Stumbling Block

Update 10:15 pm: the solution is at the end. (Spoiler: MartianCraft saves the day!)

* * *

I’m not sure what I’m doing wrong, but you might, since you’ve probably worked with Core Data more than I have. So I’ll write it up.

Vesper’s table view has an NSFetchedResultsController watching for changes to notes. Keep that in the back of your mind.

Data migration

The app has a main thread context. The data migration system uses a temporary private queue concurrency context. I use it with performBlock:. (The contexts are not parent/child contexts.)

When the data migration is finished, the private context saves. The main thread context then calls mergeChangesFromContext​DidSave​Notification (on the main thread, via performSelector​OnMainThread).

This all appears to work splendidly.

Meanwhile, back at the table view…

The table view controller gets the controllerDidChangeContent: message from its NSFetchedResultsController.

Cool — that’s how things should work. The number of fetchedObjects is correct, even. So happy.

So happy until I notice that all the objects have nil properties, with the exception of properties with default values (as defined in the data model).

They are not faults, and accessing any of the values (such as note.text) just returns nil. (If they were faults, accessing note.text and so on would work.)

And the table view appears empty.

I don’t know why.

Some things I tried

If I navigate to another screen and back, everything works correctly. Properties get their values.

If I use the main thread context for data migration, then I don’t have this problem. (But I don’t want to use the main thread context.)

The kicker

It occurred to me that perhaps I wasn’t doing the merge-changes thing properly.

So I put an NSLog in the method that gets that notification, so I could look at the to-be-merged data.

The method looks like this:

NSLog(@"userInfo: %@", [note userInfo]);
[self.mainThreadContext mergeChanges​FromContext​DidSave​Notification:note];

Guess what? It works then. The table view is not empty; objects have expected values.

Take the NSLog away, and it doesn’t work.

So then I wondered — what would happen if I take the NSLog away, but set a breakpoint on the remaining line, and do a po [note userInfo].

Well, it works in the sense that po does what I expect — but the objects have nil properties rather than expected values. It does not have the same effect NSLog has.

This is Core Data getting back at me.

P.S. On a hunch I tried one more thing instead of the NSLog: sleep(1). Guess what — it does the trick. Table view is non-empty and the objects have properties with values.

(But of course I can’t leave it that way.)

Solution - updated 10:15 pm

Stupid? Embarassing? Sure thing.

I was emailing with the awesome Trevor Squires (of MartianCraft) about this and he picked it out:

I was listening to the wrong notification. I was listening to NSManagedObjectContext​ObjectsDidChange​Notification when I should have been listening to NSManagedObjectContext​DidSave​Notification.

Total rookie mistake. (Because I am, in fact, a Core Data rookie.)

25 Feb 2014

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