Custom Section Name Crashing NSFetchedResultsController
I have a managed object with a dueDate attribute. Instead of displaying using some ugly date string as the section headers of my UITableView I created a transient attribute called "category" and defined it like so:
- (NSString*)category
{
[self willAccessValueForKey:@"category"];
NSString* categoryName;
if ([self isOverdue])
{
categoryName = @"Overdue";
}
else if ([self.finishedDate != nil])
{
categoryName = @"Done";
}
else
{
categoryName = @"In Progress";
}
[self didAccessValueForKey:@"category"];
return categoryName;
}
Here is the NSFetchedResultsController set up:
NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
NSEntityDescription *entity = [NSEntityDescription entityForName:@"Task"
inManagedObjectContext:managedObjectContext];
[fetchRequest setEntity:entity];
NSMutableArray* descriptors = [[NSMutableArray alloc] init];
NSSortDescriptor *dueDateDescriptor = [[NSSortDescriptor alloc] initWithKey:@"dueDate"
ascending:YES];
[descriptors addObject:dueDateDescriptor];
[dueDateDescriptor release];
[fetchRequest setSortDescriptors:descriptors];
fetchedResultsController = [[NSFetchedResultsController alloc] initWithFetchRequest:fetchRequest managedObjectContext:managedObjectContext sectionNameKeyPath:@"category" cacheName:@"Root"];
The table initially displays fine, showing the unfinished items whose dueDate has not passed in a section titled "In Progress". Now, the user can tap a row in the table view which pushes a new details view onto the navigation stack. In this new view the user can tap a button to indicate that the item is now "Done". Here is the handler for the button (self.task is the managed object):
- (void)taskDoneButtonTapped
{
self.task.finishedDate = [NSDate date];
}
As soon as the value of the "finishedDate" attribute changes I'm hit with this exception:
2010-03-18 23:29:52.476 MyApp[1637:207] Serious application error. Exception was caught during Core Data change processing: no section named 'Done' found with userInfo (null)
2010-03-18 23:29:52.477 MyApp[1637:207] *** Terminating app due to uncaught exception 'NSInternalInconsistencyException', reason: 'no section named 'Done' found'
I've managed to figure out that the UITableView that is currently hidden by the new details view is trying to update its rows and sections because the NSFetchedResultsController was notified that something changed in the data set. Here's my table update code (copied from either the Core Data Recipes sample or the CoreBooks sample -- I can't remember which):
- (void)controllerWillChangeContent:(NSFetchedResultsController *)controller
{
[self.tableView beginUpdates];
}
- (void)controller:(NSFetchedResultsController *)controller didChangeObject:(id)anObject atIndexPath:(NSIndexPath *)indexPath forChangeType:(NSFetchedResultsChangeType)type newIndexPath:(NSIndexPath *)newIndexPath
{
switch(type)
{
case NSFetchedResultsChangeInsert:
[self.tableView insertRowsAtIndexPaths:[NSArray arrayWithObject:newIndexPath] withRowAnimation:UITableViewRowAnimationFade];
break;
case NSFetchedResultsChangeDelete:
[self.tableView deleteRowsAtIndexPaths:[NSArray arrayWithObject:indexPath] withRowAnimation:UITableViewRowAnimationFade];
break;
case NSFetchedResultsChangeUpdate:
[self configureCell:[self.tableView cellForRowAtIndexPath:indexPath] atIndexPath:indexPath];
break;
case NSFetchedResultsChangeMove:
[self.tableView deleteRowsAtIndexPaths:[NSArray arrayWithObject:indexPath] withRowAnimation:UITableViewRowAnimationFade];
// Reloading the section inserts a new row and ensures that titles are updated appropriately.
[self.tableView reloadSections:[NSIndexSet indexSetWithIndex:newIndexPath.section] withRowAnimation:UITableViewRowAnimationFade];
break;
}
}
- (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)controllerDidChangeContent:(NSFetchedResultsController *)controller
{
[self.tableView endUpdates];
}
I put breakpoints in each of these functions and found that only controllerWillChange is called. The exception is thrown before either controller:didChangeObject:atIndexPath:forChangeType:newIndex or controller:didChangeSection:atIndex:forChangeType are called.
At this point I'm stuck. If I change my sectionNameKeyPath to just "dueDate" then everything works fine. I think that's because the dueDate attribute never changes whereas the category will be different when read back after the finishedDate attribute changes.
Please help!
UPDATE:
Here is my UITableViewDataSource code:
- (NSInteger)numberOfSectionsInTableView:(UITableView *)tableView
{
return [[self.fetchedResultsController sections] count];
}
- (NSInteger)tableView:(UITableView *)tableView numberOfRowsInSection:(NSInteger)section
{
id <NSFetchedResultsSectionInfo> sectionInfo = [[self.fetchedResultsController sections] objectAtIndex:section];
return [sectionInfo numberOfObjects];
}
- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath
{
static NSString *CellIdentifier = @"Cell";
UITableViewCell *cell = [self.tableView dequeueReusableCellWithIdentifier:CellIdentifier];
if (cell == nil)
{
cell = [[[UITableViewCell alloc] initWithStyle:UITableViewCellStyleDefault reuseIdentifier:CellIdentifier] autorelease];
}
[self configureCell:cell atIndexPath:indexPath];
return cell;
}
- (NSString *)tableView:(UITableView *)tableView titleForHeaderInSection:(NSInteger)section
{
id <NSFetchedResultsSectionInfo> sectionInfo = [[self.fetchedResultsController sections] objectAtIndex:section];
return [sectionInfo name];
}
The crash is being caused by the NSFetchedResultsController not knowing about the "done" category before hand and therefore crashing. I have seen this crash a few other times in other questions and with each one I recommend submitting a radar ticket to Apple. This is a bug in the NSFetchedResultsController .
It looks to me like your problem lies with your "category" transient property that you are using to supply the sectionNameKeyPath. The sectionNameKeyPath must order the same as the primary sort descriptor. In your case, this means that all "Overdue" tasks MUST have dates earlier than all "Done" tasks MUST have dates earlier than all "In Progress" tasks. It is possible to construct a scenario where a "Done" task has a dueDate that comes after an "In Progress" task or comes before an "Overdue" task. This scenario breaks the ordering requirement of the sectionNameKeyPath and causes the NSFetchedResultsController to throw an NSInternalConsistencyException.
I propose a solution to your problem that doesn't involve rolling your own array which then must be split into sections. Create an integer attribute in your model where you map 0 to "Overdue", 1 to "Done", and 2 to "In Progress". Make this the primary sort descriptor in your NSFetchRequest and sort this property in ascending order. Add a secondary sort descriptor to the NSFetchRequest that sorts the dueDate property in ascending order. Modify your category method to derive category names from the integer attribute you created above and use that as your sectionNameKeyPath. You will need to update the integer attribute to update tasks as they move from in progress to overdue to done, etc.
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