Undoing Core Data insertions that are performed off the main thread

Question

I'm working on some code that uses an NSOperation to import data. I'd like for the user to be able to undo the NSManagedObject instances that are created during the import operation.

From what I can tell, it's impossible to use the NSManagedObjectContext -undoManager for any operations that are performed off of the main thread. From the Core Data Programming Guide section on Use Thread Confinement to Support Concurrency, we have these two conditions:

1. Only objectID should be passed between managed object contexts (on separate threads)
2. Managed objects must be saved in a context before the objectID can be used.

This makes sense since the managed objects need to be moved from private storage (NSManagedObjectContext) to public storage (NSPersistentStore) before they can be shared.

Unfortunately, the -save: message also causes any managed objects in the undo stack to be removed. From the Memory Management Using Core Data section of the same guide:

Managed objects that have pending changes (insertions, deletions, or updates) are retained by their context until their context is sent a save:, reset , rollback, or dealloc message, or the appropriate number of undos to undo the change.

I've tried several things to work around this limitation, and everything eventually leads back to bulk of the work happening on the main thread (and spinning beach balls.) Any clues to getting undo working with objects created off the main thread would be very much appreciated.

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An enhancement Radar has been submitted: rdar://problem/8977725

Answer 1

This answer will probably be a bit of a back and forth. If I understand the issue correctly, you are doing an import but when the import is done you want the user to be able to select what gets saved from the import?

If that is not correct, please fix my assumptions and I will update this answer.

If it is correct then what you can do is:

1. Change your background object creation to

   NSEntityDescription *myEntity = ... //Entity from your context
   [[NSManagedObject alloc] initWithEntity:myEntity
            insertIntoManagedObjectContext:nil];
   

2. Store these entities in an array.
3. Pass the entities back to your main thread as needed.
4. Release on any objects you don't want to keep
5. Call [myMainContext insertObject:managedObject] on any you want to keep.
6. Perform a save on the NSManagedObjectContext.

Since these entities are not part of a NSManagedObjectContext yet they only exist in memory and should be thread safe since they are not yet tied down to a NSManagedObjectContext.

This is of course theoretical and will require testing. However it should accomplish your goal.

Answer 2

Not an expert, but I think what you're going to need to do is create a second context to perform the operations, then merge the two contexts together. You should be able to manage the merge as an undo step. Note, this only works if you're treating the entire set of operations as one undo step, as far as the user is concerned.

Answer 3

Suppose that you use a separate context for the background thread, and once it's done, push a [[backgroundContext undoManager] undo] onto the foreground thread's undo stack? I've never tried anything like that, but off the top of my head I can't think of a reason it shouldn't work.

Answer 4

One option may be to make your import thread persistent. Even when the thread is finished importing, it goes into an idle loop state. This way your threaded ManagedObjectContext is persisted in the proper thread. Then when the user wishes to undo a change, send a message to the thread to use the undomanager.

Answer 5

It's incredibly likely you've considered this and you're likely only looking for a solution using the existing undoManager, but just in case:

Since you're inserting objects and not updating existing ones, you do have the power to tag them with a transaction id as each batch is imported, deleting them in a background thread in the case of an undo. A simple incremented NSNumber is sufficient for the tag.

Inelegant, but workable.