Core Data Single Managed Object Context and two threads

Question

I am going crazy trying to figure this out. I am working on an application that is syncing up data from the webserver. There is a background thread that is pulling data from the server to the application. At the same time I am making changes to the UI. The values changed on UI are being saved to core data in foreground.

Through out the application I have one managedObjectContext that I fetch from the app delegate every time I create a fetchController . App delegate code

- (NSManagedObjectContext *)managedObjectContext
{
    if (__managedObjectContext != nil) {
        return __managedObjectContext;
    }

    NSPersistentStoreCoordinator *coordinator = [self persistentStoreCoordinator];
    if (coordinator != nil) {
        __managedObjectContext = [[NSManagedObjectContext alloc] init];
        [__managedObjectContext setPersistentStoreCoordinator:coordinator];
    }
    return __managedObjectContext;
}

Now the problem is I am getting error while trying to save the context. The errors are happening randomly in the code. I am saving the context as soon as I am making change to any entity. Also I have two relationships each in each entity one to its child that is one to many and one to its parents that is to - one. All relationship have appropriate inverse.

I think I am doing something conceptually wrong over here by mentaining one context. Could you please advice how I should manage context in a situation where both background and foreground threads are reading and writing to the coredata. Thanks.

Answer 1

Managed object contexts are not thread safe, so if you use the same one on more than one thread without considering concurrency-- you're going to have major problems. As in, crashing and/or data loss and maybe even data corruption. There are a couple of ways to deal with this:

1. Use one of the queue concurrency types when creating the context-- see docs for initWithConcurrencyType:. Then, whenever you access the data store, use either performBlock: or performBlockAndWait: to synchronize access.

2. Create a new managed object context for the background thread. Use NSManagedObjectContextDidSaveNotification and mergeChangesFromContextDidSaveNotification: to keep multiple contexts synchronized.

But whatever you do, do not just use one managed object context on more than one thread.