Question

What I’m trying todo in a nutshell is I am using a background queue to save JSON objects pulled from a web service to the Core Data Sqlite3 database. The saving takes place on a serialized background queue I’ve created via GCD, and saved to a secondary instance of NSManagedObjectContext that is created for that background queue. Once the save is complete I need to update the instance of NSManagedObjectContext that is on the main thread with the newly created/updated objects. The problem I am having though is the instance of NSManagedObjectContext on the main thread is not able to find the objects that were saved on the background context. Below is a list of actions I’m taking with code samples. Any thoughts on what I’m doing wrong?

• Create a background queue via GCD, run all pre-processing logic and then save the background context on that thread:

.

// process in the background queue
dispatch_async(backgroundQueue, ^(void){

    if (savedObjectIDs.count > 0) {
        [savedObjectIDs removeAllObjects];
    }
    if (savedObjectClass) {
        savedObjectClass = nil;
    }

    // set the thead name
    NSThread *currentThread = [NSThread currentThread];
    [currentThread setName:VS_CORE_DATA_MANAGER_BACKGROUND_THREAD_NAME];

    // if there is not already a background context, then create one
    if (!_backgroundQueueManagedObjectContext) {
        NSPersistentStoreCoordinator *coordinator = [self persistentStoreCoordinator];
        if (coordinator != nil) {
            _backgroundQueueManagedObjectContext = [[NSManagedObjectContext alloc] init];
            [_backgroundQueueManagedObjectContext setPersistentStoreCoordinator:coordinator];
        }
    }

    // save the JSON dictionary starting at the upper most level of the key path, and return all created/updated objects in an array
    NSArray *objectIds = [self saveJSON:jsonDict objectMapping:objectMapping class:managedObjectClass managedObjectContext:_backgroundQueueManagedObjectContext level:0];

    // save the object IDs and the completion block to global variables so we can access them after the save
    if (objectIds) {
        [savedObjectIDs addObjectsFromArray:objectIds];
    }
    if (completion) {
        saveCompletionBlock = completion;
    }
    if (managedObjectClass) {
        savedObjectClass = managedObjectClass;
    }

    // save all changes object context
    [self saveManagedObjectContext];
});

• The “saveManagedObjectContext” method basically looks at which thread is running and saves the appropriate context. I have verified that this method is working correctly so I will not place the code here.

• All of this code resides in a singleton, and in the singleton’s init method I am adding a listener for the “NSManagedObjectContextDidSaveNotification” and it calls the mergeChangesFromContextDidSaveNotification: method

.

// merge changes from the context did save notification to the main context
- (void)mergeChangesFromContextDidSaveNotification:(NSNotification *)notification
{
    NSThread *currentThread = [NSThread currentThread];

    if ([currentThread.name isEqual:VS_CORE_DATA_MANAGER_BACKGROUND_THREAD_NAME]) {

        // merge changes to the primary context, and wait for the action to complete on the main thread
        [_managedObjectContext performSelectorOnMainThread:@selector(mergeChangesFromContextDidSaveNotification:) withObject:notification waitUntilDone:YES];

        // on the main thread fetch all new data and call the completion block
        dispatch_async(dispatch_get_main_queue(), ^{

            // get objects from the database
            NSMutableArray *objects = [[NSMutableArray alloc] init];
            for (id objectID in savedObjectIDs) {
                NSError *error;
                id object = [_managedObjectContext existingObjectWithID:objectID error:&error];
                if (error) {
                    [self logError:error];
                } else if (object) {
                    [objects addObject:object];
                }
            }

            // remove all saved object IDs from the array
            [savedObjectIDs removeAllObjects];
            savedObjectClass = nil;

            // call the completion block
            //completion(objects);
            saveCompletionBlock(objects);

            // clear the saved completion block
            saveCompletionBlock = nil;
        });
    }
}

As you can see in the method above I am calling the “mergeChangesFromContextDidSaveNotification:” on the main thread, and I have set the action to wait until done. According to the apple documentation the background thread should wait until that action is complete before it continues with the rest of the code below that call. As I mentioned above once I run this code everything seems to work, but when I try to print out the fetched objects to the console I don’t get anything back. It seems that the merge is not in fact taking place, or possibly not finishing before the rest of my code runs. Is there another notification that I should be listening for to ensure that the merge has completed? Or do I need to save the main object context after the merge, but before the fecth?

Also, I apologize for the bad code formatting, but it seems that SO’s code tags don’t like method definitions.

Thanks guys!

UPDATE:

I’ve made the changes that were recommended below, but still having the same problem. Below is the updated code I have.

This is the code that invokes the background thread saving processes

// process in the background queue
dispatch_async(backgroundQueue, ^(void){

    if (savedObjectIDs.count > 0) {
        [savedObjectIDs removeAllObjects];
    }
    if (savedObjectClass) {
        savedObjectClass = nil;
    }

    // set the thead name
    NSThread *currentThread = [NSThread currentThread];
    [currentThread setName:VS_CORE_DATA_MANAGER_BACKGROUND_THREAD_NAME];

    // if there is not already a background context, then create one
    if (!_backgroundQueueManagedObjectContext) {
        NSPersistentStoreCoordinator *coordinator = [self persistentStoreCoordinator];
        if (coordinator != nil) {
            _backgroundQueueManagedObjectContext = [[NSManagedObjectContext alloc] init];
            [_backgroundQueueManagedObjectContext setPersistentStoreCoordinator:coordinator];
        }
    }

    // save the JSON dictionary starting at the upper most level of the key path
    NSArray *objectIds = [self saveJSON:jsonDict objectMapping:objectMapping class:managedObjectClass managedObjectContext:_backgroundQueueManagedObjectContext level:0];

    // save the object IDs and the completion block to global variables so we can access them after the save
    if (objectIds) {
        [savedObjectIDs addObjectsFromArray:objectIds];
    }
    if (completion) {
        saveCompletionBlock = completion;
    }
    if (managedObjectClass) {
        savedObjectClass = managedObjectClass;
    }

    // listen for the merge changes from context did save notification
    [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(mergeChangesFromBackground:) name:NSManagedObjectContextDidSaveNotification object:_backgroundQueueManagedObjectContext];

    // save all changes object context
    [self saveManagedObjectContext];
});

This is the code that is called with by the NSManagedObjectContextDidSaveNotification notification

    // merge changes from the context did save notification to the main context
- (void)mergeChangesFromBackground:(NSNotification *)notification
{
    // kill the listener
    [[NSNotificationCenter defaultCenter] removeObserver:self name:NSManagedObjectContextDidSaveNotification object:_backgroundQueueManagedObjectContext];

    NSThread *currentThread = [NSThread currentThread];

    // merge changes to the primary context, and wait for the action to complete on the main thread
    [[self managedObjectContext] performSelectorOnMainThread:@selector(mergeChangesFromContextDidSaveNotification:) withObject:notification waitUntilDone:YES];

    // dispatch the completion block
    dispatch_async(dispatch_get_main_queue(), ^{

        // get objects from the database
        NSMutableArray *objects = [[NSMutableArray alloc] init];
        for (id objectID in savedObjectIDs) {
            NSError *error;
            id object = [[self managedObjectContext] existingObjectWithID:objectID error:&error];
            if (error) {
                [self logError:error];
            } else if (object) {
                [objects addObject:object];
            }
        }

        // remove all saved object IDs from the array
        [savedObjectIDs removeAllObjects];
        savedObjectClass = nil;

        // call the completion block
        //completion(objects);
        saveCompletionBlock(objects);

        // clear the saved completion block
        saveCompletionBlock = nil;
    });
}

UPDATE:

So I found the solution. Turns out that the way I was saving out the object IDs on the background thread and then trying to use them on the main thread to re-fetch them wasn’t working out. So I ended up pulling the inserted/updated objects from the userInfo dictionary that is sent with the NSManagedObjectContextDidSaveNotification notification. Below is my updated code that is now working.

As before this code starts the pre-prossesing and saving logic

// process in the background queue
dispatch_async(backgroundQueue, ^(void){

    // set the thead name
    NSThread *currentThread = [NSThread currentThread];
    [currentThread setName:VS_CORE_DATA_MANAGER_BACKGROUND_THREAD_NAME];

    [self logMessage:[NSString stringWithFormat:@"(%@) saveJSONObjects:objectMapping:class:completion:", [managedObjectClass description]]];

    // if there is not already a background context, then create one
    if (!_backgroundQueueManagedObjectContext) {
        NSPersistentStoreCoordinator *coordinator = [self persistentStoreCoordinator];
        if (coordinator != nil) {
            _backgroundQueueManagedObjectContext = [[NSManagedObjectContext alloc] init];
            [_backgroundQueueManagedObjectContext setPersistentStoreCoordinator:coordinator];
        }
    }

    // save the JSON dictionary starting at the upper most level of the key path
    [self saveJSON:jsonDict objectMapping:objectMapping class:managedObjectClass managedObjectContext:_backgroundQueueManagedObjectContext level:0];

    // save the object IDs and the completion block to global variables so we can access them after the save
    if (completion) {
        saveCompletionBlock = completion;
    }

    // listen for the merge changes from context did save notification
    [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(mergeChangesFromBackground:) name:NSManagedObjectContextDidSaveNotification object:_backgroundQueueManagedObjectContext];

    // save all changes object context
    [self saveManagedObjectContext];
});

This is the modified method that handles the NSManagedObjectContextDidSaveNotification

- (void)mergeChangesFromBackground:(NSNotification *)notification
{
    // kill the listener
    [[NSNotificationCenter defaultCenter] removeObserver:self name:NSManagedObjectContextDidSaveNotification object:_backgroundQueueManagedObjectContext];

    // merge changes to the primary context, and wait for the action to complete on the main thread
    [[self managedObjectContext] performSelectorOnMainThread:@selector(mergeChangesFromContextDidSaveNotification:) withObject:notification waitUntilDone:YES];

    // dispatch the completion block
    dispatch_async(dispatch_get_main_queue(), ^{

        // pull the objects that were saved from the notification so we can get them on the main thread MOC
        NSDictionary *userInfo = [notification userInfo];
        NSMutableArray *modifiedObjects = [[NSMutableArray alloc] init];
        NSSet *insertedObject = (NSSet *)[userInfo objectForKey:@"inserted"];
        NSSet *updatedObject = (NSSet *)[userInfo objectForKey:@"updated"];

        if (insertedObject && insertedObject.count > 0) {
            [modifiedObjects addObjectsFromArray:[insertedObject allObjects]];
        }
        if (updatedObject && updatedObject.count > 0) {
            [modifiedObjects addObjectsFromArray:[updatedObject allObjects]];
        }

        NSMutableArray *objects = [[NSMutableArray alloc] init];

        // iterate through the updated objects and find them in the main thread MOC
        for (NSManagedObject *object in modifiedObjects) {
            NSError *error;
            NSManagedObject *obj = [[self managedObjectContext] existingObjectWithID:object.objectID error:&error];
            if (error) {
                [self logError:error];
            }
            if (obj) {
                [objects addObject:obj];
            }
        }

        modifiedObjects = nil;

        // call the completion block
        saveCompletionBlock(objects);

        // clear the saved completion block
        saveCompletionBlock = nil;
    });
}

Answer 1

in your case because your writing to the background moc the notification for mergeChangesFromContextDidSaveNotification will come in on the background moc, not the foreground moc.

so you’ll need to register for notifications on the background thread coming to the background moc object.

when you receive that call you can send a message to the main thread moc to mergeChangesFromContextDidSaveNotification.

andrew

update:
here’s a sample that should work

    //register for this on the background thread
    NSNotificationCenter *nc = [NSNotificationCenter defaultCenter];
    [nc addObserver:self selector:@selector(mergeChanges:) name:NSManagedObjectContextDidSaveNotification object:backgroundMOC];

- (void)mergeChanges:(NSNotification *)notification {
    NSManagedObjectContext *mainThreadMOC = [singleton managedObjectContext];

    //this tells the main thread moc to run on the main thread, and merge in the changes there
    [mainThreadMOC performSelectorOnMainThread:@selector(mergeChangesFromContextDidSaveNotification:) withObject:notification waitUntilDone:YES];
}