NSFetchedResultsController extra section [duplicate] - ios

Question

I am trying to show a table view with 2 sections. The first section will always have 1 row and the second section will have as many rows as data points. I am using Core Data and the following tableView:numberOfRowsInSection: method...
- (NSInteger)tableView:(UITableView *)tableView numberOfRowsInSection:(NSInteger)section
{
if (section == 0) {
return 1;
} else {
id <NSFetchedResultsSectionInfo> sectionInfo = [[self.fetchedResultsController sections] objectAtIndex:section];
return [sectionInfo numberOfObjects];
}
}
However, I am getting the following error:
Terminating app due to uncaught exception 'NSRangeException', reason:
'* -[__NSArrayM objectAtIndex:]: index 1 beyond bounds [0 .. 0]'
Any help will be appreciated. Thanks.
NEW -------------------------------------------------------------------------------
This is the current implementation of the relevant methods:
- (NSInteger)tableView:(UITableView *)tableView numberOfRowsInSection:(NSInteger)section
{
if (section == 0) {
return 1;
} else {
NSUInteger frcSection = section - 1;
id <NSFetchedResultsSectionInfo> sectionInfo = [[self.fetchedResultsController sections] objectAtIndex:frcSection];
return [sectionInfo numberOfObjects];
}
}
- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath
{
static NSString *CellIdentifier = #"Cell";
UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:CellIdentifier];
if (cell == nil) {
cell = [[UITableViewCell alloc] initWithStyle:UITableViewCellStyleDefault reuseIdentifier:CellIdentifier];
}
[self configureCell:cell atIndexPath:indexPath];
return cell;
}
- (void)configureCell:(UITableViewCell *)cell atIndexPath:(NSIndexPath *)indexPath
{
if (indexPath.section == 0) {
cell.textLabel.text = entityOne.name; //entityOne object passed from previous VC
} else {
entityTwo = [self.fetchedResultsController objectAtIndexPath:indexPath];
cell.textLabel.text = entityTwo.name;
}
}
- (void)controller:(NSFetchedResultsController *)controller
didChangeSection:(id <NSFetchedResultsSectionInfo>)sectionInfo
atIndex:(NSUInteger)sectionIndex
forChangeType:(NSFetchedResultsChangeType)type
{
NSUInteger frcSectionIndex = 0;
frcSectionIndex = sectionIndex + 1;
switch(type) {
case NSFetchedResultsChangeInsert:
[self.tableView insertSections:[NSIndexSet indexSetWithIndex:frcSectionIndex] withRowAnimation:UITableViewRowAnimationFade];
break;
case NSFetchedResultsChangeDelete:
[self.tableView deleteSections:[NSIndexSet indexSetWithIndex:frcSectionIndex] withRowAnimation:UITableViewRowAnimationFade];
break;
}
}
- (void)controller:(NSFetchedResultsController *)controller
didChangeObject:(id)anObject
atIndexPath:(NSIndexPath *)indexPath
forChangeType:(NSFetchedResultsChangeType)type
newIndexPath:(NSIndexPath *)newIndexPath
{
UITableView *tableView = self.tableView;
switch(type) {
case NSFetchedResultsChangeInsert:
[tableView insertRowsAtIndexPaths:#[newIndexPath] withRowAnimation:UITableViewRowAnimationFade];
break;
case NSFetchedResultsChangeDelete:
[tableView deleteRowsAtIndexPaths:#[indexPath] withRowAnimation:UITableViewRowAnimationFade];
break;
case NSFetchedResultsChangeUpdate:
[self configureCell:[tableView cellForRowAtIndexPath:indexPath] atIndexPath:indexPath];
break;
case NSFetchedResultsChangeMove:
[tableView deleteRowsAtIndexPaths:#[indexPath] withRowAnimation:UITableViewRowAnimationFade];
[tableView insertRowsAtIndexPaths:#[newIndexPath] withRowAnimation:UITableViewRowAnimationFade];
break;
}
}

Answer 1

The reason is that the fetched results controller (FRC) has only one section (section #0), which you want to be displayed in the second section (section #1) of the table view.
This is possible, but you have to map between FRC section numbers and table view section numbers, e.g.
- (NSInteger)tableView:(UITableView *)tableView numberOfRowsInSection:(NSInteger)section
{
if (section == 0) {
return 1;
} else {
NSUInteger frcSection = section - 1;
id <NSFetchedResultsSectionInfo> sectionInfo = [[self.fetchedResultsController sections] objectAtIndex:frcSection];
return [sectionInfo numberOfObjects];
}
}
The same mapping is necessary in cellForRowAtIndexPath.
In the FRC delegate methods didChangeObject, didChangeSection you have to add 1 to the section number before calling the table view methods (e.g. insertRowsAtIndexPaths).
ADDED: configureCell should look like this:
- (void)configureCell:(UITableViewCell *)cell atIndexPath:(NSIndexPath *)indexPath
{
if (indexPath.section == 0) {
cell.textLabel.text = entityOne.name; //entityOne object passed from previous VC
} else {
NSIndexPath *frcIndexPath = [NSIndexPath indexPathForRow:indexPath.row inSection:(indexPath.section - 1)];
entityTwo = [self.fetchedResultsController objectAtIndexPath:frcIndexPath];
cell.textLabel.text = entityTwo.name;
}
}
and didChangeObject like this:
- (void)controller:(NSFetchedResultsController *)controller
didChangeObject:(id)anObject
atIndexPath:(NSIndexPath *)indexPath
forChangeType:(NSFetchedResultsChangeType)type
newIndexPath:(NSIndexPath *)newIndexPath
{
UITableView *tableView = self.tableView;
NSIndexPath *tvIndexPath = [NSIndexPath indexPathForRow:indexPath.row inSection:(indexPath.section + 1)];
NSIndexPath *tvNewIndexPath = [NSIndexPath indexPathForRow:newIndexPath.row inSection:(newIndexPath.section + 1)];
switch(type) {
case NSFetchedResultsChangeInsert:
[tableView insertRowsAtIndexPaths:#[tvNewIndexPath] withRowAnimation:UITableViewRowAnimationFade];
break;
case NSFetchedResultsChangeDelete:
[tableView deleteRowsAtIndexPaths:#[tvIndexPath] withRowAnimation:UITableViewRowAnimationFade];
break;
case NSFetchedResultsChangeUpdate:
[self configureCell:[tableView cellForRowAtIndexPath:tvIndexPath] atIndexPath:tvIndexPath];
break;
case NSFetchedResultsChangeMove:
[tableView deleteRowsAtIndexPaths:#[tvIndexPath] withRowAnimation:UITableViewRowAnimationFade];
[tableView insertRowsAtIndexPaths:#[tvNewIndexPath] withRowAnimation:UITableViewRowAnimationFade];
break;
}
}
You probably get the idea:
When going from FRC index path to table view index path, add one to the section.
When going from table view index path to FRC index path, subtract one from the section.