Multiple NSManagedObjectContexts issue - ios

Question

I'm having an issue implementing a parent/child relationship between two NSManagedObjectContext's. My app imports a lot of data from a web service that was causing UI lag while saving the context. So in my AppDelegate I created a parent context (master) with NSPrivateQueueConcurrencyType:
- (NSManagedObjectContext *)masterMOC
{
if (_masterMOC != nil) {
return _masterMOC;
}
NSPersistentStoreCoordinator *coordinator = [self persistentStoreCoordinator];
if (coordinator != nil) {
_masterMOC = [[NSManagedObjectContext alloc] initWithConcurrencyType:NSPrivateQueueConcurrencyType];
[_masterMOC setPersistentStoreCoordinator:coordinator];
}
return _masterMOC;
}
...and a child context (main) with NSMainQueueConcurrencyType. I set the main MOC's parentContext to masterMOC:
- (NSManagedObjectContext *)mainMOC
{
if (_mainMOC != nil) {
return _mainMOC;
}
_mainMOC = [[NSManagedObjectContext alloc] initWithConcurrencyType:NSMainQueueConcurrencyType];
[_mainMOC setUndoManager:nil];
[_mainMOC setParentContext:[self masterMOC]];
return _mainMOC;
}
In my applicationDidFinishLaunching I kick off an import operation that queries the web service and saves the results on the master (PrivateQueue) context. I also register for the NSManagedObjectContextDidSaveNotification and attempt to merge those changes into the child mainMOC.
- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
NSNotificationCenter *notificationCenter = [NSNotificationCenter defaultCenter];
[notificationCenter addObserver:self selector:#selector(contextChanged:) name:NSManagedObjectContextDidSaveNotification object:self.masterMOC];
RequestHandler *handler = [[RequestHandler alloc] initWithManagedObjectContext:self.masterMOC];
[handler importAllViews];
...
return YES;
}
- (void) contextChanged: (NSNotification *) notification
{
// Only interested in merging from master into main.
if ([notification object] != self.masterMOC) return;
[self.mainMOC performBlock:^{
[self.mainMOC mergeChangesFromContextDidSaveNotification:notification];
}];
}
The RequestHandler class has a saveContext method that saves the master context on the correct thread:
#implementation KDBRequestHandler
...
- (void) saveContext
{
[self.managedObjectContext performBlock:^{
NSError *error;
if (![self.managedObjectContext save:&error]) {
[NSException raise:#"Unable to save build details." format:#"Error saving context: %#", error];
}
}];
}
...
#end
I've verified the import to be correctly saving its objects on a background thread with Instruments. My problem lies with the child managed object context that has NSMainQueueConcurrencyType. After the import is kicked off, application didFinishLaunching initializes the UI as is standard. The view controllers are assigned the mainMOC.
- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
// Override point for customization after application launch.
if ([[UIDevice currentDevice] userInterfaceIdiom] == UIUserInterfaceIdiomPad) {
UISplitViewController *splitViewController = (UISplitViewController *)self.window.rootViewController;
UINavigationController *navigationController = [splitViewController.viewControllers lastObject];
splitViewController.delegate = (id)navigationController.topViewController;
UINavigationController *masterNavigationController = splitViewController.viewControllers[0];
KDBMasterViewController *controller = (KDBMasterViewController *)masterNavigationController.topViewController;
controller.managedObjectContext = self.mainMOC;
} else {
UINavigationController *navigationController = (UINavigationController *)self.window.rootViewController;
KDBMasterViewController *controller = (KDBMasterViewController *)navigationController.topViewController;
controller.managedObjectContext = self.mainMOC;
}
return YES;
}
The MasterViewController is essentially the boilerplate controller created for you when creating a core data project. It's fetchedResultsController ends up performing its work on the mainMOC therefore the main thread.
- (NSFetchedResultsController *)fetchedResultsController
{
if (_fetchedResultsController != nil) {
return _fetchedResultsController;
}
NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
// Edit the entity name as appropriate.
NSEntityDescription *entity = [NSEntityDescription entityForName:#"Job" inManagedObjectContext:self.managedObjectContext];
[fetchRequest setEntity:entity];
// Set the batch size to a suitable number.
[fetchRequest setFetchBatchSize:20];
// Edit the sort key as appropriate.
NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc] initWithKey:#"name" ascending:NO];
NSArray *sortDescriptors = #[sortDescriptor];
[fetchRequest setSortDescriptors:sortDescriptors];
// Edit the section name key path and cache name if appropriate.
// nil for section name key path means "no sections".
NSFetchedResultsController *aFetchedResultsController = [[NSFetchedResultsController alloc] initWithFetchRequest:fetchRequest managedObjectContext:self.managedObjectContext sectionNameKeyPath:nil cacheName:#"Master"];
aFetchedResultsController.delegate = self;
self.fetchedResultsController = aFetchedResultsController;
NSError *error = nil;
if (![self.fetchedResultsController performFetch:&error]) {
// Replace this implementation with code to handle the error appropriately.
// abort() causes the application to generate a crash log and terminate. You should not use this function in a shipping application, although it may be useful during development.
NSLog(#"Unresolved error %#, %#", error, [error userInfo]);
abort();
}
return _fetchedResultsController;
}
On initial load when the database is fresh and empty everything works as expected. The data is downloaded in the background and populated in the MasterViewController's UITableView as expected. However on subsequent runs, the app often crashes in the MasterViewController's fetchedResultsController. The fetch fails with this error:
*** Terminating app due to uncaught exception 'NSInternalInconsistencyException', reason: '+entityForName: could not locate an NSManagedObjectModel for entity name 'Job''
*** First throw call stack:
(
0 CoreFoundation 0x01c6a1e4 __exceptionPreprocess + 180
1 libobjc.A.dylib 0x019678e5 objc_exception_throw + 44
2 CoreData 0x002c6a1b +[NSEntityDescription entityForName:inManagedObjectContext:] + 251
3 JMobile 0x00032ad4 -[KDBMasterViewController fetchedResultsController] + 340
4 JMobile 0x00031d6e -[KDBMasterViewController numberOfSectionsInTableView:] + 78
5 UIKit 0x0088e712 -[UITableViewRowData(UITableViewRowDataPrivate) _updateNumSections] + 102
6 UIKit 0x0088f513 -[UITableViewRowData invalidateAllSections] + 69
7 UIKit 0x006fa6ea -[UITableView _updateRowData] + 197
....
Since the crash does not happen 100% of the time I suspected a concurrency problem. I confirmed by noting that the mainMOC's parent context was nil at the time the fetchedResultsController was querying. How can I ensure the child context (mainMOC) is correctly set with a parent before attempting queries on it?

Answer 1

There is a race condition accessing your mainMOC:
Adding the line: [self mainMOC]; after you add the observer to the save of your masterMOC and before you start your import, will work around the MOC initialisation race.
You can read THIS for a discussion on race conditions in a similar case (regards which thread access your contexts in parallel).
The template code offered by Apple is good where you have only one thread initialising your contexts (the ones that are not temporary), otherwise, you should synchronise the initialisation using some sort of thread locking mechanism.