Multithreaded Core Data - persistentStoreCoordinator vs parentContext

Question

I'm trying to resolve issues that are being caused by accessing the same NSManagedObjectContext from multiple threads. I found the following in the Apple docs:

let moc = … //Our primary context on the main queue

let privateMOC = NSManagedObjectContext(concurrencyType: .PrivateQueueConcurrencyType)
privateMOC.parentContext = moc

privateMOC.performBlock {
    //...
}

Which seems to be what I'm after. I also found something very similar in a tutorial which has been updated for iOS 9:

let privateContext = NSManagedObjectContext(concurrencyType: .PrivateQueueConcurrencyType)
privateContext.persistentStoreCoordinator = coreDataStack.context.persistentStoreCoordinator

privateContext.performBlock { () -> Void in
  //...
}

They both seem to achieve the same, however the Apple doc's version uses parentContext instead of using the persistantStoreCoordinator directly. What is the difference between these two approaches?

Answer 1

Use parentContext. This is preferred to the method with the persistent store coordinator since iOS 5.

Prior to OS X v10.7 and iOS v5.0, the parent store is always a persistent store coordinator. In OS X v10.7 and later and iOS v5.0 and later, the parent store may be another managed object context. Ultimately the root of a context’s ancestry must be a persistent store coordinator. The coordinator provides the managed object model and dispatches requests to the various persistent stores containing the data.

In your particular case:

If a context’s parent store is another managed object context, fetch and save operations are mediated by the parent context instead of a coordinator. This pattern has a number of usage scenarios, including:

• Performing background operations on a second thread or queue.

Source: NSManagedObjectContext class reference, "Parent Store".