CODEWITHSUNDEEP
iosmultithreadingcore-datansmanagedobjectcontext
harryisaac
harryisaac

Reputation: 1141

Core Data Setting Up Multiple managedObjectContexts

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?

Upvotes: 2

Views: 197

Answers (2)

Arek Holko
Arek Holko

Reputation: 9006

If you really want to do concurrent reads and writes, you might be interested in the setup presented during WWDC 2013. It uses two NSPersistentStoreCoordinator's, each with one context. You merge changes between them manually, thus achieving nice, non UI blocking code.

It's described in the following sessions:

  • 207 - What’s New in Core Data (starting from slide number 145).
  • 211 - Core Data Performance (starting from slide number 91),

Upvotes: 0

wesley
wesley

Reputation: 859

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];

    }

Upvotes: 1

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