NSManagedObjectContext working in background - objective-c

Question

I have problem with merging changes between two context. One of them is working in background(additional context) as WebServices reading and writing data to database. While I was trying save any changes in main context, application gone stuck and nothing happend. Context from web services(singleton) I create as I showed below. When I try save main context I send notification to merge changes with WebServices context but it does not work correctly. Whats wrong? It happend when background is reading data and main context try to save something i database.
managedObjectContext = [NSManagedObjectContext new];
NSPersistentStoreCoordinator *store = [ [theDelegate managedObjectContext] persistentStoreCoordinator];
[managedObjectContext setPersistentStoreCoordinator:store];
[managedObjectContext setStalenessInterval:0.0];
Before save of Main Context I Use:
[[NSNotificationCenter defaultCenter] addObserver:self selector:#selector(mergeChanges:) name:NSManagedObjectContextDidSaveNotification object:self.managedObjectContext];
where I merge changes
[[[WebServie instance] managedObjectContext] mergeChangesFromContextDidSaveNotification:notification]
It should working but I have no idea whats going wrong, WebServices are working on background, and DataBaseManager is in main thread
Thats code of my merge context is it wrong?
NSNotificationCenter *dnc = [NSNotificationCenter defaultCenter];
[dnc addObserverForName:NSManagedObjectContextDidSaveNotification
object: self.managedObjectContext queue:nil
usingBlock:^(NSNotification *notification)
{
NSLog(#"merge");
[[[WebServiceManager instance] managedObjectContext] mergeChangesFromContextDidSaveNotification:notification];
}];
NSError *error;
NSLog(#"error");
if (![self.managedObjectContext save:&error])
{
NSLog(#"error :%#", error);// Update to handle any error appropriately.
}
NSLog(#"after error");
[dnc removeObserver:self
name:NSManagedObjectContextDidSaveNotification
object:self.managedObjectContext];

Answer 1

You need to do the merging on the queue of the context that's supposed to do the merge. Not on the queue (or thread) that it's being sent on.

Answer 2

If you are trying to keep two context synchronized, you need to listen to the NSManagedObjectContextDidSaveNotification for both context, and merge the notification of the first context to the second and viceversa. So, let's say you have your context on your applicationDelegate, and you have a context on your web singleton. You would create a method on your applicationDelegate to be fired when the singleton context saves. On that method, you would take the notification from the singleton object and merge it with the main thread context. On your singleton, you would listen to the NSManagedObjectContextDidSaveNotification from the main thread and merge it with the singleton's context.