Bad Access on Core Data deleteObject

Question

I could use some assistance in debugging a EXC_BAD_ACCESS error received on the [context deleteObject:loan]; command. The error is received in the following delegate method:

- (void)didCancelNewLoan:(Loan *)loan {
    // save the context
    NSManagedObjectContext *context = [self.fetchedResultsController managedObjectContext];

    [context deleteObject:loan];  // *** EXC_BAD_ACCESS here ***

// This method is called from a the following method in a second class:

- (IBAction)cancel:(id)sender {
    [delegate didCancelNewLoan:self.loan];
}

// The loan ivar is created by the original class
// in the below prepare for Segue method:

- (void)prepareForSegue:(UIStoryboardSegue *)segue sender:(id)sender {

    if ([segue.identifier isEqualToString:@"NewLoan"]) {
        UINavigationController *navController = (UINavigationController *)[segue destinationViewController];
        LoanViewController *loanView = (LoanViewController *)[[navController viewControllers] lastObject];
        loanView.managedObjectContext = self.managedObjectContext;
        loanView.delegate = self;

        loanView.loan = [self createNewLoan];
        loanView.newLoan = YES;
    }


// Finally, the loan is created in the above
// method's [self createNewLoan] command:

- (NSManagedObject *)createNewLoan {
    //create a new instance of the entity managed by the fetched results controller
    NSManagedObjectContext *context = [self.fetchedResultsController managedObjectContext];
    NSEntityDescription *entity = [[self.fetchedResultsController fetchRequest] entity];
    NSManagedObject *newManagedObject = [NSEntityDescription insertNewObjectForEntityForName:[entity name] inManagedObjectContext:context];
    [newManagedObject setValue:[NSDate date] forKey:@"timeStamp"];

    CFUUIDRef uuid = CFUUIDCreate(NULL);
    CFStringRef uuidstring = CFUUIDCreateString(NULL, uuid);
    //NSString *identifierValue = (__bridge_transfer NSString *)uuidstring;
    [newManagedObject setValue:(__bridge_transfer NSString *)uuidstring forKey:@"identifier"];
    CFRelease(uuid);
    CFRelease(uuidstring);

    NSError *error;
    [self.fetchedResultsController performFetch:&error];
    NSLog(@"%i items in database", [[self.fetchedResultsController fetchedObjects] count]);

    return newManagedObject;
}

Appreciate your looking at the above methods.

Answer 1

Guess #1: you are accessing a deallocated object. To debug: turn on zombies and see what happens.

Update: here's how you turn on zombies in Xcode 5:

Product > Scheme > Edit Scheme, select Diagnostics tab, check "Enable Zombie Objects"

for older Xcode

, edit your build settings, add and enable these arguments in your build scheme:

---

Guess #2: you have a multithreaded app and you are accessing a managed object context from different threads, which is a no no.

You can add an assert before your delete:

assert( [ NSThread isMainThread ] ) ;

Answer 2

I got a Bad Access because a deallocated UIViewController was a delegate of a NSFetchedResultsController it had.

The NSFetchedResultsController was deallocated - but when settings a delegate, it observes NSManagedObjectContext for changes, so when NSManagedObjectContext was saved - a bad access would occur when trying to notify the NSFetchedResultsController about the change.

Solution is to clear delegate of NSFetchedResultsController upon deallocation.

- (void)dealloc {
    fetchedResultsController.delegate = nil;
}

Answer 3

From looking at your code above, there's nothing that stands out as being done incorrectly.

I am wondering whether you are dealing with two different managed object contexts without realising it? You will have to set some breakpoints where you create the Loan object and see if that might be the case.

Also why do you have to get a reference to the context via fetchedResultsController if you already have a declared property for it in self.managedObjectContext ?

The other thing is why do you need to call the fetchedResultsController to performFetch: again when you create a new Loan object? Is your data presented in a table view and have you implemented the NSFetchedResultsController delegate methods?

That call seems unnecessary and it may be causing issues with the cache created by the fetch. See section "Modifying the fetch request" under this link http://developer.apple.com/library/ios/documentation/CoreData/Reference/NSFetchedResultsController_Class/Reference/Reference.html#//apple_ref/doc/uid/TP40008227-CH1-SW24

Finally, try your delete operation directly in the view controller that received the action rather than pass it to the delegate (just to eliminate the possibility that something has been dealloc'd without you knowing).

Here's what I'd do:

- (IBAction)cancel:(id)sender 
{
    NSError *error;
    NSManagedObjectContext *context = [self.loan managedObjectContext];
    [context deleteObject:self.loan];
    if (![context save:&error])
         NSLog (@"Error saving context: %@", error);
}