Core Data with multiple managed object contexts Photo by Free-Photo-Gallery.org 

Core Data with multiple managed object contexts Photo by Free-Photo-Gallery.org 

Matt(hew) Morey matthewmorey.com | @xzolian Senior Developer at ChaiONE Traveler Boardsport Junkie 

Agenda 1)Core Data Basics 2)Concurrency Problems 3)Concurrency Solutions 

Agenda 1)Core Data Basics 2)Concurrency Problems 3)Concurrency Solutions 

Basics 

Basics App App App App App App NSManagedObject NSManagedObject NSManagedObject NSManagedObject NSManagedObject NSManagedObject NSManagedObjectContext NSManagedObjectContext NSManagedObjectContext NSManagedObjectContext NSManagedObjectContext NSManagedObjectContext NSPersistentStoreCoordinator NSPersistentStoreCoordinator NSPersistentStoreCoordinator NSManagedObjectModel NSManagedObjectModel NSManagedObjectModel NSPersistentStore NSPersistentStore NSPersistentStore NSPersistentStore NSPersistentStore NSPersistentStore SQLite XML Binary Binary In Memory Custom 

Basics App App App App App App NSManagedObject NSManagedObject NSManagedObject NSManagedObject NSManagedObject NSManagedObject NSManagedObjectContext NSManagedObjectContext NSManagedObjectContext NSManagedObjectContext NSManagedObjectContext NSManagedObjectContext NSPersistentStoreCoordinator NSPersistentStoreCoordinator NSPersistentStoreCoordinator NSManagedObjectModel NSManagedObjectModel NSManagedObjectModel NSPersistentStore NSPersistentStore NSPersistentStore NSPersistentStore NSPersistentStore NSPersistentStore SQLite XML Binary Binary In Memory Custom 

Managed Object Model 

Managed Object Model 

Managed Object Model - (NSManagedObjectModel *)managedObjectModel { if (_managedObjectModel != nil) { return _managedObjectModel; } NSURL *modelURL = [[NSBundle mainBundle] URLForResource:@"core-data" withExtension:@"momd"]; _managedObjectModel = [[NSManagedObjectModel alloc] initWithContentsOfURL:modelURL]; return _managedObjectModel; } 

Persistent Store Coordinator 

Persistent STore Coordinator - (NSPersistentStoreCoordinator *)persistentStoreCoordinator { if (_persistentStoreCoordinator != nil) { return _persistentStoreCoordinator; } NSURL *storeURL = [[self applicationDocumentsDirectory] URLByAppendingPathComponent:@"core-data.sqlite"]; NSError *error = nil; _persistentStoreCoordinator = [[NSPersistentStoreCoordinator alloc] initWithManagedObjectModel:[self managedObjectModel]]; if (![_persistentStoreCoordinator addPersistentStoreWithType:NSSQLiteStoreType configuration:nil URL:storeURL options:nil error:&error]) { /* Replace this implementation with code to handle the error appropriately. ... */ NSLog(@"Unresolved error %@, %@", error, [error userInfo]); abort(); } return _persistentStoreCoordinator; } 

Persistent STore Coordinator - (NSPersistentStoreCoordinator *)persistentStoreCoordinator { if (_persistentStoreCoordinator != nil) { return _persistentStoreCoordinator; } NSURL *storeURL = [[self applicationDocumentsDirectory] URLByAppendingPathComponent:@"core-data.sqlite"]; NSError *error = nil; _persistentStoreCoordinator = [[NSPersistentStoreCoordinator alloc] initWithManagedObjectModel:[self managedObjectModel]]; if (![_persistentStoreCoordinator addPersistentStoreWithType:NSSQLiteStoreType configuration:nil URL:storeURL options:nil error:&error]) { /* Replace this implementation with code to handle the error appropriately. ... */ NSLog(@"Unresolved error %@, %@", error, [error userInfo]); abort(); } return _persistentStoreCoordinator; } 

Managed Object Context 

Managed Object Context - (NSManagedObjectContext *)managedObjectContext { if (_managedObjectContext != nil) { return _managedObjectContext; } NSPersistentStoreCoordinator *coordinator = [self persistentStoreCoordinator]; if (coordinator != nil) { _managedObjectContext = [[NSManagedObjectContext alloc] init]; [_managedObjectContext setPersistentStoreCoordinator:coordinator]; } return _managedObjectContext; } 

Single Context 

Single Context 

Single Context Code Example https://github.com/mmorey/CoreDataMultiContext/tree/blocking 

Agenda 1)Core Data Basics 2)Concurrency Problems 3)Concurrency Solutions 

Problems ‣Core Data Managed Objects are not thread safe ‣Must pass Object IDs to use across threads ‣Objects are locked for all operations including read ‣Objects that feed the UI must be fetched on the main thread 

Agenda 1)Core Data Basics 2)Concurrency Problems 3)Concurrency Solutions 

Traditional Multi-Context Pre-iOS 5: Thread Confinement ‣Single NSMangedObjectContext per thread ‣Manual notifications, merging, and saving ‣Fairly easy to understand, but harder to manage 

Traditional Multi-Context 

Traditional Multi-Context - (void)loadData{ dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0), ^{ // Create temp context NSManagedObjectContext *context = [[NSManagedObjectContext alloc] init]; [context setPersistentStoreCoordinator:[(AppDelegate *)[[UIApplication sharedApplication] delegate] persistentStoreCoordinator]]; // // Do lots of async work here // // Save the context. error = nil; if (![context save:&error]) { // Replace this implementation with code to handle the error appropriately. abort(); } }); } // Register for save notification in ViewDidLoad [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(contextDidSave:) name:NSManagedObjectContextDidSaveNotification object:context]; 

Traditional Multi-Context - (void)loadData{ dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0), ^{ // Create temp context NSManagedObjectContext *context = [[NSManagedObjectContext alloc] init]; [context setPersistentStoreCoordinator:[(AppDelegate *)[[UIApplication sharedApplication] delegate] persistentStoreCoordinator]]; // // Do lots of async work here // // Save the context. error = nil; if (![context save:&error]) { // Replace this implementation with code to handle the error appropriately. abort(); } }); } // Register for save notification in ViewDidLoad [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(contextDidSave:) name:NSManagedObjectContextDidSaveNotification object:context]; 

Traditional Multi-Context - (void)loadData{ dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0), ^{ // Create temp context NSManagedObjectContext *context = [[NSManagedObjectContext alloc] init]; [context setPersistentStoreCoordinator:[(AppDelegate *)[[UIApplication sharedApplication] delegate] persistentStoreCoordinator]]; // // Do lots of async work here // // Save the context. error = nil; if (![context save:&error]) { // Replace this implementation with code to handle the error appropriately. abort(); } }); } // Register for save notification in ViewDidLoad [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(contextDidSave:) name:NSManagedObjectContextDidSaveNotification object:context]; 

Traditional Multi-Context - (void)contextDidSave:(NSNotification *)notification { dispatch_async(dispatch_get_main_queue(), ^{ NSManagedObjectContext *mainContext = [self.fetchedResultsController managedObjectContext]; [mainContext mergeChangesFromContextDidSaveNotification:notification]; }); } // Or - (void)contextDidSave:(NSNotification *)notification { [self.managedObjectContext performSelectorOnMainThread: @selector(mergeChangesFromContextDidSaveNotification:) withObject:notification waitUntilDone:NO]; } 

Traditional Multi-Context - (void)contextDidSave:(NSNotification *)notification { dispatch_async(dispatch_get_main_queue(), ^{ NSManagedObjectContext *mainContext = [self.fetchedResultsController managedObjectContext]; [mainContext mergeChangesFromContextDidSaveNotification:notification]; }); } // Or - (void)contextDidSave:(NSNotification *)notification { [self.managedObjectContext performSelectorOnMainThread: @selector(mergeChangesFromContextDidSaveNotification:) withObject:notification waitUntilDone:NO]; } 

Traditional Multi-Context Code Example https://github.com/mmorey/CoreDataMultiContext/tree/notification-context 

Parent Child Context ≥ iOS 5: Parent Child Contexts ‣Grand Central Dispatch private dispatch queues ‣Threading managed for you, no manual synchronization required ‣Less complicated and more flexible than pre-iOS 5 method ‣Context can and should be nested 

Concurrency Types ‣NSConfinementConcurrencyType ‣Separate contexts for each thread ‣Default, Legacy option ‣NSPrivateQueueConcurrencyType ‣MOC maintains private serialized queue ‣Can be created from any other thread ‣Idle queues are more efficient than extra threads ‣NSMainQueueConcurrencyType ‣Similar to private queue but on the main queue NSManagedObjectContext *temporaryContext = [[NSManagedObjectContext alloc] initWithConcurrencyType:NSPrivateQueueConcurrencyType]; 

Parent Child Context 

Parent Child Context PSC PSC 

Parent Child Context __block NSManagedObjectContext *managedObjectContext = [(AppDelegate *)[[UIApplication sharedApplication] delegate] managedObjectContext]; NSManagedObjectContext *temporaryContext = [[NSManagedObjectContext alloc] initWithConcurrencyType:NSPrivateQueueConcurrencyType]; temporaryContext.parentContext = managedObjectContext; [temporaryContext performBlock:^{ // // Do lots of async work here // // Save the context. NSError *error = nil; if (![temporaryContext save:&error]) {abort();} [managedObjectContext performBlock:^{ // Save the context. NSError *error = nil; if (![managedObjectContext save:&error]) {abort();} }]; // main }]; // temp context 

Parent Child Context __block NSManagedObjectContext *managedObjectContext = [(AppDelegate *)[[UIApplication sharedApplication] delegate] managedObjectContext]; NSManagedObjectContext *temporaryContext = [[NSManagedObjectContext alloc] initWithConcurrencyType:NSPrivateQueueConcurrencyType]; temporaryContext.parentContext = managedObjectContext; [temporaryContext performBlock:^{ // // Do lots of async work here // // Save the context. NSError *error = nil; if (![temporaryContext save:&error]) {abort();} [managedObjectContext performBlock:^{ // Save the context. NSError *error = nil; if (![managedObjectContext save:&error]) {abort();} }]; // main }]; // temp context 

Parent Child Context __block NSManagedObjectContext *managedObjectContext = [(AppDelegate *)[[UIApplication sharedApplication] delegate] managedObjectContext]; NSManagedObjectContext *temporaryContext = [[NSManagedObjectContext alloc] initWithConcurrencyType:NSPrivateQueueConcurrencyType]; temporaryContext.parentContext = managedObjectContext; [temporaryContext performBlock:^{ // // Do lots of async work here // // Save the context. NSError *error = nil; if (![temporaryContext save:&error]) {abort();} [managedObjectContext performBlock:^{ // Save the context. NSError *error = nil; if (![managedObjectContext save:&error]) {abort();} }]; // main }]; // temp context Private Queue save propagates up to parent 

Parent Child Context __block NSManagedObjectContext *managedObjectContext = [(AppDelegate *)[[UIApplication sharedApplication] delegate] managedObjectContext]; NSManagedObjectContext *temporaryContext = [[NSManagedObjectContext alloc] initWithConcurrencyType:NSPrivateQueueConcurrencyType]; temporaryContext.parentContext = managedObjectContext; [temporaryContext performBlock:^{ // // Do lots of async work here // // Save the context. NSError *error = nil; if (![temporaryContext save:&error]) {abort();} [managedObjectContext performBlock:^{ // Save the context. NSError *error = nil; if (![managedObjectContext save:&error]) {abort();} }]; // main }]; // temp context Save to disc still locks PS which will block the UI during read operations 

Basics App App App App App App NSManagedObject NSManagedObject NSManagedObject NSManagedObject NSManagedObject NSManagedObject NSManagedObjectContext NSManagedObjectContext NSManagedObjectContext NSManagedObjectContext NSManagedObjectContext NSManagedObjectContext NSPersistentStoreCoordinator NSPersistentStoreCoordinator NSPersistentStoreCoordinator NSManagedObjectModel NSManagedObjectModel NSManagedObjectModel NSPersistentStore NSPersistentStore NSPersistentStore NSPersistentStore NSPersistentStore NSPersistentStore SQLite XML Binary Binary In Memory Custom 

Async Saving - Parent Child Context 

Async Saving - Parent Child Context // Child context - (NSManagedObjectContext *)managedObjectContext{ if (_managedObjectContext != nil) { return _managedObjectContext; } _managedObjectContext = [[NSManagedObjectContext alloc] initWithConcurrencyType:NSMainQueueConcurrencyType]; _managedObjectContext.parentContext = [self writerManagedObjectContext]; return _managedObjectContext; } // Parent context - (NSManagedObjectContext *)writerManagedObjectContext{ if (_writerManagedObjectContext != nil) { return _writerManagedObjectContext; } NSPersistentStoreCoordinator *coordinator = [self persistentStoreCoordinator]; if (coordinator != nil) { _writerManagedObjectContext = [[NSManagedObjectContext alloc] initWithConcurrencyType: NSPrivateQueueConcurrencyType]; [_writerManagedObjectContext setPersistentStoreCoordinator:coordinator]; } return _writerManagedObjectContext; } 

Async Saving - Parent Child Context // Child context - (NSManagedObjectContext *)managedObjectContext{ if (_managedObjectContext != nil) { return _managedObjectContext; } _managedObjectContext = [[NSManagedObjectContext alloc] initWithConcurrencyType:NSMainQueueConcurrencyType]; _managedObjectContext.parentContext = [self writerManagedObjectContext]; return _managedObjectContext; } // Parent context - (NSManagedObjectContext *)writerManagedObjectContext{ if (_writerManagedObjectContext != nil) { return _writerManagedObjectContext; } NSPersistentStoreCoordinator *coordinator = [self persistentStoreCoordinator]; if (coordinator != nil) { _writerManagedObjectContext = [[NSManagedObjectContext alloc] initWithConcurrencyType: NSPrivateQueueConcurrencyType]; [_writerManagedObjectContext setPersistentStoreCoordinator:coordinator]; } return _writerManagedObjectContext; } 

Async Saving - Parent Child Context // Child context - (NSManagedObjectContext *)managedObjectContext{ if (_managedObjectContext != nil) { return _managedObjectContext; } _managedObjectContext = [[NSManagedObjectContext alloc] initWithConcurrencyType:NSMainQueueConcurrencyType]; _managedObjectContext.parentContext = [self writerManagedObjectContext]; return _managedObjectContext; } // Parent context - (NSManagedObjectContext *)writerManagedObjectContext{ if (_writerManagedObjectContext != nil) { return _writerManagedObjectContext; } NSPersistentStoreCoordinator *coordinator = [self persistentStoreCoordinator]; if (coordinator != nil) { _writerManagedObjectContext = [[NSManagedObjectContext alloc] initWithConcurrencyType: NSPrivateQueueConcurrencyType]; [_writerManagedObjectContext setPersistentStoreCoordinator:coordinator]; } return _writerManagedObjectContext; } 

Parent Child Context __block NSManagedObjectContext *writerObjectContext = [(AppDelegate *)[[UIApplication sharedApplication] delegate] writerManagedObjectContext]; __block NSManagedObjectContext *managedObjectContext = [(AppDelegate *)[[UIApplication sharedApplication] delegate] managedObjectContext]; __block NSManagedObjectContext *temporaryContext = [[NSManagedObjectContext alloc] initWithConcurrencyType:NSPrivateQueueConcurrencyType]; temporaryContext.parentContext = managedObjectContext; [temporaryContext performBlock:^{ // // Do lots of async work here // [temporaryContext save:&error]; {abort();} // Save the context. [managedObjectContext performBlock:^{ [managedObjectContext save:&error]; {abort();} // Save the context. [writerObjectContext performBlock:^{ [writerObjectContext save:&error]; {abort();} // Save the context. }]; // writer }]; // main }]; // temp context 

Parent Child Context __block NSManagedObjectContext *writerObjectContext = [(AppDelegate *)[[UIApplication sharedApplication] delegate] writerManagedObjectContext]; __block NSManagedObjectContext *managedObjectContext = [(AppDelegate *)[[UIApplication sharedApplication] delegate] managedObjectContext]; __block NSManagedObjectContext *temporaryContext = [[NSManagedObjectContext alloc] initWithConcurrencyType:NSPrivateQueueConcurrencyType]; temporaryContext.parentContext = managedObjectContext; [temporaryContext performBlock:^{ // // Do lots of async work here // [temporaryContext save:&error]; {abort();} // Save the context. [managedObjectContext performBlock:^{ [managedObjectContext save:&error]; {abort();} // Save the context. [writerObjectContext performBlock:^{ [writerObjectContext save:&error]; {abort();} // Save the context. }]; // writer }]; // main }]; // temp context NSMainQueueConcurrency NSPrivateQueueConcurrencyType 

Asynchronous Saving - Parent Child Context Code Example https://github.com/mmorey/CoreDataMultiContext/tree/parent-context 

Still Blocking? 

Still Blocking? Small and frequent saves during import 

Still Blocking? Wait for opportunity when user won’t notice 

Still Blocking? - (void)setStalenessInterval:(NSTimeInterval)expiration 

Still Blocking? NSFetchedResultsController *aFetchedResultsController = [[NSFetchedResultsController alloc] initWithFetchRequest:fetchRequest managedObjectContext:self.managedObjectContext sectionNameKeyPath:@"sections" cacheName:@"default-cache"]; 

Still Blocking? For VERY LARGE amounts of data it may be better to generate the SQLite file on the server, download it asynchronously, and set it up as an additional persistent store. 

References Nested MOC Release Notes: http://developer.apple.com/library/mac/#releasenotes/DataManagement/RN-CoreData/index.html Core Data Programming Guide: http://developer.apple.com/library/mac/#documentation/cocoa/Conceptual/CoreData/cdProgrammingGuide.html Cocoanetics Blog: http://www.cocoanetics.com/2012/07/multi-context-coredata/ http://www.cocoanetics.com/2013/02/zarra-on-locking/ 

Thanks! Questions? Get in Touch. Twitter: @xzolian App.net: @morey Email: [email protected] URL: http://matthewmorey.com