CoreData: merging changes from background context.

gistfile1.m

// getting this error?
// "An observer of NSManagedObjectContextDidSaveNotification illegally threw an exception"

// observing the notification here 
[[NSNotificationCenter defaultCenter] addObserver:self 
                                        selector:@selector(otherContextDidSave:) 
                                            name:NSManagedObjectContextDidSaveNotification object:nil];


// FIX: you must mergeChangesFromContextDidSaveNotification: on main thread
// handling the notification occurs on background thread. 
// it needs to "merge" on the main thread.
- (void)otherContextDidSave:(NSNotification *)didSaveNotification {
    NSManagedObjectContext *context = (NSManagedObjectContext *)didSaveNotification.object;
    
    if( context.persistentStoreCoordinator == globalContext.persistentStoreCoordinator ) 
        [globalContext performSelectorOnMainThread:@selector(mergeChangesFromContextDidSaveNotification:) 
                                        withObject:didSaveNotification waitUntilDone:NO];
}