Question

I am trying to create some methods that should make it easier to perform Core Data stuff in the background. What I’m doing is:

• Use NSOperationQueue‘s -addOperationWithBlock: to create a background thread
• In the background thread, create an NSManagedObjectContext with the same persistentStoreCoordinator as the context on the main thread and with undoManager set to nil

• Add an observer to the default notification center to listen for NSManagedObjectContextDidSaveNotification notifications and handle them like this:

  - (void)mergeChanges:(NSNotification *)notification
  {
      dispatch_sync(dispatch_get_main_queue(), ^{
          [_mainContext mergeChangesFromContextDidSaveNotification:notification];
      });
  }
  

• Execute a block that was passed in and that does all the Core Data stuff. Periodically, it calls save: on the background context

• Once everything is done, remove the observer from the notification center

The described method can be found below.

Everything works as expected. I can pass in blocks that add data, update data or remove data. But: I get the following error when I close the app with some data, re-open it and then try to delete data in the background:

CoreData: error: Serious application error. An exception was caught
from the delegate of NSFetchedResultsController during a call to
-controllerDidChangeContent:. CoreData could not fulfill a fault for […]

As apparent from the error, I am using an NSFetchedResultsController to display data. The cache of the controller is set to nil.

Any suggestions on how to fix this?

---

Here is the relevant code. Note that I’m using ARC.

FJCoreDataBackgroundBlock is defined like so:

typedef void(^FJCoreDataBackgroundBlock)(NSManagedObjectContext *backgroundContext);

The method to perform a bunch of Core Data stuff in the background

- (void)performBlockInBackground:(FJCoreDataBackgroundBlock)block
{
    [FJSharedOperationQueue addOperationWithBlock:^{

        self.managedObjectContext = [[NSManagedObjectContext alloc] init];
        [_managedObjectContext setUndoManager:nil];
        [_managedObjectContext setPersistentStoreCoordinator:self.persistentStoreCoordinator];

        [self setupNotificationCenterObserverForContext:_managedObjectContext];

        FJCoreDataBackgroundBlock backgroundBlock = [block copy];
        backgroundBlock(self.managedObjectContext);

        [self saveBackgroundContext];
        [self saveMainContext];
        [self removeNotificationCenterObserver];
    }];
}

The implementation of FJSharedOperationQueue:

+ (NSOperationQueue *)sharedQueue
{
    static dispatch_once_t predicate = 0;

    __strong static NSOperationQueue *_sharedQueue = nil;

    dispatch_once(&predicate, ^{
        _sharedQueue = [[NSOperationQueue alloc] init];
        [_sharedQueue setMaxConcurrentOperationCount:1];
    });

    return _sharedQueue;
}

+ (void)addOperationWithBlock:(void (^)(void))block
{
    [[FJSharedOperationQueue sharedQueue] addOperationWithBlock:block];
}

Answer 1

Now that is weird: I was shuffling around code lines in the block that deletes objects. Before it looked like this:

FJCoreDataHelper *helper = [[FJCoreDataHelper alloc] initUsingMainManagedObjectContext:[self mainContext]];

[helper performBlockInBackground:^(NSManagedObjectContext *backgroundContext) {

    // [Fetch objets here...]

    // Delete:

    const int kSaveThreshold = 50;
    for (int card = 0, count = [allCards count]; card < count; card++)
    {
        [backgroundContext deleteObject:[allCards objectAtIndex:card]];

        if (card % kSaveThreshold == 0)
        {
            [helper saveBackgroundContext];
        }
    }
    }];

To fix it, I simply had to save the main context after saving the background context:

[helper performBlockInBackground:^(NSManagedObjectContext *backgroundContext) {

    // [Fetch objets here...]

    // Delete:

    const int kSaveThreshold = 50;
    for (int card = 0, count = [allCards count]; card < count; card++)
    {
        [backgroundContext deleteObject:[allCards objectAtIndex:card]];

        if (card % kSaveThreshold == 0)
        {
            [helper saveBackgroundContext];
            [helper saveMainContext];
        }
    }
    }];

If I save only the background context, it crashes. If I save the main context first, it also crashes. I find this weird because I thought that merging the background context into the main context would also save the main context…