How do I create a child NSManagedObjectContext?

Question

I've seen a few videos / threads that say it's possible to create 'children' MOCs -- MOCs that use other MOCs as their persistant stores. Useful, for example, in a context where you're threading your application, and want to have a single master MOC that can save / rollback the changes that the child threads create. (From what I understand, a MOC and it's managedObjects MUST all be used on the same thread)

The question is, how do I create a child MOC? I can't track down the WWDC videos I was watching that introduced them, and everything I"ve seen has been talking about how to use them ONCE they're made. I can easily alloc a new MOC, but how do I set it's persistent store to be another MOC? The reference doesn't show any functions that do that!

Inductee

Answer 1

Create a new MOC for which you are in total control of the synchronization. This is the same as calling init and the same behavior as pre-parent/child relationships.

NSManagedObjectContext *moc = [[NSManagedObjectContext alloc]
       initWithConcurrencyType:NSConfinementConcurrencyType];

You parent a MOC to another MOC by setting its property:

moc.parentContext = someOtherMocThatIsNowMyParent;

Here, the child chooses the parent. I'm sure my kids wish they were NSManagedObjectContexts. A parent context must be of either NSPrivateQueueConcurrencyType or NSMainQueueConcurrencyType.

You can create a MOC that is "bound" to a private queue:

NSManagedObjectContext *moc = [[NSManagedObjectContext alloc]
       initWithConcurrencyType:NSPrivateQueueConcurrencyType];

which means you should only access it via the performBlock or performBlockAndWait API. You can call those methods from any thread as they will ensure proper serialization of the code in the block. For example...

NSManagedObjectContext *moc = [[NSManagedObjectContext alloc]
       initWithConcurrencyType:NSPrivateQueueConcurrencyType];
moc.parentContext = someOtherMocThatIsNowMyParent;
[moc performBlock:^{
    // All code running in this block will be automatically serialized
    // with respect to all other performBlock or performBlockAndWait
    // calls for this same MOC.
    // Access "moc" to your heart's content inside these blocks of code.
}];

The difference between performBlock and performBlockAndWait is that performBlock will create a block of code, and schedule it with Grand Central Dispatch to be executed asynchronously at some time in the future, on some unknown thread. The method call will return immediately.

performBlockAndWait will do some magic synchronization with all the other performBlock calls, and when all the blocks that have been presented prior to this one are done, this block will execute. The calling thread will pend until this call has completed.

You can also create a MOC that is "bound" to the main thread, just like private concurrency.

NSManagedObjectContext *moc = [[NSManagedObjectContext alloc]
       initWithConcurrencyType:NSMainQueueConcurrencyType];
moc.parentContext = someOtherMocThatIsNowMyParent;
[moc performBlock:^{
    // All code running in this block will be automatically serialized
    // with respect to all other performBlock or performBlockAndWait
    // calls for this same MOC.  Furthermore, it will be serialized with
    // respect to the main thread as well, so this code will run in the
    // main thread -- which is important if you want to do UI work or other
    // stuff that requires the main thread.
}];

which means you should only access it directly if you know you are on the main thread, or via the performBlock or performBlockAndWait API.

Now, you can use the "main concurrency" MOC either via the performBlock methods, or directly if you know you are already running in the main thread.

University

Thank you for such an (incredibly) thorough answer. – Inductee

Hello, I know it's really old but my question isn't big enough to create a new one, I was wondering where I should call save on the context, inside the performBlock or outside ? Thank you ! – Nisus

Thank you for an excellent answer. It helped me out since Apple's documentation claims that parentContext is "read-only", which it clearly isn't. developer.apple.com/library/prerelease/ios/documentation/Cocoa/… – Coles

Answer 2

Initialize child MOC then:

[_childMOC performBlockAndWait:^{
            [_childMOC setParentContext:parentMOC]; 
 }];

Astrophysics

So... I can ACCESS MOCs accross threads, I'm just not allowed to operate (insert, delete, save, rollback) accross them? – Inductee

You can call performBlock and performBlockAndWait from other threads. Not much else. – University