Question

I have the following setup:

NSManagedObjectContext *parent = [[NSManagedObjectContext alloc] 
                            initWithConcurrencyType:NSMainQueueConcurrencyType];
// other setup for parent

NSManagedObjectContext *child = [[NSManagedObjectContext alloc] 
                          initWithConcurrencyType:NSPrivateQueueConcurrencyType];
[child setParentContext:parent];

What I want is to save the parent whenever the child saves, so currently I do something like this:

[child performBlock^{
      [child save:nil];
      [parent performBlock:^{
          [parent save:nil];
      }
}];

That’s me being safe, and calling save within the context’s own queue. Is that necessary? Could I just do:

[child performBlock^{
     [child save:nil];
     [parent save:nil];
}];

Answer 1

No, you cannot use the second variant. You would execute the save operation for the parent context on the queue that is associated with the child context, instead of the main queue.

This means that the save operation would be executed on a (potentially) different thread than the main thread, which is not allowed because managed object contexts are not thread safe.

See also Concurrency Support for Managed Object Contexts
in the Core Data Release Notes for OS X v10.7 and iOS 5.0:

You use contexts using the queue-based concurrency types in
conjunction with two new methods: performBlock: and
performBlockAndWait:. … The one exception is: if your code is
executing on the main thread, you can invoke methods on the main queue
style contexts directly instead of using the block based API.

performBlock: and performBlockAndWait: ensure the block operations
are executed on the queue specified for the context. …