Question

Possible Duplicate:
How to implement re-ordering of CoreData records?

I’m trying to find a code sample that shows how to handle moving/rearranging cells in a tableView when the cell uses a fetchedResultsController (i.e. in conjunction with Core Data). I’m getting the moveRowAtIndexPath: call to my data source, but I can’t find the right combination of black magic to get the table/data to recognize the change properly.

For example, when I move row 0 to row 2 and then let go, it “looks” correct. Then I click “Done”. The row (1) that had slid up to fill row 0 still has it’s editing mode appearance (minus and move icons), while the other rows below slide back to normal appearance. If I then scroll down, as row 2 (originally 0, remember?) nears the top, it completely disappears.

WTF. Do I need to somehow invalidate the fetchedResultsController? Whenever I set it to nil, I get crashes. Should I release it instead? Am I in the weeds?

Here’s what I’ve currently got in there…

- (void)tableView:(UITableView *)tableView moveRowAtIndexPath:(NSIndexPath *)fromIndexPath toIndexPath:(NSIndexPath *)toIndexPath {

    NSManagedObjectContext *context = [fetchedResultsController managedObjectContext];

    /*
     Update the links data in response to the move.
     Update the display order indexes within the range of the move.
     */

    if (fromIndexPath.section == toIndexPath.section) {

        NSInteger start = fromIndexPath.row;
        NSInteger end = toIndexPath.row;
        NSInteger i = 0;
        if (toIndexPath.row < start)
            start = toIndexPath.row;
        if (fromIndexPath.row > end)
            end = fromIndexPath.row;
        for (i = start; i <= end; i++) {
            NSIndexPath *tempPath = [NSIndexPath indexPathForRow:i inSection:toIndexPath.section];
            LinkObj *link = [fetchedResultsController objectAtIndexPath:tempPath];
            //[managedObjectContext deleteObject:[fetchedResultsController objectAtIndexPath:tempPath]];
            link.order = [NSNumber numberWithInteger:i];
            [managedObjectContext refreshObject:link mergeChanges:YES];
            //[managedObjectContext insertObject:link];
        }

    }
    // Save the context.
    NSError *error;
    if (![context save:&error]) {
        // Handle the error...
    }

}

- (void)controllerWillChangeContent:(NSFetchedResultsController *)controller {

    // The fetch controller is about to start sending change notifications, so prepare the table view for updates.
    if (self.theTableView != nil)
        [self.theTableView beginUpdates];
}

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

Answer 1

Here’s what’s officially working now, with deletes, moves, and inserts. I “validate” the order any time there’s an edit action affecting the order.

- (void)tableView:(UITableView *)tableView commitEditingStyle:(UITableViewCellEditingStyle)editingStyle forRowAtIndexPath:(NSIndexPath *)indexPath {
    if (indexPath.section != kHeaderSection) {

        if (editingStyle == UITableViewCellEditingStyleDelete) {

            @try {
                LinkObj * link = [self.fetchedResultsController objectAtIndexPath:indexPath];

                debug_NSLog(@"Deleting at indexPath %@", [indexPath description]);
            //debug_NSLog(@"Deleting object %@", [link description]);

                if ([self numberOfBodyLinks] > 1) 
                    [self.managedObjectContext deleteObject:link];

            }
            @catch (NSException * e) {
                debug_NSLog(@"Failure in commitEditingStyle, name=%@ reason=%@", e.name, e.reason);
            }

        }
        else if (editingStyle == UITableViewCellEditingStyleInsert) {
            // we need this for when they click the "+" icon; just select the row
            [theTableView.delegate tableView:tableView didSelectRowAtIndexPath:indexPath];
        }
    }
}

- (BOOL)validateLinkOrders {        
    NSUInteger index = 0;
    @try {      
        NSArray * fetchedObjects = [self.fetchedResultsController fetchedObjects];

        if (fetchedObjects == nil)
            return NO;

        LinkObj * link = nil;       
        for (link in fetchedObjects) {
            if (link.section.intValue == kBodySection) {
                if (link.order.intValue != index) {
                    debug_NSLog(@"Info: Order out of sync, order=%@ expected=%d", link.order, index);

                    link.order = [NSNumber numberWithInt:index];
                }
                index++;
            }
        }
    }
    @catch (NSException * e) {
        debug_NSLog(@"Failure in validateLinkOrders, name=%@ reason=%@", e.name, e.reason);
    }
    return (index > 0 ? YES : NO);
}


- (void)tableView:(UITableView *)tableView moveRowAtIndexPath:(NSIndexPath *)fromIndexPath toIndexPath:(NSIndexPath *)toIndexPath {
    NSArray * fetchedObjects = [self.fetchedResultsController fetchedObjects];  
    if (fetchedObjects == nil)
        return;

    NSUInteger fromRow = fromIndexPath.row + NUM_HEADER_SECTION_ROWS;
    NSUInteger toRow = toIndexPath.row + NUM_HEADER_SECTION_ROWS;

    NSInteger start = fromRow;
    NSInteger end = toRow;
    NSInteger i = 0;
    LinkObj *link = nil;

    if (toRow < start)
        start = toRow;
    if (fromRow > end)
        end = fromRow;

    @try {

        for (i = start; i <= end; i++) {
            link = [fetchedObjects objectAtIndex:i]; //
            //debug_NSLog(@"Before: %@", link);

            if (i == fromRow)   // it's our initial cell, just set it to our final destination
                link.order = [NSNumber numberWithInt:(toRow-NUM_HEADER_SECTION_ROWS)];
            else if (fromRow < toRow)
                link.order = [NSNumber numberWithInt:(i-1-NUM_HEADER_SECTION_ROWS)];        // it moved forward, shift back
            else // if (fromIndexPath.row > toIndexPath.row)
                link.order = [NSNumber numberWithInt:(i+1-NUM_HEADER_SECTION_ROWS)];        // it moved backward, shift forward
            //debug_NSLog(@"After: %@", link);
        }
    }
    @catch (NSException * e) {
        debug_NSLog(@"Failure in moveRowAtIndexPath, name=%@ reason=%@", e.name, e.reason);
    }
}


- (void)controller:(NSFetchedResultsController *)controller didChangeObject:(id)anObject atIndexPath:(NSIndexPath *)indexPath forChangeType:(NSFetchedResultsChangeType)type newIndexPath:(NSIndexPath *)newIndexPath {    
    @try {
        switch (type) {
            case NSFetchedResultsChangeInsert:
                [theTableView insertRowsAtIndexPaths:[NSArray arrayWithObject:newIndexPath] withRowAnimation:UITableViewRowAnimationFade];
                [self validateLinkOrders];
                break;
            case NSFetchedResultsChangeUpdate:
                break;
            case NSFetchedResultsChangeMove:
                self.moving = YES;
                [self validateLinkOrders];
                break;
            case NSFetchedResultsChangeDelete:
                [theTableView deleteRowsAtIndexPaths:[NSArray arrayWithObject:indexPath] withRowAnimation:UITableViewRowAnimationFade];
                [self validateLinkOrders];
                break;
            default:
                break;
        }
    }
    @catch (NSException * e) {
        debug_NSLog(@"Failure in didChangeObject, name=%@ reason=%@", e.name, e.reason);
    }
}

- (void)controller:(NSFetchedResultsController *)controller didChangeSection:(id <NSFetchedResultsSectionInfo>)sectionInfo atIndex:(NSUInteger)sectionIndex forChangeType:(NSFetchedResultsChangeType)type {
    switch(type) {
        case NSFetchedResultsChangeInsert:
            [self.theTableView insertSections:[NSIndexSet indexSetWithIndex:sectionIndex] withRowAnimation:UITableViewRowAnimationFade];
            break;

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

- (void)controllerDidChangeContent:(NSFetchedResultsController *)controller {
    // The fetch controller has sent all current change notifications, so tell the table view to process all updates.
    @try {
        if (self.theTableView != nil) {
            //[self.theTableView endUpdates];
            if (self.moving) {
                self.moving = NO;
                [self.theTableView reloadData];
                //[self performSelector:@selector(reloadData) withObject:nil afterDelay:0.02];
            }
            [self performSelector:@selector(save) withObject:nil afterDelay:0.02];
        }   

    }
    @catch (NSException * e) {
        debug_NSLog(@"Failure in controllerDidChangeContent, name=%@ reason=%@", e.name, e.reason);
    }
}