Core Data managed object context thread synchronisation

Question

I'm have an issue where i'm updating a many-to-many relationship in a background thread, which works fine in that threa, but when I send the object back to the main thread the changes do not show. If I close the app and reopen the data is saved fine and the changes show on the main thread. Also using [context lock] instead of a different managed object context works fine.

I have tried NSManagedObjectContext:

- (BOOL)save:(NSError **)error;
- (void)refreshObject:(NSManagedObject *)object mergeChanges:(BOOL)flag;    

at different stages throughout the process but it doesn't seem to help.

My core data code uses the following getter to ensure any operations are thread safe:

- (NSManagedObjectContext *) managedObjectContext 
{   

    NSThread * thisThread = [NSThread currentThread];
    if (thisThread == [NSThread mainThread]) 
    {
        //Main thread just return default context
        return managedObjectContext;
    }
    else 
    {
        //Thread safe trickery
        NSManagedObjectContext * threadManagedObjectContext = [[thisThread threadDictionary] objectForKey:CONTEXT_KEY]; 
        if (threadManagedObjectContext == nil)
        {
            threadManagedObjectContext = [[[NSManagedObjectContext alloc] init] autorelease];
            [threadManagedObjectContext setPersistentStoreCoordinator: [self persistentStoreCoordinator]];
            [[thisThread threadDictionary] setObject:threadManagedObjectContext forKey:CONTEXT_KEY];
        }

        return threadManagedObjectContext;
    }
}

and when I pass object between threads i'm using

-(NSManagedObject*)makeSafe:(NSManagedObject*)object
{
    if ([object managedObjectContext] != [self managedObjectContext])
    {               
        NSError * error = nil;
        object =  [[self managedObjectContext] existingObjectWithID:[object objectID] error:&error];

        if (error) 
        {
            NSLog(@"Error makeSafe: %@", error);
        }
    }

    return object;
}

Any help appreciated

Answer 1

If you save the background context on the background thread and then listen for NSManagedObjectContextObjectsDidChangeNotification on the main thread you can call -mergeChangesFromContextDidSaveNotification: on the main context (on the main thread) and the changes will show up as soon as you perform the save on the background thread.