![Subscribe, and ye shall receive
[[NSNotificationCenter defaultCenter] addObserver:self
selector:@selector(contextDidSave:)
name:NSManagedObjectContextDidSaveNotification
object:SomeManagedObjectContext];](https://image.slidesharecdn.com/coredataobserver-140502172203-phpapp02/85/Create-a-Core-Data-Observer-in-10mins-5-320.jpg)
Create a Core Data Observer in 10mins
- 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