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I am writing a WatchKit extension for an iPhone application. I share Core Data using App Groups. This is working, the data is shared. When a new NSManagedObject is created using watch I send a notification to the iPhone app that a new object was created. To do that I use MMWormhole. The iPhone app receives the MMWormhole notification and now I have to do the last step - refresh NSManagedObjectContext. How can I do it?

I was trying to forward notification of NSManagedObjectContextDidSaveNotification inside MMWormhole notification and to use mergeChangesFromContextDidSaveNotification in the iPhone app, but it doesn’t work as MMWormhole serialize the notification and NSManagedObject doesn’t support it.

sash
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1 Answers1

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The simple way is just to have the app reload its data. Re-do any fetches so that you get the latest data from the persistent store.

If you want to make it more sophisticated, do something like this:

In the watch extension, for every new/changed/deleted object,

  • Call objectID to get the NSManagedObjectID
  • Convert the object ID to a string URIRepresentation
  • Pass these strings in the MMWormhole message

In the app, when receiving the message,

  • Use [NSPersistentStoreCoordinator managedObjectIDForURIRepresentation:] to convert the strings back to an NSManagedObjectID
  • Use [NSManagedObjectContext existingObjectWithID:] to get the managed object corresponding to the object ID.

Now you know which objects need refreshing.

Tom Harrington
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  • What exactly do you mean by "reload its data". How can I do it? I have NSFetchedResultsController and even if I performFetch, nothing happens. I believe that NSManagedObjectContext on the iPhone app doesn't know about changes inside the persistent store. In that case, will existingObjectWithID work? – sash Apr 10 '15 at 18:49
  • If the changes have been saved to the persistent store, `existingObjectWithID:` will work. You may need to use `shouldRefreshRefetchedObjects` to get fresh values from the store file. Also, after redoing the fetch, make sure to update the UI (table view, probably) displaying the data. – Tom Harrington Apr 10 '15 at 20:54
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    Thanks for the help Tom. It worked, but I also had to set stalenessInterval=0. If not, context didn’t see new objects. Not sure if it is correct way to do? – sash Apr 12 '15 at 19:22
  • Cool. Thanks for the help. It would be nice if you could update the answer. – sash Apr 13 '15 at 08:25