tableView:cellForRowAtIndexPath called with nil indexPath after deleting item

问题

I have a rather vanilla UITableView managed by an NSFetchedResultsController to display all instances of a given Core Data entity.

When the user deletes an entry in the table view by swiping over it, tableView:cellForRowAtIndexPath: eventually gets called on my UITableViewController with a nil indexPath. Since I had not expected it to be called with a nil indexPath, the app crashes.

I can work around the crash by checking for that nil value and then returning an empty cell. This seems to work, but I still worry that I may have handled something wrong. Any ideas? Has anybody ever seen tableView:cellForRowAtIndexPath: called with a nil indexPath?

Note that this only happens when the user deletes from the table view by swiping over the cell. When deleting an item using the table view editing mode, it doesn't happen. What would be different between the two ways to delete a cell?

So is it really an OK situation to get a nil indexPath in a table view delegate method?

My view controller code is really standard. Here is the deletion:

- (void)tableView:(UITableView *)tableView commitEditingStyle:(UITableViewCellEditingStyle)editingStyle forRowAtIndexPath:(NSIndexPath *)indexPath
{
    if (editingStyle == UITableViewCellEditingStyleDelete) {
        // Delete the row from the data source
        NSManagedObject *managedObject = [self.fetchedResultsController objectAtIndexPath:indexPath];
        [self.moc deleteObject:managedObject];
        NSError *error = NULL;
        Boolean success = [self.moc save:&error];
        if (!success) { <snip> }
        // actual row deletion from table view will be handle from Fetched Result Controller delegate
        // [tableView deleteRowsAtIndexPaths:[NSArray arrayWithObject:indexPath] withRowAnimation:UITableViewRowAnimationFade];
    } else { <snip> }   
}

This will lead to the NSFetchedResultsController delegate method being called:

- (void)controller:(NSFetchedResultsController *)controller didChangeObject:(id)anObject
   atIndexPath:(NSIndexPath *)indexPath forChangeType:(NSFetchedResultsChangeType)type
  newIndexPath:(NSIndexPath *)newIndexPath
{
    UITableView *tableView = self.tableView;
    switch(type) {

        case NSFetchedResultsChangeDelete:
            [tableView deleteRowsAtIndexPaths:[NSArray arrayWithObject:indexPath] withRowAnimation:UITableViewRowAnimationFade];
        break;

        case NSFetchedResultsChangeInsert: <snip> break;
        case NSFetchedResultsChangeUpdate: <snip> break;
        case NSFetchedResultsChangeMove: <snip> break;
    }
}

And of course, the data source methods are handled by the NSFetchedResultsController, e.g.:

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

Many thanks.

回答1:

It seems like you are deleting the indexPath from table but table data source is not updating. Did you verify the data source udation process by NSFetchedResultsController is correctly updationg the table data source.?

回答2:

I would do like this, since you are populating the table directly from your managed context, why not on delete first delete the object from the managed context and then imediately update the table from the context using reloadData. But using your approach i think you need to add beginUpdates and endUpdates:

#pragma mark -
#pragma mark Fetched results controller delegate


- (void)controllerWillChangeContent:(NSFetchedResultsController *)controller {
[self.tableView beginUpdates];
}


- (void)controller:(NSFetchedResultsController *)controller didChangeSection:(id <NSFetchedResultsSectionInfo>)sectionInfo
       atIndex:(NSUInteger)sectionIndex forChangeType:(NSFetchedResultsChangeType)type {

switch(type) {
    case NSFetchedResultsChangeInsert:
        [self.tableView insertSections:[NSIndexSet indexSetWithIndex:sectionIndex] withRowAnimation:UITableViewRowAnimationFade];
        break;

    case NSFetchedResultsChangeDelete:
        [self.tableView deleteSections:[NSIndexSet indexSetWithIndex:sectionIndex] withRowAnimation:UITableViewRowAnimationFade];
        break;
}
}


- (void)controller:(NSFetchedResultsController *)controller didChangeObject:(id)anObject
   atIndexPath:(NSIndexPath *)indexPath forChangeType:(NSFetchedResultsChangeType)type
  newIndexPath:(NSIndexPath *)newIndexPath {

UITableView *tableViews = self.tableView;

switch(type) {

    case NSFetchedResultsChangeInsert:
        [tableViews insertRowsAtIndexPaths:[NSArray arrayWithObject:newIndexPath] withRowAnimation:UITableViewRowAnimationFade];
        break;

    case NSFetchedResultsChangeDelete:
        [tableViews deleteRowsAtIndexPaths:[NSArray arrayWithObject:indexPath] withRowAnimation:UITableViewRowAnimationFade];
        break;

    case NSFetchedResultsChangeUpdate:
        [_delegate configureCell:[tableViews cellForRowAtIndexPath:indexPath] atIndexPath:indexPath];
        break;

    case NSFetchedResultsChangeMove:
        [tableViews deleteRowsAtIndexPaths:[NSArray arrayWithObject:indexPath] withRowAnimation:UITableViewRowAnimationFade];
        [tableViews insertRowsAtIndexPaths:[NSArray arrayWithObject:newIndexPath]withRowAnimation:UITableViewRowAnimationFade];
        break;
}
}


- (void)controllerDidChangeContent:(NSFetchedResultsController *)controller {
    [self.tableView endUpdates];
    NSIndexPath *indexPath = [NSIndexPath indexPathForRow:0 inSection:0];
    [self.tableView scrollToRowAtIndexPath:indexPath
                      atScrollPosition:UITableViewScrollPositionMiddle
                              animated:NO];
}

回答3:

I use CoreData in table views and allow the users to delete records and update them all the time, I have them in 4 apps without encountering the problem you mentioned.

I have these FetchedResultsController Delegate Method in my code

     - (void)controllerWillChangeContent:(NSFetchedResultsController *)controller {
  // The fetch controller is about to start sending change notifications, so prepare the table view for updates.
    [self.myTableView beginUpdates];
 }


- (void)controllerDidChangeContent:(NSFetchedResultsController *)controller {
    // The fetch controller has sent all current change notifications, so tell the table view to process all updates.
        [self.myTableView endUpdates];

}

来源：https://stackoverflow.com/questions/6941391/tableviewcellforrowatindexpath-called-with-nil-indexpath-after-deleting-item

标签

ios

null

nsindexpath

uitableview