Showing a progress HUD while deleting UITableViewCell - iphone

Question

I am attempting to delete a UITableViewCell while showing a progress HUD (in this case MBProgressHUD). This is necessary, as the Core Data entity being deleted is relatively large. When I run this code I get the following error message:
Terminating app due to uncaught exception 'NSInternalInconsistencyException', reason: 'Invalid update: invalid number of rows in section 0. The number of rows contained in an existing section after the update (2) must be equal to the number of rows contained in that section before the update (2), plus or minus the number of rows inserted or deleted from that section (0 inserted, 1 deleted) and plus or minus the number of rows moved into or out of that section (0 moved in, 0 moved out).'
The code being executed is:
- (void)tableView:(UITableView *)tableView commitEditingStyle:(UITableViewCellEditingStyle)editingStyle forRowAtIndexPath:(NSIndexPath *)indexPath
{
if (editingStyle == UITableViewCellEditingStyleDelete) {
[self.tableView beginUpdates];
[SVProgressHUD showWithStatus:#"Deleting..." maskType:SVProgressHUDMaskTypeGradient];
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
Garden *gardenToDelete = [self.fetchedResultsController objectAtIndexPath:indexPath];
NSLog(#"Deleting garden '%#'", gardenToDelete.gardenName);
[self.managedObjectContext deleteObject:gardenToDelete];
[self.managedObjectContext save:nil];
dispatch_async(dispatch_get_main_queue(), ^{
NSLog(#"Dismissing progress HUD");
NSLog(#"delete animation");
NSLog(#"deleting row");
[self.tableView deleteRowsAtIndexPaths:[NSArray arrayWithObject:indexPath] withRowAnimation:UITableViewRowAnimationFade];
NSLog(#"performFetch");
[self performFetch];
[SVProgressHUD dismiss];
});
});
[self.tableView endUpdates];
}
}
When it runs, I see the HUD appear, then the app proceeds to hang.
I am sure that this has to do with the structure of my multitasking.

Answer 1

managedObjectContext is not thread-safe. You need to create it for each thread.

Answer 2

When you call dispatch_async the first time, you're detaching that thread and allowing the method to continue to execute. This means that [self.tableView endUpdates]; is being called before [self.tableView deleteRowsAtIndexPaths:[NSArray arrayWithObject:indexPath] withRowAnimation:UITableViewRowAnimationFade];
Instead, place your calls to [self.tableView beginUpdates]; and [self.tableView endUpdates]; inside your second call to dispatch_async when you reenter the main thread.