CoreData error driving me crazy... CoreData: Serious application error. An exception caught from delegate of NSFetchedResultsController

Question

My application has two tab bars... Each takes the user to a tableviewcontroller that presents him with a list of items. The first view lets the user record entries in the database. The other tab/view reads from the database and presents those items to the user as well, however, no updates are made to the coreData/persistant store from this second view.

When I add a new item via the first viewcontroller, it shows up perfectly in the view. However, as soon as I tap on the other tab bar to see the new item appear in that viewcontroller, I get the error listed below, and the newly added item does not appear... Note: if I stop the app and reload/re-run it, and start by tapping the 2nd tab-bar, the new item shows up fine, so I know the model is being updated fine.

*** Assertion failure in -[UITableView _endCellAnimationsWithContext:], /SourceCache/UIKit_Sim/UIKit-1912.3/UITableView.m:1046
2011-10-20 20:56:15.117 Gtrac[72773:fb03] 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)

Code from the delegate application where the managedObjectContext is passed to the two viewControllers.

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {


    // get a point to the master database context
    NSManagedObjectContext *context = [self managedObjectContext];
    if (!context) {
        // Handle the error.
    }

    // create tab bar controller and array to hold each of the tab-based view controllers that will appear as icons at bottom of screen
    tabBarController = [[UITabBarController alloc] init];
    NSMutableArray *localControllersArray = [[NSMutableArray alloc] initWithCapacity:5];    


    //
    // setup first tab bar item
    //
    //
    // alloc the main view controller - the one that will be the first one shown in the navigation control
    RootViewController *rootViewController = [[RootViewController alloc] initWithTabBar];

    // Pass the managed object context to the view controller.
    rootViewController.managedObjectContext = context;

    // create the navigation control and stuff the rootcontroller inside it
    UINavigationController *aNavigationController = [[UINavigationController alloc] initWithRootViewController:rootViewController];

    // set the master navigation control 
    self.navigationController = aNavigationController;

    // add the navigaton controller as the first tab for the tab bar
    [localControllersArray addObject:aNavigationController];

    [rootViewController release];
    [aNavigationController release];    


    //
    // setup the other tab bar
    //
    //

    // alloc the view controller 
    vcSimulator *vcSimulatorController = [[vcSimulator alloc] initWithTabBar];

    UINavigationController *blocalNavigationController = [[UINavigationController alloc] initWithRootViewController:vcSimulatorController];

    // Pass the managed object context to the view controller.
    vcSimulatorController.managedObjectContext = context;

    // add this controller to the array of controllers we are building
    [localControllersArray addObject:blocalNavigationController];

    // release these guys, they are safely stored in the array - kill these extra references
    [blocalNavigationController release];
    [vcSimulatorController release];


    //
    //
    // ok, all the tab bars are in the array - get crackin
    //
    //
    // load up our tab bar controller with the view controllers
    tabBarController.viewControllers = localControllersArray;

    // release the array because the tab bar controller now has it
    [localControllersArray release];

    [window addSubview:[tabBarController view]];
    [window makeKeyAndVisible];

    return YES;




When I add a new item via the first viewcontroller, it shows up perfectly in the view.  However, as soon as I tap on the other tab bar to see the new item appear in that viewcontroller, I get the error listed above, and the newly added item does not appear...   Note: if I stop the app and reload/re-run it, and start by tapping the 2nd tabbar, the new item shows up fine, so I know the model is being updated fine.

Here are the tableview delegate methods from the 2nd view controller.



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

    [self.tableView beginUpdates];
}


- (void)controller:(NSFetchedResultsController *)controller didChangeObject:(id)anObject atIndexPath:(NSIndexPath *)indexPath forChangeType:(NSFetchedResultsChangeType)type newIndexPath:(NSIndexPath *)newIndexPath {

    NSLog(@">>> Entering %s [Line %d] ", __PRETTY_FUNCTION__, __LINE__);    
    UITableView *tableView = self.tableView;

    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:[tableView cellForRowAtIndexPath:indexPath] 
                    atIndexPath:indexPath];
            break;

        case NSFetchedResultsChangeMove:
            [tableView deleteRowsAtIndexPaths:[NSArray arrayWithObject:indexPath] withRowAnimation:UITableViewRowAnimationFade];
            [tableView insertRowsAtIndexPaths:[NSArray arrayWithObject:newIndexPath] withRowAnimation:UITableViewRowAnimationFade];

            break;
    }

}


- (void)controller:(NSFetchedResultsController *)controller didChangeSection:(id <NSFetchedResultsSectionInfo>)sectionInfo atIndex:(NSUInteger)sectionIndex forChangeType:(NSFetchedResultsChangeType)type {
    NSLog(@">>> Entering %s [Line %d] ", __PRETTY_FUNCTION__, __LINE__);    

    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];

}

Any help you can provide would be very appreciated.

I have searched this site and found many instances of this error but none seem to quite fit. I've also seen references inferring that this error I'm seeing is actually a known bug in the Apple code...

* UPDATED INFORMATION *

I've gone back and set break points in the code and am editing the original question with this additional information. When the user adds a new item to the database he is transitioned from the rootview to the listCourses view. The add transaction works flawlessly and the listCourses View UITableView is updated perfectly.

When I click on the other view that also reads data from the same core data model, it's viewcontroller runs through the following sequence but never ends adding the new item to the tableview. Here is the sequence it goes through.

Simulator VC:

- controllerWillChangeContent which runs...
        [self.tableView beginUpdates];

    - didChangeObject 
        ..with message: NSFetchedResultsChangeUpdate
        ..which ran:
        [self configureCell:[tableView cellForRowAtIndexPath:indexPath] 

    - controllerDidChangeContent:   
        [self.tableView endUpdates];

The other viewcontroller that works great, goes through this sequence immediately after the record is added to the database.

ListCourses VC:

- didChangeSection
    ...with message:  NSFetchedResultsChangeInsert
...which ran:
    [self.tableView insertSections:[NSIndexSet indexSetWithIndex:sectionIndex] withRowAnimation:UITableViewRowAnimationFade];

- didChangeObject
    ..with message: NSFetchedResultsChangeInsert
    ..which ran:
    [tableView insertRowsAtIndexPaths:[NSArray arrayWithObject:newIndexPath] withRowAnimation:UITableViewRowAnimationFade];

Why does the one viewcontroller get the NSFetchedResultsChangeInsert message but the other one does not?

Here are the delegate methods from the faulty viewcontroller.

// Override to support editing the table view.
- (void)tableView:(UITableView *)tableView commitEditingStyle:(UITableViewCellEditingStyle)editingStyle forRowAtIndexPath:(NSIndexPath *)indexPath
{
    if (editingStyle == UITableViewCellEditingStyleDelete) {
        // Delete the row from the data source
        [tableView deleteRowsAtIndexPaths:[NSArray arrayWithObject:indexPath] withRowAnimation:UITableViewRowAnimationFade];
    }   
    else if (editingStyle == UITableViewCellEditingStyleInsert) {
        // Create a new instance of the appropriate class, insert it into the array, and add a new row to the table view
    }   
}





- (void)controllerWillChangeContent:(NSFetchedResultsController *)controller {
    // The fetch controller is about to start sending change notifications, so prepare the table view for updates.
    NSLog(@">>> Entering %s [Line %d] ", __PRETTY_FUNCTION__, __LINE__);    

    [self.tableView beginUpdates];
}


- (void)controller:(NSFetchedResultsController *)controller didChangeObject:(id)anObject atIndexPath:(NSIndexPath *)indexPath forChangeType:(NSFetchedResultsChangeType)type newIndexPath:(NSIndexPath *)newIndexPath {
    NSLog(@">>> Entering %s [Line %d] ", __PRETTY_FUNCTION__, __LINE__);    


    UITableView *tableView = self.tableView;

    switch(type) {

        case NSFetchedResultsChangeInsert:
            [tableView insertRowsAtIndexPaths:[NSArray arrayWithObject:newIndexPath] withRowAnimation:UITableViewRowAnimationFade];
            break;

        case NSFetchedResultsChangeDelete:
            [tableView deleteRowsAtIndexPaths:[NSArray arrayWithObject:indexPath] withRowAnimation:UITableViewRowAnimationFade];
            break;

        case NSFetchedResultsChangeUpdate:

            //[tableView insertRowsAtIndexPaths:[NSArray arrayWithObject:newIndexPath] withRowAnimation:UITableViewRowAnimationFade];
            [self configureCell:[tableView cellForRowAtIndexPath:indexPath] 
                    atIndexPath:indexPath];
            //[tableView reloadData];


            break;

        case NSFetchedResultsChangeMove:
            [tableView deleteRowsAtIndexPaths:[NSArray arrayWithObject:indexPath] withRowAnimation:UITableViewRowAnimationFade];
            [tableView insertRowsAtIndexPaths:[NSArray arrayWithObject:newIndexPath] withRowAnimation:UITableViewRowAnimationFade];

            // Reloading the section inserts a new row and ensures that titles are updated appropriately.
            // [tableView reloadSections:[NSIndexSet indexSetWithIndex:newIndexPath.section] withRowAnimation:UITableViewRowAnimationFade];

            break;
    }
    NSLog(@"vc>>>  about to reload data");
    //  [self.tableView reloadData];

}


- (void)controller:(NSFetchedResultsController *)controller didChangeSection:(id <NSFetchedResultsSectionInfo>)sectionInfo atIndex:(NSUInteger)sectionIndex forChangeType:(NSFetchedResultsChangeType)type {
    NSLog(@">>> Entering %s [Line %d] ", __PRETTY_FUNCTION__, __LINE__);    

    switch(type) {

        case NSFetchedResultsChangeInsert:
            [self.tableView insertSections:[NSIndexSet indexSetWithIndex:sectionIndex] withRowAnimation:UITableViewRowAnimationFade];
            break;

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

}


- (void)controllerDidChangeContent:(NSFetchedResultsController *)controller {
    // The fetch controller has sent all current change notifications, so tell the table view to process all updates.
    NSLog(@">>> Entering %s [Line %d] ", __PRETTY_FUNCTION__, __LINE__);    
    [self.tableView endUpdates];

}

Thanks, phil

Answer 1

The UITableView sanity checking works like this:

At the line [self.tableView beginUpdates]; the tableView calls your tableView:numberOfRowsInSection: delegate method, which appears to be returning 3. At the line [self.tableView endUpdates]; it calls it again and it seems to be returning 4. Therefore, the tableView is expecting you to insert 1 row between these two lines. In fact, no rows are being inserted so the tableView fails an assertion. (You can see the expected and actual row counts in the assertion message).

The increase from 3 rows to 4 rows shows that your NSFetchedResultsController is noticing the newly inserted Core Data item correctly. What you need to do is put a breakpoint at the start of your controller:didChangeObject:atIndexPath:forChangeType: method and step through it when you switch to the 2nd tab after inserting an item. You should see the NSFetchedResultsChangeInsert: case of the switch statement being performed but this is obviously not happening.

Hopefully, you can figure out why the insertion is not taking place - otherwise come back and let us know what you actually saw when stepping through this method.

EDITED TO ADD:

OK, so your NSFetchedResultsController delegate methods in the 2nd view controller are called when you switch to that tab rather than immediately when the new item is inserted on tab 1. This means that the 2nd view controller is not seeing the insert (which should happen immediately) and is actually responding to some other Core Data update notification later that occurs when you switch to tab 2. The fetched results controller is working with stale information at the beginUpdates line (there are actually 4 items in the result set here not 3). By the time it gets to the endUpdates line it has refreshed its fetch and found an unexpected insert.

The NSFetchedResultsController delegate methods are really designed to update UI in-place while you are making changes AND the controller's view is visible. In your case, you are making changes and THEN displaying the new view controller. The pattern that you really should be using is to refresh the tableview in your viewWillAppear method of controller 2. Something like this should do it:

- (void)viewWillAppear:(BOOL)animated
{
    [super viewWillAppear:animated];

    NSError *error = nil;
    [resultsController performFetch:&error]; // Refetch data
    if (error != nil) {
        // handle error
    }

    [self.tableView reloadData];
}

This will make sure that whenever you switch to tab 2 it is working with fresh data from the model.