Core Data and threads / Grand Central Dispatch

Rui Lopes picture Rui Lopes · Sep 24, 2011 · Viewed 14.3k times · Source

I'm a beginner with Grand Central Dispatch (GCD) and Core Data, and I need your help to use Core Data with CGD, so that the UI is not locked while I add 40.000 records to Core Data.

I know that CD is not thread safe, so I have to use another context, and then save the data and merge contexts, as far as I was able to understand from some articles.

What I couldn't do yet, is put the pieces together.

So, in my code, I need your help on how to to that.

I have:

/*some other code*/

for (NSDictionary *memberData in arrayWithResult) {

    //get the Activities for this member
    NSArray *arrayWithMemberActivities = [activitiesDict objectForKey:[memberData objectForKey:@"MemberID"]];

    //create the Member, with the NSSet of Activities
    [Members createMemberWithDataFromServer:memberData
                         andActivitiesArray:arrayWithMemberActivities
                              andStaffArray:nil
                           andContactsArray:nil
                     inManagedObjectContext:self.managedObjectContext];
}

How can I transform this to work on the background, and then, when done saving, save the data and update the UI, without blocking the UI while saving the 40.000 objects?

Answer

Rog picture Rog · Sep 25, 2011

Here's a good example for you to try. Feel free to come back if you have any questions:

self.mainThreadContext... // This is a reference to your main thread context
NSPersistentStoreCoordinator *mainThreadContextStoreCoordinator = [self.mainThreadContext persistentStoreCoordinator];
dispatch_queue_t request_queue = dispatch_queue_create("com.yourapp.DescriptionOfMethod", NULL);
dispatch_async(request_queue, ^{

    // Create a new managed object context
    // Set its persistent store coordinator
    NSManagedObjectContext *newMoc = [[NSManagedObjectContext alloc] init];
    [newMoc setPersistentStoreCoordinator:mainThreadContextStoreCoordinator]];

    // Register for context save changes notification
    NSNotificationCenter *notify = [NSNotificationCenter defaultCenter];
    [notify addObserver:self 
               selector:@selector(mergeChanges:) 
                   name:NSManagedObjectContextDidSaveNotification 
                 object:newMoc];

    // Do the work
    // Your method here
    // Call save on context (this will send a save notification and call the method below)
    BOOL success = [newMoc save:&error];
    if (!success)
        // Deal with error
    [newMoc release];
});
dispatch_release(request_queue);

And in response to the context save notification:

- (void)mergeChanges:(NSNotification*)notification 
{
    dispatch_async(dispatch_get_main_queue(), ^{
        [self.mainThreadContext mergeChangesFromContextDidSaveNotification:notification waitUntilDone:YES];
    });
}

And don't forget to remove the observer from the notification center once you are done with the background thread context.

[[NSNotificationCenter defaultCenter] removeObserver:self];