NSFetchedResultsController with backgroundContext and mainContext, nasty error

1
votes

I'm struggling with an error for about 4 hours already. Here is what I have:

  • I have a TableViewController with an NSFetchedResultsController, the TableViewController class implements all the fetchedResultsController delegate methods
  • The very first time I run the app my core data is empty
  • In a background thread I download stuff and add those to core data, I have a background Managed Object Context for this.
  • In the Table View Controller I register for NSManagedObjectContextDidSaveNotification and when I save the background Context my notification callback gets called and I attempt to merge the main context with the background one.

enter image description here

Now the nasty stuff happens here

  • If my NSFetchedResultsController's delegate is nil and I do mergeChangesFromContextDidSaveNotification: everything works but my UI doesn't get updated and I have to do another fetchRequest on my fetchResultsController

  • If I assign a delegate to the NSFetchedResultsController the app crashes when it gets to the mergeChangesFromContextDidSaveNotification: line (see screenshot below)

enter image description here

Any pointers on how to make the NSFetchedResultsController work and fix this problem will be very appreciated.

Thank you for taking the time to read my question!

1
iphonemultithreadingcore-datansfetchedresultscontroller
What are the details of the crash? Any console logs or other information that you can provide? Does the merge work when, perhaps, populating data programmatically in the background to see if the issue maybe with your data download? - timthetoolman
The only output I get in the console is this one: 2011-10-25 20:56:31.326 ArticleViewer[9345:207] Merge context 2011-10-25 20:56:31.327 ArticleViewer[9345:207] Controller Will Change Content And these are NSLogs I've inserted in code, the first one is right before mergeContext is fired and the second one is in the controllerWillChangeContent: delegate method of NSFetchedResultsController The problem is not in the contents of the CoreData because the second time I run the app the CD contents are displayed properly - Horatiu Paraschiv
The EXC_BAD_ACCESS means that you, or another object, are sending a message to an object that no longer exists. You may want to google "CocaDev NSZombieEnabled" to set the right options for determining what Dealloc'd object is receiving the message. From the stack trace you have in the screen shot it would seem to be something associated with your context or NSFetchedResultsController. - timthetoolman
It is infinitely more helpful to paste the code and the console logs in as text instead of images. Those images are nearly impossible to read. - Marcus S. Zarra
Per Marcus and TechZen, can you post some of your delegate methods? - timthetoolman

1 Answers

2
votes

If my NSFetchedResultsController's delegate is nil and I do mergeChangesFromContextDidSaveNotification: everything works but my UI doesn't get updated and I have to do another fetchRequest on my fetchResultsController

That is the expected behavior as the delegate is the only object that forces the UI to update.

If I assign a delegate to the NSFetchedResultsController the app crashes when it gets to the mergeChangesFromContextDidSaveNotification: line (see screenshot below)

The process of merging context will trigger notifications while it is ongoing so the NSFetchedResultsControllerdelegate methods can be called while the merge is ongoing. If assigning the delegate is the only change that triggers the crash, then the error has to be in the NSFetchedResultsController delegate methods regardless of where the debugger displays the error.