The document discusses creating a custom data observer in Core Data to handle NSManagedObject changes and saves. It highlights the limitations of NSFetchedResultsController and the need for more granular UI updates or background actions, utilizing relevant notifications from NSManagedObjectContext. It also addresses concurrency considerations and provides a reference implementation for managing these notifications effectively.

1. Build your own
Core Data Observer
Zach McArtor, @zmcartor, http://hackazach.net

2. Wat?
• Small component that notifies us of
NSManagedObject changes/saves
• React to those changes via method invocation

3. Why?
• Sometimes, an NSFetchedResultsController
won’t cut it
• Want to update a tiny piece of UI or run a
background action
• Be reactive!

4. Relevant Notifications
Fired by NSManagedObjectContext
• NSManagedObjectContextObjectsDidChangeNotification
• NSManagedObjectContextDidSaveNotification

5. Subscribe, and ye shall receive
[[NSNotificationCenter defaultCenter] addObserver:self
selector:@selector(contextDidSave:)
name:NSManagedObjectContextDidSaveNotification
object:SomeManagedObjectContext];

6. An NSDict of Changes!
• NSDict keyed by these constants:
• { NSInsertedObjectsKey,
• NSUpdatedObjectsKey
• NSDeletedObjectsKey. }

7. Last step, add a predicate
• Subscribing to didChange/DidSave triggers a
callback for every model save/change. We only
care about a subset of models.
• Abstract notification subscription into Class
• Predicate filter models returned in NSDict. (Protip!)

8. Dealing Concurrency With
• There are different ways to set up a concurrent
Core Data stack :(
!
!
!
http://floriankugler.com/blog/2013/4/2/theconcurrentcoredatastack
https://www.cocoanetics.com/2012/07/multi-context-coredata/

9. Which context to subscribe?
• ‘didSave’ notifications fired during context save
(duh.) Going up chain child->parent
• ’didChange’ notifications fired when context
merges changes (MagicalRecord merges down into default context)

10. Watch out!
• The managedObjects received via notification
may not be in the correct context for use on
main thread.
• Use existingObjectWithId:error: to pull into
correct context
• Subscribe to a single context, don’t listen for
every event within a nested system

11. Thanks for
observing this
talk!
Reference implementation :
https://github.com/mzarra/ZDS_Shared/blob/master/
ZSContextWatcher.m