ios - NSFetchedResultsController: Multiple FRCs, Delegate Error when Updating

Question

Objective: Using FRC, sort Section's by startDate, an NSDate attribute, but want Today's date Section to appear before Upcoming dates Section.

I followed Apple's code using a transient property sectionIdentifier. Apple's sample code. and started with this project first: OneFRC

I soon realized that this may not be possible with just one FRC (I could be wrong).

Next, I decided to take a stab at this with 3 FRCs: ThreeFRC.

TableView sections now appears in the Order that I want:

Section 0: Today

Section 1: Upcoming

Section 2: Past

However, adding data triggers FRC delegates, and I get the following error:

CoreData: error: Serious application error.  An exception was caught from the 
delegate of NSFetchedResultsController during a call to 
-controllerDidChangeContent:.  Invalid update: invalid number of rows in section 0.
The number of rows contained in an existing section after the update (4) must be 
equal to the number of rows contained in that section before the update (3), plus 
or minus the number of rows inserted or deleted from that section (0 inserted, 
0 deleted) and plus or minus the number of rows moved into or out of that section 
(0 moved in, 0 moved out). with userInfo (null)

Again, I would love to be able to accomplish my objective with 1 FRC, but I can't seem to figure out how.

I have been trying to resolve this for 4 days now! If this issue doesn't get resolved on SO, I think I may reach out to Apple for Developer support. And in the event that I do, I'll post the resolution here so others can benefit.

Projects are available on Github:

One FRC

Three FRC

EDIT

Thanks to @blazejmar, I was able get rid of the rows error. However, now I get an error when I attempt to add sections.

2014-11-03 16:39:46.852 FRC[64305:60b] CoreData: error: Serious application error.  
An exception was caught from the delegate of NSFetchedResultsController during a 
call to -controllerDidChangeContent:.  Invalid update: invalid number of sections.  
The number of sections contained in the table view after the update (2) must be 
equal to the number of sections contained in the table view before the update (1), 
plus or minus the number of sections inserted or deleted (0 inserted, 0 deleted). 
with userInfo (null)

Steps to reproduce the error in Three FRC:

1. Launch App -> 
2. Tap Generate Data -> 
3. Tap View in FRC -> 
4. Tap back to the RootVC ->
5. Change the system date to a month from Today -> 
6. Tap View in FRC and only one section `Past` should appear. -> 
7. Tap `Add Data`.  
8. The error should appear in the log. 

Answer 1

In your ThreeFRC project there are some issues:

- (void)controllerWillChangeContent:(NSFetchedResultsController *)controller {
    [self.tableView beginUpdates];
    self.numberOfSectionsInTV = 0;
    [self fetchData];
}

- (void)controllerDidChangeContent:(NSFetchedResultsController *)controller {
    [self.tableView reloadData];
    [self.tableView endUpdates];
}

You shouldn't use fetchData inside FRC delegate. Methods are called in proper order (before, during and after update) so inside callbacks you have consistent state of context. Also it's not the best idea to use reloadData before endUpdates(it's applying all changes you provided earlier) and reloadData is erasing everything and building it from scratch. This is most likely causing the crash.

Other thing I've spotted that may be buggy is handling of updates. If you have 3 separate FRC without sections you won't get section update callback in FRC delegate. But if some objects appear in one of the FRC's then you should detect that and manually insert them.

Using just reloadData in controllerDidChangeContent would be enough, but this isn't the best solution, as you won't get any animations. The proper way would be to handle all the cases: deleting all objects from one of FRCs (and then deleting section manually from TableView), inserting first object into FRC (then you should create new section at proper indexPath).