When to use Core Data's NSMainQueueConcurrencyType?

Question

Is initializing a NSManagedObjectContext using NSMainQueueConcurrencyType only for the situation where that MOC has a child MOC that was initialized using NSPrivateQueueConcurrencyType?

To give some background: my app has a traditional structure whereby the main table view is driven by a NSFetchedResultsController and data are imported asynchronously from a web service using an NSOperation subclass that has its own MOC. I wasn't sure whether both MOCs in that situation should use NSConfinementConcurrencyType (the default, I believe) or whether the MOC associated with the fetched results controller on the main thread should use NSMainQueueConcurrencyType and the background MOC should use NSConfinementConcurrencyType.

Answer 1

First a recipe on Core Data new context types.

NSMainQueueConcurrencyType creates a context that is associated with the main dispatch queue and thus the main thread. You could use such a context to link it to objects that are required to run on the main thread, for example UI elements.

NSPrivateQueueConcurrencyType creates and manages a private dispatch queue to operate on. You must use the new methods performBlock: or performBlockAndWait:. The context will then execute the passed blocks on its own private queue.

Finally, NSConfinementConcurrencyType is the default type and can be used only within the thread where it has been created. So, within your NSOperation, you have used it in the right manner. A simple note. If you want to use it as a child context, you need to have a "queue context" (NSMainQueueConcurrencyType or NSPrivateQueueConcurrencyType).

Now, about your question.

Is initializing a NSManagedObjectContext using NSMainQueueConcurrencyType only for the situation where that MOC has a child MOC that was initialized using NSPrivateQueueConcurrencyType?

No, not necessary. Yes, you could set up a private context that does some work in background and then pushes the retrieved objects to the main one, but I will do the contrary: use a NSPrivateQueueConcurrencyType as the master context and the NSMainQueueConcurrencyType as a child context for the former. In this way, the main context will deal only with objects that are in memory. Save to disk are performed by the private queue only.

This approach is used by the UIManagedDocument class. Save to disk are performed in a background thread (a private queue). In this manner the UI is not freezed.