Core Data Setting Up Multiple managedObjectContexts

Question

In an app that I am working on I need to access and write to Core Data at the same time. I have been able to gather that this means that I need to user multiple managedObjectContexts but I don't understand how I am supposed to setup those two managedObjectContexts.

I understand that once I have them setup I need to do the write operations on a background thread on its managedObjectContext and then merge the data through an operation like this: Core Data and threads / Grand Central Dispatch .

So my question is, how would I go about initiating two separate managedObjectContexts so that I can then use them as described?

Answer 1

You have to create two separate NSManagedObjectContexts with the same NSPersistentStoreCoordinator like this,

First create two NSManagedObjectContexts name as backgroundManagedObjectContext and mainBackgroundManagedObjectContext in your model class like this

+ (NSManagedObjectContext *)backgroundManagedObjectContext
    {
        static NSManagedObjectContext * backgroundManagedObjectContext;
        if(backgroundManagedObjectContext != nil){
            return backgroundManagedObjectContext;
        }
        @try {
            NSPersistentStoreCoordinator *coordinator = [self persistentStoreCoordinator];
            if (coordinator != nil) {
                backgroundManagedObjectContext = [[NSManagedObjectContext alloc] initWithConcurrencyType:NSPrivateQueueConcurrencyType];
                [backgroundManagedObjectContext setPersistentStoreCoordinator: [self persistentStoreCOordinator]];
            }
        }
        @catch (NSException *exception) {
            NSLog(@"Exception occur %@",exception);
        }
        return backgroundManagedObjectContext;

    }

then both will need get the same persistentStoreCoordinator then need to merge your backgroungManagedObjectContext to mainBackgroundManagedObjectContext, for that create NSNotification whenever you save the data into backgroundManageObjectContext like this

[[NSNotificationCenter defaultCenter] addObserver:self
                                             selector:@selector(contextDidSave:)
                                                 name:NSManagedObjectContextDidSaveNotification
                                               object:nil];

then you have to implement this notification method for updating your mainManagedObjectContext like this

- (void)contextDidSave:(NSNotification *)notification
    {
        SEL selector = @selector(mergeChangesFromContextDidSaveNotification:);
        [[self mainManagedObjectContext] performSelectorOnMainThread:selector withObject:notification waitUntilDone:YES];

    }