Core Data background sync conflict issue

Question

I'm running into a crash issue while developing an iPhone app with Core Data.

The app syncs data with a webservice on a background thread.

When the app first launches, the existing data in the Core Data DB will be displayed to the user in a UITableView, while a background thread is kicked off the grab the latest data from the web service API. Lets call the core data models User and Items, just for discussion purposes with a User having many items. The items are what is displayed in the UITableView.

When the API results come back, the users existing Item records are deleted from Core data and the new set of Item records is inserted into Core Data. Once all items are parsed, a message is sent back to the main thread to merge in the core data changes and refetch the data from CD.

However, I keep crashing in my configure table cell routine, and I'm sure its because of out of scope objects. Meaning, the main thread is trying to display objects from Core Data, while the background thread is deleting those same objects from Core data and replacing them with new ones.

What is the best way to handle these kind of conflicts? Do I have to put in some kind of mechanism to not kick off the background thread updating until the main view has loaded/displayed all its data? If so, how do I accomplish this? Can I still keep my same methodology and just handle the deletion of displayed items better?

Answer 1

Are those Core Data operations so expensive that you can't do this from the main thread? Just do the webservice part on another thread. Otherwise, it probably will get more difficult - maybe put some controller class in between that caches the relevant data or synchronizes access to Core Data.

Answer 2

How are you accessing your data for the table? If you're using an NSFetchedResultsController, it should be automatically updating the cells in a safe manner.

In the scope of your original question:
Unless your data is saved somewhere else, redrawing the cells as you scroll through the table accesses the data. You could copy all of the data you're using to populate the cells, but that would spike your memory usage.

Answer 3

Are the main thread and the background thread each using their own Core Data managed object context? You also might want to review the Core Data multithreading tips at

http://developer.apple.com/iphone/library/documentation/Cocoa/Conceptual/CoreData/Articles/cdMultiThreading.html

Answer 4

Check your NSFetchedResultsControllerDelegate methods controllerWillChangeContent: and controllerDidChangeContent:.

You are responsible for notifying the table that you will be altering it with the beginUpdates and endUpdates messages. When the fetched results controller is going to modified the returned data, the table needs to freeze updating itself until the controller is done. Otherwise, the table is trying to represent a constantly shifting list of data.

Answer 5

Refer to the TopSongs project SongsViewController implementation Specifically viewDidLoad and handleSaveNotification

- (void)handleSaveNotification:(NSNotification *)aNotification {
[managedObjectContext mergeChangesFromContextDidSaveNotification:aNotification];
[self fetch];
}

Your viewController's handleSaveNotification: is not being called because you registered for notifications from the MOC that it owns, not all MOC's. It works for the the TopSongs sample as the app delegate and viewController's MOC are the same object. (appDelegate passes the moc to songsViewController).

Replace:

// ViewController.m viewDidLoad
[[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(handleSaveNotification:) name:NSManagedObjectContextDidSaveNotification object:self.managedObjectContext];

With:

[[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(handleSaveNotification:) name:NSManagedObjectContextDidSaveNotification object:nil];

Replace:

// ViewController.m viewDidUnload
[[NSNotificationCenter defaultCenter] removeObserver:self name:NSManagedObjectContextDidSaveNotification object:self.managedObjectContext];

With:

[[NSNotificationCenter defaultCenter] removeObserver:self name:NSManagedObjectContextDidSaveNotification object:nil];

Answer 6

Have you resolved this ?

Check that you a) Are not using caching - i.e your call when you initially create the FRC does not use caching:

 NSFetchedResultsController *aFetchedResultsController = [[NSFetchedResultsController alloc] initWithFetchRequest:fetchRequest managedObjectContext:managedObjectContext sectionNameKeyPath:nil cacheName:nil]

(See cacheName:nil)

b) Are you using a Predicate in the FRC ? I'm having all sorts of problems with this now, trying to do the same thing as your code.