How to use ManagedObjectContext with threads - objective-c

Question

This is probably a very straight forward application, but I am new to Objective-C (coming from Java) and the whole memory management and "EXC_BAD_ACCESS" errors are breaking my heart.
I have a normal NavigationController iPhone App, with Core Data. in the AppDelegate the NSManagedObjectContext is created and passed to the RootViewController. A view things are looked up directly from the main thread to populate the table, and that seems to work fine.
The App is somekind of RSS-type reader, so as soon as the App starts I fire a thread to fetch new data and update the view:
-(void)updateData:(id)sender {
UIActivityIndicatorView *activityIndicator =
[[UIActivityIndicatorView alloc] initWithFrame:CGRectMake(0, 0, 20, 20)];
[activityIndicator startAnimating];
UIBarButtonItem *activityItem =
[[UIBarButtonItem alloc] initWithCustomView:activityIndicator];
[activityIndicator release];
self.navigationItem.leftBarButtonItem = activityItem;
[activityItem release];
// Start thread to update the data
[NSThread detachNewThreadSelector:#selector(doUpdateData) toTarget:self withObject:nil];
}
-(void)doUpdateData{
NSLog(#"Update data Thread (in 5 sec.)");
[NSThread sleepForTimeInterval:5];
NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];
DataManager *data = [[DataManager alloc] initWithContext:managedObjectContext];
[data updateData];
[data release];
data=nil;
[self performSelectorOnMainThread:#selector(finishUpdateData) withObject:nil waitUntilDone:NO];
[pool release];
}
-(void)finishUpdateData{
self.navigationItem.leftBarButtonItem = updateBttn;
DataManager *data = [[DataManager alloc] initWithContext:managedObjectContext];
objects = [data getArticles];
[data release];
data=nil;
NSLog(#"Amount of records after update: %d", [objects count]);
[self.tableView reloadData];
}
The problem is that this doesn't work. In the DataManager, first settings need to be retrieved, and as soon as the NSEntityDescription is created I get the "EXC_BAD_ACCESS":
- (NSFetchedResultsController *)fetchedResultsController {
// Set up the fetched results controller if needed.
if (fetchedResultsController == nil) {
NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
NSEntityDescription *entity = [NSEntityDescription entityForName:#"Setting" inManagedObjectContext:managedObjectContext];
[fetchRequest setEntity:entity];
NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc] initWithKey:#"key" ascending:YES];
NSArray *sortDescriptors = [[NSArray alloc] initWithObjects:sortDescriptor, nil];
[fetchRequest setSortDescriptors:sortDescriptors];
[fetchRequest setFetchLimit:1];
.
NSFetchedResultsController *aFetchedResultsController = [[NSFetchedResultsController alloc] initWithFetchRequest:fetchRequest managedObjectContext:managedObjectContext sectionNameKeyPath:nil cacheName:nil];
aFetchedResultsController.delegate = self;
self.fetchedResultsController = aFetchedResultsController;
[aFetchedResultsController release];
[fetchRequest release];
[sortDescriptor release];
[sortDescriptors release];
}
return fetchedResultsController;
}
I guess the pointer to the ManagedObjectContext is wrong, as a result from running in a different thread and memory-pool. So how do you create such an application if that is the issue), how do I get a reference to the original ManagedObjectContext format he thread?
[EDIT]
I also tried to use
iDomsAppDelegate *appDelegate = (iDomsAppDelegate *)[[UIApplication sharedApplication] delegate];
DataManager *data = [[DataManager alloc] initWithContext:appDelegate.managedObjectContext];
in doUpdateData (as hinted by other posts), but that gives the same result

Answer 1

Managed Object Contexts are not thread safe. Apple's guidelines indicate that you must have a separate instance of NSManagedObjectContext per thread.