Core data saving in thread - iphone

Question

I'm developing a client server application in iPad. I need to save quite a number of data the server sends me. it sends me a long string, and i have to break it up into small records and save it in core data. it sends me a total of probably 20 messages and each messages has roughly 100 over records.
The problem now is the user has to wait for all the messages to be saved into the core data, before the UI unfreezes, as its all running in the main thread.
Question, Can i receive the message from server, and throw the breaking up of data and saving into core data into threads? i keep getting the sigbart error when the context is save. I checked the core data, it saves about 4 records before hitting that error.
Can multiple threads access / save into core data at the same time?
Sorry i'm really lost. tried the open source Magical Records but it keep having errors.

Answer 1

A Core Data managed object context is not thread safe. While you can have a single Core Data store, you need to create a separate managed object context for each thread. If you need to pass references to managed objects between threads, you need to pass the object ID and then read the object from the local managed object context, rather than trying to pass the object itself.
Doing this will enable you to save in the background using Core Data.
Beware when saving on background threads however that the app could exit before the background thread is finished saving. See this discussion.

Answer 2

Since Core Data requires one Managed Object Context per thread, a possible solution would be to track a context per Thread in a global manager, then track save notifications and propagate to all Threads:
Assuming:
#property (nonatomic, strong) NSDictionary* threadsDictionary;
Here is how to get the managed object (per thread):
- (NSManagedObjectContext *) managedObjectContextForThread {
// Per thread, give one back
NSString* threadName = [NSString stringWithFormat:#"%d",[NSThread currentThread].hash];
NSManagedObjectContext * existingContext = [self.threadsDictionary objectForKey:threadName];
if (existingContext==nil){
existingContext = [[NSManagedObjectContext alloc] init];
[existingContext setPersistentStoreCoordinator: [self persistentStoreCoordinator]];
[self.threadsDictionary setValue:existingContext forKey:threadName];
}
return existingContext;
}
At some point in the init method of your global manager (I used a singleton):
[[NSNotificationCenter defaultCenter] addObserver:self selector:#selector(backgroundContextDidSave:) name:NSManagedObjectContextDidSaveNotification object:nil];
Then to receive save notifications and propagate to all other managed context objects:
- (void)backgroundContextDidSave:(NSNotification *)notification {
/* Make sure we're on the main thread when updating the main context */
if (![NSThread isMainThread]) {
[self performSelectorOnMainThread:#selector(backgroundContextDidSave:)
withObject:notification
waitUntilDone:NO];
return;
}
/* merge in the changes to the main context */
for (NSManagedObjectContext* context in [self.threadsDictionary allValues]){
[context mergeChangesFromContextDidSaveNotification:notification];
}
}
(some other methods were removed for clarity)