ios - NSFetchedResultsController: delete 1 row crashes the app

Question

Deleting a row from a UITableView fed by an NSFetchedResultsController causes my app to crash.

Error is:

* Assertion failure in -[UITableView _endCellAnimationsWithContext:], /SourceCache/UIKit/UIKit-2903.23/UITableView.m:1330
* 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 (1) must be equal to the number of rows contained in that section before the update (1), 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).'

I want only swipe-to-delete. My deletion code goes like this:

- (void)tableView:(UITableView *)tableView commitEditingStyle:(UITableViewCellEditingStyle)editingStyle forRowAtIndexPath:(NSIndexPath *)indexPath
{  
    [self.tableView beginUpdates];  
    SequenceData *sd = [self.fetchedResultsController objectAtIndexPath:indexPath];  
    [self.managedObjectContext deleteObject:sd];  
    [self.tableView deleteRowsAtIndexPaths:[NSArray arrayWithObject:indexPath]
                          withRowAnimation:UITableViewRowAnimationFade];  
    [self.tableView endUpdates];  
}

My NSFetchedResultsController is set up just like in Ray Wanderlich's tutorial (http://www.raywenderlich.com/999/core-data-tutorial-for-ios-how-to-use-nsfetchedresultscontroller)

Number of rows is determined by:

- (NSInteger)tableView:(UITableView *)tableView numberOfRowsInSection:(NSInteger)section {    
    id sectionObjects = [[self.fetchedResultsController sections] objectAtIndex:section];  
    NSInteger nbObjects = [sectionObjects numberOfObjects];  
    return nbObjects;  
}

It looks like the fetch is not updating (number of rows does not vary). Do I need to fetch myself (isn't this taking care of by the fetch controller)?

There is obviously something basic I am missing here. Don't hesitate to suggest basic answers.

I first implemented this with the controller: didChangeObject: ... method. Error was the same, but some details differed.

Edit I believe my problem is fixed. Both answers (From CX and Martin) helped me find it. Martin got the answer because of the explanations that helped me understand a little bit better...

Answer 1

Don't call

[self.tableView deleteRowsAtIndexPaths:[NSArray arrayWithObject:indexPath] withRowAnimation:UITableViewRowAnimationFade];

if you use a fetched results controller. Only delete the object with

[self.managedObjectContext deleteObject:sd];

The fetched results controller delegate method didChangeObject: is then called automatically, and that calls deleteRowsAtIndexPaths:.

So in your case, the row was deleted twice, and that caused the exception.

Note that you don't need beginUpdates/endUpdates here.