Core Data with Multiple Managed Object Contexts

When using Core Data for persisting app data multiple managed object contexts (MOC) are often required to avoid blocking UI. Typically you would create a background MOC and listen for changes on the main MOC, merging changes as necessary. With iOS 5, MOCs now have parent context and the ability to set concurrency types. These new features greatly simplify dealing with Core Data on background queues. During this presentation Matt will cover the pros and cons of this new method of dealing with Core Data.

Transcript

1. Core Data with multiple managed object contexts Photo by Free-Photo-Gallery.org
2. Core Data with multiple managed object contexts Photo by Free-Photo-Gallery.org
3. Matt(hew) Morey matthewmorey.com | @xzolian Senior Developer at ChaiONE Traveler Boardsport Junkie
4. Agenda 1)Core Data Basics 2)Concurrency Problems 3)Concurrency Solutions
5. Agenda 1)Core Data Basics 2)Concurrency Problems 3)Concurrency Solutions
6. Basics
7. 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
8. 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
9. Managed Object Model
10. Managed Object Model
11. 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; }
12. Persistent Store Coordinator
13. 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; }
14. 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; }
15. Managed Object Context
16. 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; }
17. Single Context
18. Single Context
19. Single Context Code Example https://github.com/mmorey/CoreDataMultiContext/tree/blocking
20. Agenda 1)Core Data Basics 2)Concurrency Problems 3)Concurrency Solutions
21. 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
22. Agenda 1)Core Data Basics 2)Concurrency Problems 3)Concurrency Solutions
23. 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
24. Traditional Multi-Context
25. 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];
26. 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];
27. 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];
28. 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]; }
29. 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]; }
30. Traditional Multi-Context Code Example https://github.com/mmorey/CoreDataMultiContext/tree/notification-context
31. 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
32. 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];
33. Parent Child Context
34. Parent Child Context PSC PSC
35. 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
36. 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
37. 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
38. 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
39. 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
40. Async Saving - Parent Child Context
41. 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; }
42. 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; }
43. 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; }
44. 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
45. 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
46. Asynchronous Saving - Parent Child Context Code Example https://github.com/mmorey/CoreDataMultiContext/tree/parent-context
47. Still Blocking?
48. Still Blocking? Small and frequent saves during import
49. Still Blocking? Wait for opportunity when user won’t notice
50. Still Blocking? - (void)setStalenessInterval:(NSTimeInterval)expiration
51. Still Blocking? NSFetchedResultsController *aFetchedResultsController = [[NSFetchedResultsController alloc] initWithFetchRequest:fetchRequest managedObjectContext:self.managedObjectContext sectionNameKeyPath:@"sections" cacheName:@"default-cache"];
52. 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.
53. 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/
54. Thanks! Questions? Get in Touch. Twitter: @xzolian App.net: @morey Email: matt@matthewmorey.com URL: http://matthewmorey.com