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.

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
Senior Developer at ChaiONE
Traveler
Boardsport Junkie
matthewmorey.com | @xzolian

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
NSManagedObject
NSManagedObjectContext
NSPersistentStoreCoordinator NSManagedObjectModel
NSPersistentStore
SQLite XML Binary In Memory Custom

8. Basics
App
NSManagedObject
NSManagedObjectContext
NSPersistentStoreCoordinator NSManagedObjectModel
NSPersistentStore
SQLite XML 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
NSManagedObjectContext *temporaryContext = [[NSManagedObjectContext alloc]
initWithConcurrencyType:NSPrivateQueueConcurrencyType];
‣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

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:^{
Private Queue save
//
// Do lots of async work here
//
propagates up to parent
// 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

38. Parent Child Context
__block NSManagedObjectContext *managedObjectContext = [(AppDelegate *)[[UIApplication sharedApplication]
delegate] managedObjectContext];
NSManagedObjectContext *temporaryContext = [[NSManagedObjectContext alloc]
initWithConcurrencyType:NSPrivateQueueConcurrencyType];
temporaryContext.parentContext = managedObjectContext;
[temporaryContext performBlock:^{ Save to disc still locks PS
// which will block the UI during
// Do lots of async work here
// read operations
// 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

39. Basics
App
NSManagedObject
NSManagedObjectContext
NSPersistentStoreCoordinator NSManagedObjectModel
NSPersistentStore
SQLite XML 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:^{ NSPrivateQueueConcurrencyType
//
// Do lots of async work here
// NSMainQueueConcurrency
[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

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