NSFetchedResultsController: Moved Objects Sometimes Reported as Updated

In some situations, an instance of NSFetchedResultsController may report moved objects using a NSFetchedResultsChangeUpdate change notification instead of NSFetchedResultsChangeMove.

A fetched results controller only sends object change notifications tagged as NSFetchedResultsChangeMove when the original index path is different from the new index path. It's possible that a series of object changes can result in an object being moved (such as moving to a new section), yet its relative index path remain the same (if other object changes happened in objects that appear before the moved object). This scenario will result in a NSFetchedResultsChangeUpdate notification being sent to the delegate.

A possible workaround is to maintain an extra (non modeled) instance variable in your object that indicates when an object has changed section (due to a change in its property that determines the section). For example, you might declare a custom class as follows: 

@interface MyClass : NSManagedObject
 
{
 
    BOOL _changedSection;
 
}
 
 
// The modeled property that determines the object's section.
 
@property (nonatomic, retain) NSString *theSectionKey;
 
 
// The unmodeled property that tracks if a section was recently changed.
 
@property BOOL changedSection;
 
 
@end

The corresponding implementation might be: 

@synthesize changedSection=_ changedSection;
 
 
- (void)setTheSectionKey:(id)value
 
{
 
    if (value != theSectionKey) {
 
        changedSection = YES;
 
    }
 
 
    [self willChangeValueForKey:@"theSectionKey"];
 
    [self setPrimitiveTheSectionKey:value];
 
    [self didChangeValueForKey:@"theSectionKey"];
 
}

You can then use this property to determine if a change flagged as NSFetchedResultsChangeUpdate should actually be treated as a NSFetchedResultsChangeMove by implementing controller:didChangeObject:atIndexPath:forChangeType:newIndexPath: as illustrated in this example: 

- (void)controller:(NSFetchedResultsController *)controller didChangeObject:(id)anObject atIndexPath:(NSIndexPath *)indexPath forChangeType:(NSFetchedResultsChangeType)type newIndexPath:(NSIndexPath *)newIndexPath {
 
 
    UITableView *tableView = self.tableView;
 
 
    // This is the workaround.
 
    MyClass *myInstance = (MyClass *)anObject;
 
    if ( (NSFetchedResultsChangeUpdate == type) && ([myInstance changedSection]) ) {
 
        [myInstance  setChangedSection:NO];
 
        type = NSFetchedResultsChangeMove;
 
        newIndexPath = indexPath;
 
    }
 
    switch(type) {
 
        case NSFetchedResultsChangeInsert:
 
            [tableView insertRowsAtIndexPaths:[NSArray arrayWithObject:newIndexPath] withRowAnimation:UITableViewRowAnimationFade];
 
            break;
 
        case NSFetchedResultsChangeDelete:
 
            [tableView deleteRowsAtIndexPaths:[NSArray arrayWithObject:indexPath] withRowAnimation:UITableViewRowAnimationFade];
 
            break;
 
        case NSFetchedResultsChangeUpdate:
 
            [self configureCell:(ExpenseCell *)[self.tableView cellForRowAtIndexPath:indexPath] atIndexPath:indexPath];
 
            break;
 
        case NSFetchedResultsChangeMove:
 
            [tableView deleteRowsAtIndexPaths:[NSArray arrayWithObject:indexPath] withRowAnimation:UITableViewRowAnimationFade];
 
            [tableView insertRowsAtIndexPaths:[NSArray arrayWithObject:newIndexPath] withRowAnimation:UITableViewRowAnimationFade];
 
            break;
 
    }
 
}