC straight into your applications MOC (then passes through to get saved).

There is some penalty for the pass-through, but all the heavy lifting gets done in the MASTER when it hits the disk. And if you kick those saves on the master with performBlock, then main thread just sends off the request, and returns immediately.

Please let me know how it goes!