Question

My iOS app hangs on savaging when i segue between 2 screens a couple of times.
it belongs to store the data but than this just freeze with no errors.
Somebody any idea how i can fix this ?

self.fetchF = dispatch_queue_create(label, NULL);
dispatch_async(self.fetchF, ^{

    NSArray *feeds = [FeedFetcher getData:self.pageTitle downloadBy:@"up"];

    NSManagedObjectContext *newContext = self.managedObject;

    for (NSDictionary *feedInfo in feeds) {
        [Feed FeedWithInfo:feedInfo InManageObject:newContext];
    }

    dispatch_async(dispatch_get_main_queue(), ^{

        NSError *error = nil;
        if (newContext != self.managedObject)
            [newContext save:&error];

        if (error)
            NSLog(@"Error save : %@", error);

        [self setupFetchedResultsController];

        [self downloadImages:feeds];
    });        
});

Edit :

I change my managedobjectcontext so it has every thread a new one.
But now when i segue between 2 screens a couple of times it freeze on [self.fetchedResultsController performFetch:&error]; With no errors..
Does somebody a idea for a solution ?

- (void)performFetch
{
    if (self.fetchedResultsController) {
        if (self.fetchedResultsController.fetchRequest.predicate) {
            if (self.debug) NSLog(@"[%@ %@] fetching %@ with predicate: %@", NSStringFromClass([self class]), NSStringFromSelector(_cmd), self.fetchedResultsController.fetchRequest.entityName, self.fetchedResultsController.fetchRequest.predicate);
        } else {
            if (self.debug) NSLog(@"[%@ %@] fetching all %@ (i.e., no predicate)", NSStringFromClass([self class]), NSStringFromSelector(_cmd), self.fetchedResultsController.fetchRequest.entityName);
        }
        NSError *error;
        if (self.fetchedResultsController != nil)
            [self.fetchedResultsController performFetch:&error];

        if (error) NSLog(@"[%@ %@] %@ (%@)", NSStringFromClass([self class]), NSStringFromSelector(_cmd), [error localizedDescription], [error localizedFailureReason]);
    } else {
        if (self.debug) NSLog(@"[%@ %@] no NSFetchedResultsController (yet?)", NSStringFromClass([self class]), NSStringFromSelector(_cmd));
    }
    [self.tableView reloadData];
}

Answer 1

Core Data is generally not thread-safe. Rule of thumb is to create a NSManagedObjectContext per thread. I believe you’re reusing your context among threads and that’s causing the problem.

Docs on Core Data concurrency:

Core Data uses thread (or serialized queue) confinement to protect
managed objects and managed object contexts (see “Concurrency with
Core Data”). A consequence of this is that a context assumes the
default owner is the thread or queue that allocated it—this is
determined by the thread that calls its init method. You should not,
therefore, initialize a context on one thread then pass it to a
different thread. Instead, you should pass a reference to a persistent
store coordinator and have the receiving thread/queue create a new
context derived from that. If you use NSOperation, you must create the
context in main (for a serial queue) or start (for a concurrent
queue).